Build a change once: the check builds what its merge moves, the walk takes it by key

A merge built every moved module a second time although its check had judged
the same tree (issue 649). The check now builds into the scratch namespace
under a reuse key (source fingerprint plus the builder's own digest), and the
walk of the trunk commit copies a build of the same key by digest instead of
building, saying per module why when it cannot (novox/hq ADR 0350).

cmd/mesh-builder/holder.go

@@ -41,6 +41,9 @@ type holder struct {
 	say func(link.HolderState) error
 	// remove runs what a kill needs outside the build: the containers left behind.
 	remove builder.Runner
+	// scratch is where check builds are kept (novox/hq ADR 0350 §5): what the week's sweep lets go of. Nil where
+	// this machine publishes to no artifact store.
+	scratch builder.Scratch
 
 	mu      sync.Mutex
 	paused  bool
@@ -131,6 +134,34 @@ func (h *holder) stateWhilePaused(ctx context.Context, every time.Duration) {
 	}
 }
 
+// expireChecks lets go of every check build older than a week (novox/hq ADR 0350 §5), at start and every while
+// after. Every holder sweeps the one store; what one let go of, another finds gone.
+func (h *holder) expireChecks(ctx context.Context, every time.Duration, now func() time.Time) {
+	sweep := func() {
+		if h.scratch == nil {
+			return
+		}
+		n, err := h.scratch.Expire(ctx, builder.CheckBuildsKept, now())
+		switch {
+		case err != nil:
+			fmt.Fprintf(os.Stderr, "check builds older than %s: %d let go of, and not all: %v\n", builder.CheckBuildsKept, n, err)
+		case n > 0:
+			fmt.Fprintf(os.Stderr, "check builds older than %s: %d check(s)' builds let go of\n", builder.CheckBuildsKept, n)
+		}
+	}
+	sweep()
+	tick := time.NewTicker(every)
+	defer tick.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-tick.C:
+			sweep()
+		}
+	}
+}
+
 // begin records the build this machine took, with the cancel that ends it.
 func (h *holder) begin(request link.BuildRequest, cancel context.CancelFunc) *running {
 	r := &running{request: request, started: time.Now(), cancel: cancel, done: make(chan struct{})}

cmd/mesh-builder/main.go

@@ -97,6 +97,9 @@ func run() error {
 	if err := notReady(); err != nil {
 		return err
 	}
+	// **The builder's own version is part of every reuse key** (novox/hq ADR 0350 §1): a builder that builds
+	// differently never takes a build another one made.
+	builder.Version = builder.SelfVersion(version)
 	credential, err := brokerFrom()
 	if err != nil {
 		return err
@@ -174,6 +177,9 @@ func run() error {
 
 	fmt.Fprintf(os.Stderr, "building for the mesh, publishing to %s\n", registry)
 	publisher := builder.Registry{Address: registry, Run: builder.Command}
+	// Check builds are let go of after a week (novox/hq ADR 0350 §5).
+	h.scratch = publisher
+	go h.expireChecks(ctx, time.Hour, time.Now)
 
 	return machine.Take(ctx, func(ctx context.Context, work link.Build) {
 		answer(ctx, publisher, on, workspace, work, h)
@@ -262,6 +268,7 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 	if err == nil {
 		say("packages", saysRegistry(npmrc))
 	}
+	registryHere, isRegistry := publisher.(builder.Registry)
 	var built builder.Result
 	if request.Check != nil {
 		// **A pull request's merge check, not a build** (novox/hq to-be 45 §9): nothing is built,
@@ -271,12 +278,21 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 		if r, ok := publisher.(builder.Registry); ok {
 			registry = r.Address
 		}
+		spec := checkSpecOf(request)
+		// What the merge would move, built by the check for the walk to take (novox/hq ADR 0350 §1): into the
+		// scratch namespace. **Never with the package registry's credential**: this is code nobody has approved
+		// yet, and a recipe could read the credential it is handed (issue 462). A module that resolves packages
+		// at build time has no reuse key anyway, so nothing is lost by refusing it here.
+		if isRegistry && len(spec.Builds) > 0 {
+			spec.BuildWith = &builder.CheckBuilding{Publish: publisher, Scratch: registryHere, Held: request.Held,
+				Seats: request.Seats}
+		}
 		var v builder.CheckVerdict
-		v, err = builder.Check(building, builder.Command, checkSpecOf(request), workspace, registry, forgeFrom(), say)
+		v, err = builder.Check(building, builder.Command, spec, workspace, registry, forgeFrom(), say)
 		if err == nil {
 			result.Check = &link.CheckOutcome{Verdict: v.Verdict, Summary: v.Summary, Report: v.Report,
 				Took: v.Took.Round(time.Second).String(), Gate: layerOf(v.Gate), RepoCheck: layerOf(v.Repo),
-				Head: v.Head, Trunk: v.Trunk, Beside: v.Beside}
+				Head: v.Head, Trunk: v.Trunk, Beside: v.Beside, Built: checkBuiltOf(v.Built)}
 			// What each of a group's heads was judged as (novox/hq ADR 0323 §6).
 			for _, m := range v.Members {
 				result.Check.Members = append(result.Check.Members, link.CheckedMember{Owner: m.Owner, Repo: m.Repo,
@@ -289,9 +305,20 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 		// after a clone that then fails at npm ci.
 		// Every container it starts is labelled with its id, so a kill finds what outlived the
 		// docker client (ADR 0219).
-		built, err = builder.Build(building, builder.Labelled(builder.Command, request.ID), publisher,
-			request.Repository, request.Path, request.Ref, workspace, request.Held, npmrc,
-			forgeFrom(), say, request.Seats)
+		if request.Reuse != nil && isRegistry && !request.DryRun {
+			// **A walk's build takes the check's build of the same key** (novox/hq ADR 0350 §2).
+			reuse := builder.Reuse{Scratch: registryHere}
+			for _, p := range request.Reuse.Pulls {
+				reuse.Pulls = append(reuse.Pulls, builder.PullRef{Owner: p.Owner, Repo: p.Repo, Number: p.Number})
+			}
+			built, err = builder.BuildReusing(building, builder.Labelled(builder.Command, request.ID), publisher, reuse,
+				request.Repository, request.Path, request.Ref, workspace, request.Held, npmrc,
+				forgeFrom(), say, request.Seats)
+		} else {
+			built, err = builder.Build(building, builder.Labelled(builder.Command, request.ID), publisher,
+				request.Repository, request.Path, request.Ref, workspace, request.Held, npmrc,
+				forgeFrom(), say, request.Seats)
+		}
 	}
 	// What it copied into the store, whatever became of the build (novox/hq ADR 0257).
 	result.Mirrored = built.Mirrored
@@ -327,6 +354,19 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 			}
 			result.Against = built.Against
 			result.SourceFingerprint = built.Source
+			// What the walk did instead of building, or why it built (novox/hq ADR 0350 §3, §4).
+			result.ReuseKey = built.ReuseKey
+			if built.Reused != nil {
+				f := built.Reused.From
+				result.Reused = built.Reused.Words()
+				result.ReusedFrom = &link.ReusedFrom{Pull: link.PullRef{Owner: f.Pull.Owner, Repo: f.Pull.Repo,
+					Number: f.Pull.Number}, Head: f.Head, Trunk: f.Trunk, Check: f.Check, Key: f.Key}
+			} else if request.Reuse != nil {
+				result.BuiltWhy = built.Why
+				if result.BuiltWhy == "" {
+					result.BuiltWhy = "it has no artifact to build"
+				}
+			}
 			result.Trunk, result.OnTrunk, result.Branches = built.Trunk, built.OnTrunk, built.Branches
 			for _, r := range built.Read {
 				result.Read = append(result.Read, link.ReadRepository{Repository: r.Repository, Ref: r.Ref})
@@ -377,6 +417,10 @@ func checkSpecOf(request link.BuildRequest) builder.CheckSpec {
 	for dir, b := range c.Beside {
 		spec.Beside[dir] = builder.Beside{Repository: b.Repository, Ref: b.Ref}
 	}
+	for _, b := range c.Builds {
+		spec.Builds = append(spec.Builds, builder.CheckBuildSpec{Module: b.Module, Repository: b.Repository, Path: b.Path,
+			Ref: b.Ref, Merged: b.Merged, Tier: b.Tier, Skip: b.Skip})
+	}
 	for _, m := range c.Members {
 		spec.Group = append(spec.Group, builder.GroupHead{Owner: m.Owner, Repo: m.Repo, Repository: m.Repository,
 			Ref: m.Ref, Paths: m.Paths, Base: m.Base})
@@ -384,6 +428,19 @@ func checkSpecOf(request link.BuildRequest) builder.CheckSpec {
 	return spec
 }
 
+// checkBuiltOf is a check's builds for its merge as the outcome carries them (novox/hq ADR 0350 §1).
+func checkBuiltOf(built []builder.CheckBuilt) []link.CheckBuilt {
+	var out []link.CheckBuilt
+	for _, b := range built {
+		c := link.CheckBuilt{Module: b.Module, Key: b.Key, NotBuilt: b.NotBuilt, Took: b.Took.Round(time.Second).String()}
+		for _, m := range b.Made {
+			c.Made = append(c.Made, link.MadeArtifact{Name: m.Name, Kind: m.Kind, Reference: m.Reference})
+		}
+		out = append(out, c)
+	}
+	return out
+}
+
 // layerOf is one layer of a check as the outcome carries it.
 func layerOf(l *builder.Layer) *link.CheckLayer {
 	if l == nil {

cmd/mesh-controller/build.go

@@ -15,6 +15,7 @@ import (
 	"github.com/nats-io/nats.go"
 
 	"github.com/novox/mesh-controller/internal/broker"
+	"github.com/novox/mesh-controller/internal/builder"
 	"github.com/novox/mesh-controller/internal/catalogue"
 	"github.com/novox/mesh-controller/internal/inventory"
 	"github.com/novox/mesh-controller/internal/link"
@@ -483,6 +484,7 @@ func buildOneAsked(ctx context.Context, source buildSource, path, ref string, wa
 		Seats:      seatBases(ctx),
 		DryRun:     dryRun,
 	}
+	request.Reuse = reuseAsked(ctx, dryRun)
 	fmt.Printf("asked for %s", source)
 	if source.Seat != "" {
 		fmt.Printf(" (%s)", repository)
@@ -669,6 +671,12 @@ func takeIn(ctx context.Context, inv *inventory.Inventory, result link.BuildResu
 		return manifest, kept, fmt.Errorf("%s built %s (%s), recorded and not registered: %w", result.On,
 			manifest.Module, short(result.Commit), err)
 	}
+	// **Never a build in the scratch namespace** (novox/hq ADR 0239 §9, ADR 0350 §2): what a check builds is
+	// registered only as the walk's copy in the module's own namespace.
+	if scratch := inScratch(result.Made); scratch != "" {
+		return manifest, kept, fmt.Errorf("%s built %s (%s), recorded and not registered: %s is in the scratch "+
+			"namespace, which only a walk's copy leaves (ADR 0350)", result.On, manifest.Module, short(result.Commit), scratch)
+	}
 	// **A build that failed its gate is never registered again** (novox/hq ADR 0236): an outcome heard
 	// twice, or replayed, would otherwise make the build a rollback put back what the module is again,
 	// and the next push would send it.
@@ -714,6 +722,16 @@ func takeIn(ctx context.Context, inv *inventory.Inventory, result link.BuildResu
 	return manifest, kept, nil
 }
 
+// inScratch is the first artifact a build made into the scratch namespace; empty when none is.
+func inScratch(made []link.MadeArtifact) string {
+	for _, m := range made {
+		if path, kept := catalogue.InArtifactStore(catalogue.Recorded(m.Reference)); kept && strings.HasPrefix(path, builder.ScratchPrefix) {
+			return m.Reference
+		}
+	}
+	return ""
+}
+
 // errOffTheTrunk is a build of a commit off its repository's trunk, which is never published.
 var errOffTheTrunk = errors.New("the commit is not on its repository's trunk: a commit off the trunk is checked, " +
 	"never published (ADR 0238) — merge it, and the merge builds it")

cmd/mesh-controller/check_builds.go

@@ -0,0 +1,87 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"strconv"
+
+	"github.com/novox/mesh-controller/internal/builder"
+	"github.com/novox/mesh-controller/internal/inventory"
+)
+
+// A pull request's check builds go when it closes (novox/hq ADR 0350 §5): the build seat keeps them in the artifact
+// store's scratch namespace for the walk of its merge to take. **Let go of where the moment is heard**: the
+// controller ends the walk answering a merge — after that walk took what it reuses — and the delivery's owner, which
+// hears the forge say a pull request closed unmerged, asks the `forget-checks` verb; both let go through the store,
+// as the controller's own collection does. The build seat's holders let go of anything a week old on their own,
+// so a word lost here leaves a week's builds, never more.
+
+// checkBuildsIn is the scratch namespace as the controller reaches the artifact store.
+func checkBuildsIn(ctx context.Context, inv *inventory.Inventory) (builder.Scratch, error) {
+	shelf, err := inv.Catalogue(ctx)
+	if err != nil {
+		return nil, err
+	}
+	address, err := artifactStoreAddress(ctx, inv, shelf, "")
+	if err != nil {
+		return nil, err
+	}
+	if address == "" {
+		return nil, errors.New("the artifact store is not on the mesh's network")
+	}
+	return builder.Registry{Address: address}, nil
+}
+
+// forgetChecks lets go of a pull request's check builds and answers what it did.
+var forgetChecks = func(ctx context.Context, inv *inventory.Inventory, pull builder.PullRef) (string, error) {
+	scratch, err := checkBuildsIn(ctx, inv)
+	if err != nil {
+		return "", err
+	}
+	n, err := scratch.Forget(ctx, pull)
+	if err != nil {
+		return "", fmt.Errorf("%s: %d check(s)' builds let go of, and not all: %w", pull, n, err)
+	}
+	return fmt.Sprintf("%s: %d check(s)' builds let go of", pull, n), nil
+}
+
+// forgetChecksOfWalk lets go, on the save that ends a walk, of the check builds of every pull request whose merge
+// it answers: the walk took what it reuses. Said, never fatal.
+func forgetChecksOfWalk(ctx context.Context, inv *inventory.Inventory, p inventory.Plan) {
+	if p.WalkSaid != inventory.WalkEnded || p.Delivery == nil {
+		return
+	}
+	for _, pull := range reuseAskOf(&p).Pulls {
+		said, err := forgetChecks(ctx, inv, builder.PullRef{Owner: pull.Owner, Repo: pull.Repo, Number: pull.Number})
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "%s: its check builds stay until the week's sweep: %v\n", p.ID, err)
+			continue
+		}
+		fmt.Printf("%s: %s\n", p.ID, said)
+	}
+}
+
+// forgetChecksCommand is `forget-checks <owner> <repo> <number>`: the verb the delivery's owner asks when the forge
+// says a pull request closed unmerged (novox/hq ADR 0350 §5).
+func forgetChecksCommand(ctx context.Context, args []string) error {
+	if len(args) != 3 {
+		return errors.New("forget-checks <owner> <repo> <number>")
+	}
+	number, err := strconv.Atoi(args[2])
+	if err != nil || number <= 0 {
+		return fmt.Errorf("%q is not a pull request's number", args[2])
+	}
+	open, err := openStores(ctx)
+	if err != nil {
+		return err
+	}
+	defer open.Close()
+	said, err := forgetChecks(ctx, open.inventory, builder.PullRef{Owner: args[0], Repo: args[1], Number: number})
+	if err != nil {
+		return err
+	}
+	fmt.Println(said)
+	return nil
+}

cmd/mesh-controller/check_builds_test.go

@@ -0,0 +1,190 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"reflect"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/novox/mesh-controller/internal/builder"
+	"github.com/novox/mesh-controller/internal/catalogue"
+	"github.com/novox/mesh-controller/internal/inventory"
+	"github.com/novox/mesh-controller/internal/link"
+)
+
+// One build per change (novox/hq ADR 0350), as the controller asks it and takes it in.
+
+// The check of a change is asked to build what a merge of it would move, in the walk's order: a module of the
+// repository checked from the merge, one standing on it from its own repository as the walk builds it, and one
+// that reads the repository through a context not at all. A change that moves no module builds nothing.
+func TestACheckIsAskedToBuildWhatTheMergeWouldMove(t *testing.T) {
+	entries := []inventory.Entry{
+		{Manifest: catalogue.Manifest{Module: "app"}, Source: inventory.Source{Repository: "novox/shop", Seat: "git", Path: "modules/app"}},
+		{Manifest: catalogue.Manifest{Module: "web"}, Source: inventory.Source{Repository: "novox/web", Seat: "git", Ref: "main"}},
+		{Manifest: catalogue.Manifest{Module: "docs"}, Source: inventory.Source{Repository: "novox/docs", Seat: "git"}},
+	}
+	p := link.PullUpdated{Owner: "novox", Repo: "shop", Number: 7, Base: "main", Commit: "abc"}
+	scope := checkScope{Reach: mergeReach{Packaging: entries[2:], Plan: inventory.Plan{Tiers: [][]string{{"app", "docs"}, {"web"}}}}}
+	clone := func(s inventory.Source) (string, error) { return "http://forge.invalid/" + s.Repository + ".git", nil }
+
+	got := checkBuildsOf(p, scope, entries, clone)
+	want := []link.CheckBuild{
+		{Module: "app", Path: "modules/app", Merged: true, Tier: 0},
+		{Module: "docs", Tier: 0, Skip: "its build reads novox/shop through a context, which a check cannot hand the merge"},
+		{Module: "web", Repository: "http://forge.invalid/novox/web.git", Ref: "main", Tier: 1},
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("the check is asked to build\n%+v\nwant\n%+v", got, want)
+	}
+	if got := checkBuildsOf(p, checkScope{}, entries, clone); got != nil {
+		t.Fatalf("a change that moves no module asks for builds: %+v", got)
+	}
+}
+
+// A walk's ask carries its leave to take a check's build, naming the pull requests of the merges it answers; a dry
+// run never does, and neither does an ask that is not a walk's.
+func TestOnlyAWalksAskTakesAChecksBuildAndADryRunNever(t *testing.T) {
+	p := &inventory.Plan{Delivery: &inventory.PlanDelivery{Merges: []inventory.PlanMerge{
+		{Repository: "novox/shop", Number: 7}, {Repository: "http://forge.invalid/novox/web.git", Number: 3},
+		{Repository: "novox/shop", Number: 7}, {Repository: "novox/shop"}}}}
+	asked := reuseAskOf(p)
+	if want := []link.PullRef{{Owner: "novox", Repo: "shop", Number: 7}, {Owner: "novox", Repo: "web", Number: 3}}; !reflect.DeepEqual(asked.Pulls, want) {
+		t.Fatalf("the walk names %+v", asked.Pulls)
+	}
+	walk := withReuse(t.Context(), asked)
+	if reuseAsked(walk, false) != asked {
+		t.Fatal("a walk's ask does not carry its leave")
+	}
+	if reuseAsked(walk, true) != nil {
+		t.Fatal("a dry run carries a leave to take a check's build")
+	}
+	if reuseAsked(t.Context(), false) != nil {
+		t.Fatal("an ask that is not a walk's carries a leave")
+	}
+}
+
+// A walk ended lets go of its merges' check builds — only once it ended, after it took what it reuses.
+func TestAWalkEndedLetsGoOfItsMergesCheckBuilds(t *testing.T) {
+	var forgot []builder.PullRef
+	was := forgetChecks
+	forgetChecks = func(_ context.Context, _ *inventory.Inventory, pull builder.PullRef) (string, error) {
+		forgot = append(forgot, pull)
+		return "", nil
+	}
+	t.Cleanup(func() { forgetChecks = was })
+	p := inventory.Plan{ID: "plan-1", Delivery: &inventory.PlanDelivery{Merges: []inventory.PlanMerge{
+		{Repository: "novox/shop", Number: 7}}}}
+
+	p.WalkSaid = inventory.WalkStarted
+	forgetChecksOfWalk(t.Context(), nil, p)
+	p.WalkSaid = ""
+	forgetChecksOfWalk(t.Context(), nil, p)
+	if len(forgot) != 0 {
+		t.Fatalf("a walk not ended let go of %v, which it may still take", forgot)
+	}
+	p.WalkSaid = inventory.WalkEnded
+	forgetChecksOfWalk(t.Context(), nil, p)
+	if want := []builder.PullRef{{Owner: "novox", Repo: "shop", Number: 7}}; !reflect.DeepEqual(forgot, want) {
+		t.Fatalf("the walk ended let go of %v", forgot)
+	}
+}
+
+// A build in the scratch namespace is never registered: only the walk's copy in the module's own namespace is.
+func TestABuildInTheScratchNamespaceIsNeverRegistered(t *testing.T) {
+	if got := inScratch([]link.MadeArtifact{{Name: "server", Reference: "store.internal:5000/app/server@sha256:aa"}}); got != "" {
+		t.Fatalf("a module's own build reads as a scratch build: %s", got)
+	}
+	scratch := "store.internal:5000/scratch/app/server@sha256:aa"
+	if got := inScratch([]link.MadeArtifact{{Name: "server", Reference: scratch}}); got != scratch {
+		t.Fatalf("a scratch build was not found: %q", got)
+	}
+	if got := inScratch([]link.MadeArtifact{{Name: "look", Reference: "http://store.internal:5000/v2/scratch/app/look/blobs/sha256:bb"}}); got == "" {
+		t.Fatal("a scratch archive was not found")
+	}
+}
+
+// The walk says per module what it did: "reused from …" or "built: <why>" — and a build seat from before says
+// neither, which is said.
+func TestAWalksBuildSaysWhetherItReusedOrBuilt(t *testing.T) {
+	if got := (link.BuildResult{Reused: "reused from the check of novox/shop#7"}).MadeWords(); got != "reused from the check of novox/shop#7" {
+		t.Fatalf("said %q", got)
+	}
+	if got := (link.BuildResult{BuiltWhy: "no check built it"}).MadeWords(); got != "built: no check built it" {
+		t.Fatalf("said %q", got)
+	}
+	if got := (link.BuildResult{}).MadeWords(); !strings.Contains(got, "predates ADR 0350") {
+		t.Fatalf("said %q", got)
+	}
+}
+
+// A walk's module the check's build stood in for is registered by the digest the check built — the copy in the
+// module's namespace — and the walk keeps "reused from …" for it; its first-node gate judges that registered
+// build as it judges one the walk built. A scratch reference is refused. Needs the store (MESH_TEST_POSTGRES), and
+// is named in testdata/store-tests.
+func TestAReusedCheckBuildIsRegisteredByItsDigestAndGatedAsABuiltOne(t *testing.T) {
+	g := aGateMesh(t)
+	ctx := t.Context()
+	inv := g.open.inventory
+	gateEvery = 0
+
+	digest := "sha256:" + strings.Repeat("c", 64)
+	reused := func(id, reference string) link.BuildResult {
+		manifest := mustJSON(t, catalogue.Manifest{Module: "app", Version: "c3", Resources: []map[string]any{
+			{"id": "svc", "type": "container", "name": "app", "image": reference}}})
+		return link.BuildResult{ID: id, Repository: "novox/mesh-catalog", Path: "modules/app", Ref: "main", On: "anchor",
+			Commit: "c3", Manifest: manifest, Trunk: "main", OnTrunk: true, Branches: []string{"main"},
+			Made:   []link.MadeArtifact{{Name: "server", Kind: "image", Reference: reference}},
+			Source: &link.SourceOnSeat{Seat: "git", Repository: "novox/mesh-catalog"},
+			Reused: "reused from the check of novox/mesh-catalog#9 at head abcdefabcdef (merged into trunk 123456123456): server " + digest,
+			ReusedFrom: &link.ReusedFrom{Pull: link.PullRef{Owner: "novox", Repo: "mesh-catalog", Number: 9},
+				Head: "abcdefabcdef", Trunk: "123456123456", Key: "r1:" + strings.Repeat("1", 64)}}
+	}
+	if _, _, err := takeIn(ctx, inv, reused(link.NewBuildID(time.Now()), "store.internal:5000/scratch/app/server@"+digest)); err == nil ||
+		!strings.Contains(err.Error(), "scratch") {
+		t.Fatalf("a build in the scratch namespace was registered: %v", err)
+	}
+	id := link.NewBuildID(time.Now().Add(time.Second))
+	result := reused(id, "store.internal:5000/app/server@"+digest)
+	if _, _, err := takeIn(ctx, inv, result); err != nil {
+		t.Fatal(err)
+	}
+	current, err := inv.CurrentBuilds(ctx)
+	if err != nil || current["app"].Commit != "c3" {
+		t.Fatalf("the reused build is not what app is registered at: %+v %v", current["app"], err)
+	}
+	shelf, err := inv.Catalogue(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	raw, _ := json.Marshal(shelf["app"].Resources)
+	if !strings.Contains(string(raw), "app/server@"+digest) {
+		t.Fatalf("app is registered as %s, not by the check's digest in its own namespace", raw)
+	}
+
+	// The walk asked it: the walk keeps what its build did, and its gate judges the registered build.
+	asked := time.Now().UTC()
+	p := g.plan(t)
+	p.Commit = "c3"
+	p.Modules["app"] = &inventory.PlanModule{State: "asked", AskedAt: &asked, Build: id}
+	if err := inv.SavePlan(ctx, &p); err != nil {
+		t.Fatal(err)
+	}
+	planBuiltSaying(ctx, g.open, "app", "c3", "", asked, id, result.MadeWords())
+	p = g.plan(t)
+	if m := p.Modules["app"]; m.State != "built" || !strings.HasPrefix(m.Made, "reused from the check of novox/mesh-catalog#9") {
+		t.Fatalf("the walk keeps %+v", m)
+	}
+	for i := 0; i < 6; i++ {
+		advancePlans(ctx, g.open)
+	}
+	p = g.plan(t)
+	gate := p.Modules["app"].Gate
+	if gate == nil || gate.To != "c3" || gate.Verdict != inventory.GatePassed {
+		t.Fatalf("the gate judged %+v, not the reused build", gate)
+	}
+	if sent, _, _ := inv.SentBuilds(ctx, "anchor"); sent["app"] != "c3" {
+		t.Fatalf("the first machine was sent %v, not the reused build", sent)
+	}
+}

cmd/mesh-controller/checks.go

@@ -282,10 +282,59 @@ func checkRequestFor(ctx context.Context, open *stores, p link.PullUpdated, scop
 		Source:     sourceOnSeat(source),
 		Check: &link.CheckRequest{Owner: p.Owner, Repo: p.Repo, Number: p.Number, Base: p.Base,
 			Paths: p.Paths, Beside: beside, Modules: scope.Modules, Dependents: scope.Dependents, New: scope.New,
-			Manifests: scope.Manifests, Judge: scope.Judge},
+			Manifests: scope.Manifests, Judge: scope.Judge, Builds: checkBuildsOf(p, scope, entries, clone)},
 	}, nil
 }
 
+// checkBuildsOf are the modules a merge of the change would build, in the walk's order, as the check builds them
+// for the walk to take (novox/hq ADR 0350 §1): a module built from the repository checked, at the branch it merges
+// into, from the merge the check makes; any other — a module standing on one — from its own repository at the
+// branch it follows, as the walk builds it. A module whose build reads the repository checked through a context,
+// or that the merge deletes, cannot be built as the walk would: said, and nothing after it is built. A change that
+// moves no module builds nothing.
+func checkBuildsOf(p link.PullUpdated, scope checkScope, entries []inventory.Entry,
+	clone func(inventory.Source) (string, error)) []link.CheckBuild {
+	if len(scope.Reach.Plan.Tiers) == 0 {
+		return nil
+	}
+	m := link.SourceMoved{Owner: p.Owner, Repo: p.Repo, Base: p.Base, CloneURL: p.CloneURL}
+	byName := map[string]inventory.Entry{}
+	for _, e := range entries {
+		byName[e.Manifest.Module] = e
+	}
+	skip := map[string]string{}
+	for _, e := range scope.Reach.Packaging {
+		skip[e.Manifest.Module] = "its build reads " + p.Owner + "/" + p.Repo + " through a context, which a check " +
+			"cannot hand the merge"
+	}
+	for _, e := range scope.Reach.Deleted {
+		skip[e.Manifest.Module] = "the merge deletes it"
+	}
+	var out []link.CheckBuild
+	for tier, names := range scope.Reach.Plan.Tiers {
+		for _, name := range names {
+			e, known := byName[name]
+			b := link.CheckBuild{Module: name, Tier: tier, Skip: skip[name]}
+			switch {
+			case !known:
+				b.Skip = "the catalogue does not hold it"
+			case b.Skip != "":
+			case sourceIs(e.Source, m):
+				b.Merged, b.Path = true, e.Source.Path
+			default:
+				url, err := clone(e.Source)
+				if err != nil {
+					b.Skip = "its repository cannot be cloned: " + err.Error()
+					break
+				}
+				b.Repository, b.Path, b.Ref = url, e.Source.Path, followedBranch(e.Source.Ref)
+			}
+			out = append(out, b)
+		}
+	}
+	return out
+}
+
 // besideRefs is the ref each repository is cloned at beside a check, by the directory it is found under:
 // a core repository at the commit the mesh runs of the module built from it (`running`) — but the
 // catalogue, whose checkout beside is what tests read its files from, at its main, what the next merge
@@ -382,6 +431,7 @@ func checkedOf(result link.BuildResult) link.Checked {
 		// What was judged (novox/hq ADR 0323 §3): the head merged into the trunk commit, and the commits of the
 		// repositories read beside it.
 		c.Trunk, c.Beside, c.Took = result.Check.Trunk, result.Check.Beside, result.Check.Took
+		c.Built = result.Check.Built
 		if c.Gate == nil {
 			// A build seat from before the layers: its verdict is the gate's.
 			c.Gate = &link.CheckLayer{Verdict: c.Verdict, Summary: c.Summary}

cmd/mesh-controller/main.go

@@ -104,6 +104,8 @@ func run() error {
 		return replayCommand(ctx, args[1:])
 	case "kill":
 		return killCommand(ctx, args[1:])
+	case "forget-checks":
+		return forgetChecksCommand(ctx, args[1:])
 	case "pause", "resume":
 		return pauseCommand(ctx, args[0], args[1:])
 	case "collection":
@@ -429,8 +431,14 @@ func (b builds) Built(ctx context.Context, result link.BuildResult) error {
 	}
 	fmt.Printf("%s: %s %s registered, built on %s from %s\n",
 		result.ID, manifest.Module, manifest.Version, result.On, short(result.Commit))
+	// What the walk's build did, kept on the walk whose ask it answers (novox/hq ADR 0350 §4); a build seat from
+	// before it says nothing, and that is what is kept.
+	made := result.MadeWords()
+	if result.Reused != "" || result.BuiltWhy != "" {
+		fmt.Printf("%s: %s %s\n", result.ID, manifest.Module, made)
+	}
 	saysWhenThePolicyActs(ctx, b.inv, manifest.Module)
-	planBuilt(ctx, b.open, manifest.Module, result.Commit, "", asked, result.ID)
+	planBuiltSaying(ctx, b.open, manifest.Module, result.Commit, "", asked, result.ID, made)
 	// A module registered may be one a machine is now behind: `status` is composed again.
 	statusFrom.nudge()
 	return nil

cmd/mesh-controller/push.go

@@ -249,7 +249,13 @@ func serve(ctx context.Context) (err error) {
 	// (novox/hq to-be 45 §9).
 	checkEvents = bus
 	// And every walk this controller keeps in a new state, for the delivery it walks (novox/hq ADR 0239).
-	inventory.PlanSaved = func(_ *inventory.Inventory, p inventory.Plan) { sayPlanMoved(ctx, bus, open.inventory, p) }
+	inventory.PlanSaved = func(_ *inventory.Inventory, p inventory.Plan) {
+		sayPlanMoved(ctx, bus, open.inventory, p)
+		// A walk ended has taken what it reuses: its merges' check builds go (novox/hq ADR 0350 §5).
+		if p.WalkSaid == inventory.WalkEnded {
+			go forgetChecksOfWalk(context.WithoutCancel(ctx), open.inventory, p)
+		}
+	}
 	if err := server.Checks(following{open}); err != nil {
 		return err
 	}

cmd/mesh-controller/release_plan.go

@@ -313,6 +313,59 @@ func init() {
 	}
 }
 
+// reuseKey carries a walk's leave to take a check's build from askModule to the ask it makes (novox/hq ADR 0350 §2).
+type reuseKey struct{}
+
+// withReuse is ctx carrying a walk's leave to take a check's build.
+func withReuse(ctx context.Context, r *link.ReuseAsk) context.Context {
+	return context.WithValue(ctx, reuseKey{}, r)
+}
+
+// reuseIn is the leave ctx carries; nil for every ask but a walk's.
+func reuseIn(ctx context.Context) *link.ReuseAsk {
+	r, _ := ctx.Value(reuseKey{}).(*link.ReuseAsk)
+	return r
+}
+
+// reuseAsked is the leave an ask carries: a walk's, and never a dry run's — a dry run registers nothing, so it
+// takes nothing either (novox/hq ADR 0350 §2, §6).
+func reuseAsked(ctx context.Context, dryRun bool) *link.ReuseAsk {
+	if dryRun {
+		return nil
+	}
+	return reuseIn(ctx)
+}
+
+// reuseAskOf is a walk's leave to take a check's build: the pull requests of the merges it answers.
+func reuseAskOf(p *inventory.Plan) *link.ReuseAsk {
+	r := &link.ReuseAsk{}
+	if p.Delivery == nil {
+		return r
+	}
+	seen := map[link.PullRef]bool{}
+	for _, m := range p.Delivery.Merges {
+		pull, ok := pullOf(m)
+		if !ok {
+			continue
+		}
+		if !seen[pull] {
+			seen[pull] = true
+			r.Pulls = append(r.Pulls, pull)
+		}
+	}
+	return r
+}
+
+// pullOf is a merge's pull request: owner and repository, the last two parts of however the repository is spelled
+// (owner/repo, a path on the git seat, a URL), and its number. False when the merge names none.
+func pullOf(m inventory.PlanMerge) (link.PullRef, bool) {
+	parts := strings.Split(strings.Trim(strings.TrimSuffix(m.Repository, ".git"), "/"), "/")
+	if len(parts) < 2 || m.Number <= 0 || parts[len(parts)-2] == "" || parts[len(parts)-1] == "" {
+		return link.PullRef{}, false
+	}
+	return link.PullRef{Owner: parts[len(parts)-2], Repo: parts[len(parts)-1], Number: m.Number}, true
+}
+
 // askModule asks the build machine for one module of a plan and marks it asked, with the id it was
 // asked under (novox/hq ADR 0219) — or failed, with the plan, when it could not be asked.
 func askModule(ctx context.Context, p *inventory.Plan, name string, byName map[string]inventory.Entry) {
@@ -340,7 +393,9 @@ func askModule(ctx context.Context, p *inventory.Plan, name string, byName map[s
 	if p.Delivery != nil && p.Delivery.Alone && carried != "" {
 		ref = carried
 	}
-	id, err := askABuild(ctx, source, e.Source.Path, ref)
+	// **The walk takes the check's build of the same key** (novox/hq ADR 0350 §2): its ask says so, with the pull
+	// requests it answers, whose checks name the part of a differing key.
+	id, err := askABuild(withReuse(ctx, reuseAskOf(p)), source, e.Source.Path, ref)
 	if err != nil {
 		state.State = "failed"
 		state.Why = err.Error()
@@ -359,7 +414,7 @@ func askModule(ctx context.Context, p *inventory.Plan, name string, byName map[s
 	state.State = "asked"
 	state.AskedAt = &now
 	state.Build = id
-	state.Why, state.Commit, state.BuiltAt = "", "", nil
+	state.Why, state.Commit, state.BuiltAt, state.Made = "", "", nil, ""
 }
 
 // planBuilt marks a module built (or failed) in every open plan whose current tier holds it, and
@@ -371,6 +426,12 @@ func askModule(ctx context.Context, p *inventory.Plan, name string, byName map[s
 // would send machines, and the next tier, what the later merge replaced. asked is zero when the
 // build's request time is not known, and such an outcome is taken as before.
 func planBuilt(ctx context.Context, open *stores, module, commit, failed string, asked time.Time, id string) {
+	planBuiltSaying(ctx, open, module, commit, failed, asked, id, "")
+}
+
+// planBuiltSaying is planBuilt keeping what the build did — "reused from <check build>" or "built: <why>" (novox/hq
+// ADR 0350 §4) — on the module of the walk it answers.
+func planBuiltSaying(ctx context.Context, open *stores, module, commit, failed string, asked time.Time, id, made string) {
 	inv := open.inventory
 	// One controller works the plans at a time (novox/hq issue 213); an outcome waits its turn rather
 	// than write over what the holder is about to save. Not taken, it is still in the build records,
@@ -436,6 +497,7 @@ func planBuilt(ctx context.Context, open *stores, module, commit, failed string,
 			state.State = "built"
 			state.BuiltAt = &now
 			state.Commit = commit
+			state.Made = made
 		}
 		if err := inv.SavePlan(ctx, p); err != nil {
 			fmt.Printf("%s: cannot keep the plan: %v\n", p.ID, err)

cmd/mesh-controller/seatverbs.go

@@ -452,6 +452,11 @@ func (a *verbArguments) commandLine() ([]string, error) {
 	case "queue":
 		// As data, the table its output (novox/hq issue 455).
 		return []string{"queue", "--json"}, nil
+	case "forget-checks":
+		if err := need("owner", "repo", "number"); err != nil {
+			return nil, err
+		}
+		return []string{"forget-checks", str("owner"), str("repo"), str("number")}, nil
 	case "cancel", "kill":
 		if err := need("id"); err != nil {
 			return nil, err

internal/artifacts/scratch.go

@@ -0,0 +1,91 @@
+package artifacts
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+)
+
+// Listing and letting go of tagged documents: the check builds the scratch namespace keeps (novox/hq ADR 0350
+// §5). A check build is found by its tag, and the build seat's holder lets each go when its pull request closes
+// or it is seven days old, so the store keeps a week of check builds and no more.
+
+// Tags is every tag repository holds, in the store's order. None for a repository the store does not hold.
+func (s Store) Tags(ctx context.Context, repository string) ([]string, error) {
+	if s.Address == "" {
+		return nil, fmt.Errorf("this mesh has no artifact store on its network to list %s in", repository)
+	}
+	var tags []string
+	next := "http://" + s.Address + "/v2/" + repository + "/tags/list?n=1000"
+	for next != "" {
+		request, err := http.NewRequestWithContext(ctx, http.MethodGet, next, nil)
+		if err != nil {
+			return nil, err
+		}
+		response, err := s.client().Do(request)
+		if err != nil {
+			return nil, fmt.Errorf("cannot reach the artifact store at %s: %w", s.Address, err)
+		}
+		switch response.StatusCode {
+		case http.StatusOK:
+		case http.StatusNotFound:
+			response.Body.Close()
+			return tags, nil
+		default:
+			response.Body.Close()
+			return nil, fmt.Errorf("the artifact store answered %s for the tags of %s", response.Status, repository)
+		}
+		var page struct {
+			Tags []string `json:"tags"`
+		}
+		err = json.NewDecoder(io.LimitReader(response.Body, 8<<20)).Decode(&page)
+		link := response.Header.Get("Link")
+		response.Body.Close()
+		if err != nil {
+			return nil, fmt.Errorf("the tags of %s could not be read: %w", repository, err)
+		}
+		tags = append(tags, page.Tags...)
+		next = nextPage(link, s.Address)
+	}
+	return tags, nil
+}
+
+// nextPage is the registry's `Link: </v2/…>; rel="next"`, made whole; empty when there is no next page.
+func nextPage(link, address string) string {
+	if !strings.Contains(link, `rel="next"`) {
+		return ""
+	}
+	start, end := strings.Index(link, "<"), strings.Index(link, ">")
+	if start < 0 || end <= start+1 {
+		return ""
+	}
+	target := link[start+1 : end]
+	if strings.HasPrefix(target, "/") {
+		return "http://" + address + target
+	}
+	return target
+}
+
+// DeleteTagged lets go of the document tag names in repository: its manifest, by digest, so the next
+// collection takes its layer. Gone when the tag names nothing.
+func (s Store) DeleteTagged(ctx context.Context, repository, tag string) error {
+	if s.Address == "" {
+		return fmt.Errorf("this mesh has no artifact store on its network to let go of %s:%s in", repository, tag)
+	}
+	digest, err := s.manifestDigest(ctx, repository, tag)
+	if err != nil {
+		return err
+	}
+	if digest == "" {
+		return Gone
+	}
+	err = s.remove(ctx, s.url(repository, "manifests", digest), repository+":"+tag)
+	if errors.Is(err, Gone) {
+		return Gone
+	}
+	return err
+}

internal/builder/builder.go

@@ -91,6 +91,16 @@ type Result struct {
 	// (novox/hq issue 280).
 	Source string
 
+	// ReuseKey is the build's reuse key (novox/hq ADR 0350 §1): its source fingerprint's parts with the builder's
+	// version, and ReuseParts those parts; empty, with NoKey saying why, where the source does not pin the build.
+	ReuseKey   string
+	ReuseParts map[string]string
+	NoKey      string
+	// Reused is the check build a walk took instead of building (ADR 0350 §2), and Why, when it built, why it did
+	// not take one (§3). Both empty for a build not asked to reuse.
+	Reused *Reused
+	Why    string
+
 	// Sources are what this build was made from, as files (novox/hq ADR 0267 rule 1): per repository, the
 	// entries a changed file is tested against (SourceHolds). The module's own repository has an empty
 	// Repository. Said only for a build of a commit on the trunk, and only for a repository whose every
@@ -136,14 +146,41 @@ func Build(ctx context.Context, run Runner, publish Publisher,
 	// one record that it is there.
 	var mirrored []string
 	result, err := build(ctx, run, publish, repository, path, ref, workspace, held, npmrc, forge, log,
-		&mirrored, seats...)
+		&mirrored, nil, seats...)
+	result.Mirrored = mirrored
+	return result, err
+}
+
+// Reuse is what a walk's build may take instead of building (novox/hq ADR 0350 §2): the scratch namespace's check
+// builds, and the pull requests the walk answers, whose checks name a differing key.
+type Reuse struct {
+	Scratch Scratch
+	Pulls   []PullRef
+	// keyFirst is a check's build (§1): its key is taken before anything is built, and a module with none is not
+	// built at all — nothing could ever take it.
+	keyFirst bool
+}
+
+// errNoKey is a check's build of a module whose source does not pin it: not built (ADR 0350 §1).
+var errNoKey = errors.New("it has no reuse key")
+
+// BuildReusing is Build for the walk of a trunk commit (novox/hq ADR 0350 §2): before building, it takes the reuse
+// key of the commit's tree, and where a check build holds the same key it copies that build into the module's
+// namespace by digest and answers with it, building nothing. Otherwise it builds, and Result.Why says why (§3).
+// Only a commit on its trunk takes a check build: anything else builds, as Build does.
+func BuildReusing(ctx context.Context, run Runner, publish Publisher, reuse Reuse,
+	repository, path, ref, workspace string, held map[string]string, npmrc Npmrc,
+	forge GitCredential, log Log, seats ...map[string]string) (Result, error) {
+	var mirrored []string
+	result, err := build(ctx, run, publish, repository, path, ref, workspace, held, npmrc, forge, log,
+		&mirrored, &reuse, seats...)
 	result.Mirrored = mirrored
 	return result, err
 }
 
 func build(ctx context.Context, run Runner, publish Publisher,
 	repository, path, ref, workspace string, held map[string]string, npmrc Npmrc,
-	forge GitCredential, log Log, mirrored *[]string, seats ...map[string]string) (Result, error) {
+	forge GitCredential, log Log, mirrored *[]string, reuse *Reuse, seats ...map[string]string) (Result, error) {
 	// The clone base of each seat a context may name (novox/hq ADR 0155); variadic so the callers
 	// that hand none — tests of everything but contexts — read as they did.
 	var seatBases map[string]string
@@ -284,6 +321,9 @@ func build(ctx context.Context, run Runner, publish Publisher,
 	var built []catalogue.Built
 	// stoodOn is every base the build was handed, as resolved — the edges the catalogue derives.
 	var stoodOn []string
+	// reused is the check build a walk took, and builtWhy why it built instead (novox/hq ADR 0350).
+	var reused *Reused
+	builtWhy := ""
 	if manifest.Build != nil {
 		// What this module said it stands on, answered with what this mesh actually holds. Done
 		// before anything is built, so a missing base is refused in front of the person who can
@@ -327,7 +367,35 @@ func build(ctx context.Context, run Runner, publish Publisher,
 		// Ordered, so two builds of one commit do the same work in the same sequence and their
 		// logs can be compared.
 		sort.Slice(artifacts, func(i, j int) bool { return artifacts[i].Name < artifacts[j].Name })
+		// **A walk takes the check's build of the same key** (novox/hq ADR 0350 §2): only a commit on its trunk, and
+		// only when asked — a check, a dry run, `build --ref` and `rebuild` never are. Its key is taken by the same
+		// steps a build takes, stopped before anything is built; what it takes is copied by digest.
+		if reuse != nil && reuse.keyFirst {
+			_, key, err := keyFor(ctx, run, publish, manifest, within, workspace, commit, credentials, artifacts, args,
+				held, npmrcPath, npmrc, seatBases, src, say)
+			if err == nil && key.Key == "" {
+				err = fmt.Errorf("%w: %s", errNoKey, key.Why)
+			}
+			if err != nil {
+				say("reuse", "not built for the merge: %v", err)
+				return Result{Commit: commit, NoKey: err.Error()}, err
+			}
+		} else if reuse != nil && reuse.Scratch != nil {
+			taken, keyed, why := takeACheckBuild(ctx, run, publish, *reuse, manifest, within, workspace, commit,
+				credentials, artifacts, args, held, npmrcPath, npmrc, seatBases, src, onTrunk, say)
+			if taken != nil {
+				built = taken.Made
+				src = keyed
+				reused = taken
+			} else {
+				builtWhy = why
+				say("reuse", "built: %s", why)
+			}
+		}
 		for _, a := range artifacts {
+			if reused != nil {
+				break
+			}
 			say("artifact", "%s (%s%s) — starting", a.Name, a.Kind, langSuffix(a))
 			made, err := one(ctx, run, publish, manifest.Module, within, workspace, commit, credentials, a, args, held, npmrcPath, npmrc, seatBases, src, say)
 			if err != nil {
@@ -363,9 +431,102 @@ func build(ctx context.Context, run Runner, publish Publisher,
 			say("source", "%d path(s) of %s", len(s.Paths), where)
 		}
 	}
+	// The reuse key (novox/hq ADR 0350 §1): what a check keeps its build under, and what a walk takes one by.
+	key := keyOf(src, manifest, Version)
 	return Result{Manifest: resolved, Commit: commit, Built: built,
 		Against: against(within, manifest, stoodOn), Read: readBy(manifest), Source: fingerprint,
-		Trunk: trunk, OnTrunk: onTrunk, Branches: branches, Sources: sources}, nil
+		Trunk: trunk, OnTrunk: onTrunk, Branches: branches, Sources: sources,
+		ReuseKey: key.Key, ReuseParts: key.Parts, NoKey: key.Why, Reused: reused, Why: builtWhy}, nil
+}
+
+// takeACheckBuild is the walk's look for a check build of its key (novox/hq ADR 0350 §2, §3): the key taken by
+// the steps a build takes, each artifact stopped before it is built; the build kept under it copied by digest into
+// the module's namespace. Answers what it took and the source inputs its key was taken from, or why it builds. A
+// copy that fails is said, and the walk builds (§3): it never fails the walk on its own.
+func takeACheckBuild(ctx context.Context, run Runner, publish Publisher, reuse Reuse, manifest catalogue.Manifest,
+	within, workspace, commit, credentials string, artifacts []catalogue.Artifact, args []string,
+	held map[string]string, npmrcPath string, npmrc Npmrc, seats map[string]string, src *sourceInputs, onTrunk bool,
+	say func(step, format string, args ...any)) (*Reused, *sourceInputs, string) {
+	if !onTrunk {
+		return nil, nil, "the commit is not on its trunk, and only a trunk commit's walk takes a check's build"
+	}
+	keyed, key, err := keyFor(ctx, run, publish, manifest, within, workspace, commit, credentials, artifacts, args,
+		held, npmrcPath, npmrc, seats, src, say)
+	if err != nil {
+		return nil, nil, err.Error()
+	}
+	if key.Key == "" {
+		return nil, nil, "it has no reuse key: " + key.Why
+	}
+	found, ok, err := reuse.Scratch.Find(ctx, manifest.Module, key.Key)
+	if err != nil {
+		return nil, nil, "the scratch namespace could not be read: " + err.Error()
+	}
+	if !ok {
+		return nil, nil, notKeptWhy(ctx, reuse, manifest.Module, key)
+	}
+	taken := &Reused{From: found}
+	for _, made := range found.Made {
+		copied, err := reuse.Scratch.Take(ctx, manifest.Module, made)
+		if err != nil {
+			why := fmt.Sprintf("copying %s's %s from %s failed (%v)", manifest.Module, made.Name, found.Said(), err)
+			say("reuse", "%s; built instead", why)
+			return nil, nil, why
+		}
+		taken.Made = append(taken.Made, copied)
+	}
+	// Every artifact the manifest declares, or none: a check build missing one is not this build.
+	if len(taken.Made) != len(artifacts) {
+		return nil, nil, fmt.Sprintf("%s holds %d artifact(s) of %s, which declares %d", found.Said(), len(taken.Made),
+			manifest.Module, len(artifacts))
+	}
+	say("reuse", "%s", taken.Words())
+	return taken, keyed, ""
+}
+
+// keyFor takes a build's reuse key by the steps a build takes, each artifact stopped before anything is built or
+// published (ADR 0350 §2): its own copy of the source inputs, so the build's are untouched.
+func keyFor(ctx context.Context, run Runner, publish Publisher, manifest catalogue.Manifest,
+	within, workspace, commit, credentials string, artifacts []catalogue.Artifact, args []string,
+	held map[string]string, npmrcPath string, npmrc Npmrc, seats map[string]string, src *sourceInputs,
+	say func(step, format string, args ...any)) (*sourceInputs, reuseKey, error) {
+	keyed := src.copy()
+	keyed.onlyKey = true
+	for _, a := range artifacts {
+		if _, err := one(ctx, run, publish, manifest.Module, within, workspace, commit, credentials, a, args, held,
+			npmrcPath, npmrc, seats, keyed, say); err != nil {
+			return nil, reuseKey{}, fmt.Errorf("its reuse key could not be taken: %w", err)
+		}
+	}
+	keyed.onlyKey = false
+	return keyed, keyOf(keyed, manifest, Version), nil
+}
+
+// notKeptWhy is why no check build has the key (§3): no check of the walk's pull requests built the module, the
+// key differs — naming the part — or the build it made is gone.
+func notKeptWhy(ctx context.Context, reuse Reuse, module string, key reuseKey) string {
+	if len(reuse.Pulls) == 0 {
+		return "no check built it with this key"
+	}
+	builds, err := reuse.Scratch.OfPulls(ctx, reuse.Pulls)
+	if err != nil {
+		return "no check build has this key, and which checks built it could not be read: " + err.Error()
+	}
+	for _, b := range builds {
+		if b.Module != module {
+			continue
+		}
+		if b.Key == key.Key {
+			return b.Said() + " built it with this key, and it was removed"
+		}
+		return "the key differs from " + b.Said() + "'s in " + strings.Join(differing(b.Parts, key.Parts), ", ")
+	}
+	var pulls []string
+	for _, p := range reuse.Pulls {
+		pulls = append(pulls, p.String())
+	}
+	return "no check of " + strings.Join(pulls, ", ") + " built it (a module its check did not move, or a check " +
+		"that did not finish)"
 }
 
 // branchesHolding is every branch of a fresh clone's origin the commit is on, without `origin/`.
@@ -754,6 +915,10 @@ func one(ctx context.Context, run Runner, publish Publisher,
 
 	switch a.Kind {
 	case catalogue.ArtifactUpstream:
+		// Its key is the image it copies, read from the manifest (keyOf): nothing to take here.
+		if src.keyOnly() {
+			return catalogue.Built{Name: a.Name, Kind: a.Kind}, nil
+		}
 		// Mirrored, not built: copied under a name of the mesh's own, so what a machine fetches is
 		// pinned by a digest this registry assigned rather than by a tag somebody else can move.
 		//
@@ -882,6 +1047,10 @@ func one(ctx context.Context, run Runner, publish Publisher,
 		} else {
 			src.ownWhole()
 		}
+		// Taken for a reuse key (novox/hq ADR 0350 §2): everything its fingerprint reads is read; nothing is built.
+		if src.keyOnly() {
+			return catalogue.Built{Name: a.Name, Kind: a.Kind}, nil
+		}
 		invocation := append([]string{"build", "-f", recipePath, "-t", local}, args...)
 		if a.Target != "" {
 			invocation = append(invocation, "--target", a.Target)
@@ -949,6 +1118,9 @@ func one(ctx context.Context, run Runner, publish Publisher,
 				src.notPinned(a.Name + " resolves packages of its own at build time")
 			}
 		}
+		if src.keyOnly() {
+			return catalogue.Built{Name: a.Name, Kind: a.Kind}, nil
+		}
 		// The module's own packages first, where the compiler and the bundler resolve them from
 		// (dependencies.go); nothing at all for a module whose package.json names only the SDK.
 		if err := installOwn(ctx, run, tree, chain, base, registry, say); err != nil {
@@ -992,6 +1164,9 @@ func one(ctx context.Context, run Runner, publish Publisher,
 		say("package", "building and publishing %s (%s)", a.Name, a.Language)
 		src.ownWhole()
 		src.notPinned(a.Name + " is a package, built from what the registry holds when it is built")
+		if src.keyOnly() {
+			return catalogue.Built{Name: a.Name, Kind: a.Kind}, nil
+		}
 		reference, err := publishPackage(ctx, run, module, tree, a, npmrc, say)
 		if err != nil {
 			return catalogue.Built{}, fmt.Errorf("%s: publishing %s failed: %w", module, a.Name, err)
@@ -1000,6 +1175,12 @@ func one(ctx context.Context, run Runner, publish Publisher,
 		return catalogue.Built{Name: a.Name, Kind: a.Kind, Reference: reference}, nil
 
 	case catalogue.ArtifactArchive:
+		if src.keyOnly() {
+			if a.URL == "" {
+				src.ownHas(cleanEntry(a.From) + "/**")
+			}
+			return catalogue.Built{Name: a.Name, Kind: a.Kind}, nil
+		}
 		var body []byte
 		var err error
 		if a.URL != "" {

internal/builder/check.go

@@ -98,6 +98,11 @@ type CheckSpec struct {
 	// and composed with it by the gate as one future state. The repository's own check is not run for a
 	// group: each member's pull request runs its own.
 	Group []GroupHead
+	// Builds are the modules the delivery plan moves, in the walk's order, and BuildWith what they are built with
+	// (novox/hq ADR 0350 §1): built from the merge the check judged into the scratch namespace once it has judged,
+	// for the walk to take. None for a group's composed check, whose members' own checks build.
+	Builds    []CheckBuildSpec
+	BuildWith *CheckBuilding
 }
 
 // GroupHead is one other head of a delivery group's composed check.
@@ -155,6 +160,8 @@ type CheckVerdict struct {
 	// Members are a delivery group's other heads as the composed check judged them, each with the trunk
 	// commit it was merged into (ADR 0323 §6).
 	Members []JudgedHead
+	// Built are the builds it made for its merge (novox/hq ADR 0350 §1), each with its key, or why not.
+	Built []CheckBuilt
 }
 
 // Layer is one layer of a check, judged.
@@ -572,9 +579,38 @@ func Check(ctx context.Context, run Runner, spec CheckSpec, workspace, registry
 	v.Report, v.Took = redact.redact(withWhatFailed(out.String(), v.Repo)), time.Since(began)
 	say("check", "gate %s — %s; repository %s — %s (%s)", strings.ToUpper(v.Gate.Verdict), v.Gate.Summary,
 		strings.ToUpper(v.Repo.Verdict), v.Repo.Summary, v.Took.Round(time.Second))
+
+	// **What the merge would move, built once, here** (novox/hq ADR 0350 §1): after the judging, from the merge it
+	// judged, into the scratch namespace — so the walk of the merge commit takes it rather than building again.
+	if len(spec.Builds) > 0 && spec.BuildWith != nil && len(spec.Group) == 0 {
+		if why := skipsTheBuilds(v); why != "" {
+			say("check", "%s", why)
+		} else {
+			began := time.Now()
+			v.Built = buildForTheMerge(ctx, run, spec, *spec.BuildWith, workspace, tree, judged.Merge, judged, forge,
+				func(step, message string) {
+					if log != nil {
+						log(step, redact.redact(message))
+					}
+				}, say)
+			say("check", "built for the merge: %d of %d module(s) (%s)", builtCount(v.Built), len(v.Built),
+				time.Since(began).Round(time.Second))
+		}
+	}
 	return named(v), nil
 }
 
+// builtCount is how many of a check's builds for its merge were built.
+func builtCount(built []CheckBuilt) int {
+	n := 0
+	for _, b := range built {
+		if b.NotBuilt == "" {
+			n++
+		}
+	}
+	return n
+}
+
 // ownCheck runs the repository's own check, part by part, each in its toolchain's own container: the
 // layer's verdict. Nil when the check was ended from outside (not past its bound), which the caller
 // answers as the context's error.

internal/builder/check_builds.go

@@ -0,0 +1,151 @@
+package builder
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// The builds a merge check makes for its merge (novox/hq ADR 0350 §1): every artifact of every module the delivery
+// plan moves, built from the merged tree the check judged into the scratch namespace, kept under its reuse key, so
+// the walk of the merge commit takes it instead of building again. Built after the check has judged, and only when
+// it did not fail: a change that fails is not merged, and nothing it built would be taken.
+
+// CheckBuildSpec is one module a check builds for its merge, in the walk's order (Tier).
+type CheckBuildSpec struct {
+	Module string
+	// Repository, Path and Ref are where it is built from; Merged says from the merge the check made instead — the
+	// module is built from the repository checked, at the branch merged into.
+	Repository string
+	Path       string
+	Ref        string
+	Merged     bool
+	Tier       int
+	// Skip says why it cannot be built as the walk would build it: not built, and nothing after it.
+	Skip string
+}
+
+// CheckBuilding is what a check builds with: where it publishes and keeps its builds, and what a build is handed.
+type CheckBuilding struct {
+	Publish Publisher
+	Scratch Scratch
+	Held    map[string]string
+	Npmrc   Npmrc
+	Seats   map[string]string
+}
+
+// CheckBuilt is one module a check built for its merge, as its verdict lists it (§1): its key and its artifacts, or
+// why it was not built.
+type CheckBuilt struct {
+	Module string
+	Key    string
+	Made   []catalogue.Built
+	// NotBuilt says why it was not: no key, a build that failed, or a module after one of those.
+	NotBuilt string
+	Took     time.Duration
+}
+
+// skipsTheBuilds is why a check builds nothing for its merge; empty when it builds.
+func skipsTheBuilds(v CheckVerdict) string {
+	for _, l := range []*Layer{v.Gate, v.Repo} {
+		if l != nil && (l.Verdict == "fail" || l.Verdict == "error") {
+			return "the check did not pass, so nothing is built for its merge"
+		}
+	}
+	return ""
+}
+
+// buildForTheMerge builds each module the plan moves, tier by tier, into the scratch namespace, and keeps what it
+// built under its keys. A module standing after one not built is not built either: what it stands on in the walk
+// is not what it would stand on here. tree is the check's clone and merge the merge commit it made there.
+func buildForTheMerge(ctx context.Context, run Runner, spec CheckSpec, with CheckBuilding, workspace, tree, merge string,
+	judged CheckVerdict, forge GitCredential, log Log, say func(step, format string, args ...any)) []CheckBuilt {
+	if len(spec.Builds) == 0 || with.Publish == nil || with.Scratch == nil {
+		return nil
+	}
+	ref := merge
+	if ref == "" {
+		ref = spec.Ref
+	}
+	held := map[string]string{}
+	for k, v := range with.Held {
+		held[k] = v
+	}
+	var out []CheckBuilt
+	var kept []CheckBuild
+	stopAt, stopWhy := -1, ""
+	for _, b := range spec.Builds {
+		began := time.Now()
+		if stopAt >= 0 && b.Tier > stopAt {
+			out = append(out, CheckBuilt{Module: b.Module, NotBuilt: stopWhy})
+			continue
+		}
+		if b.Skip != "" {
+			say("build", "%s not built for the merge: %s", b.Module, b.Skip)
+			out = append(out, CheckBuilt{Module: b.Module, NotBuilt: b.Skip})
+			if stopAt < 0 || b.Tier < stopAt {
+				stopAt, stopWhy = b.Tier, "it stands after "+b.Module+", which was not built for the merge"
+			}
+			continue
+		}
+		repository, at := b.Repository, b.Ref
+		if b.Merged {
+			repository, at = tree, ref
+		}
+		say("build", "for the merge: %s (tier %d)", b.Module, b.Tier)
+		var mirrored []string
+		result, err := build(ctx, run, scratchPublisher{to: with.Publish}, repository, b.Path, at,
+			filepath.Join(workspace, "for-the-merge"), held, with.Npmrc, forge, log, &mirrored, &Reuse{keyFirst: true},
+			with.Seats)
+		built := CheckBuilt{Module: b.Module, Key: result.ReuseKey, Took: time.Since(began)}
+		switch {
+		case errors.Is(err, errNoKey):
+			built.NotBuilt = err.Error()
+		case err != nil:
+			built.NotBuilt = "its build failed: " + err.Error()
+		case result.ReuseKey == "":
+			built.NotBuilt = "it has no reuse key: " + result.NoKey
+		}
+		if built.NotBuilt != "" {
+			say("build", "%s not built for the merge: %s", b.Module, built.NotBuilt)
+			out = append(out, built)
+			if stopAt < 0 || b.Tier < stopAt {
+				stopAt, stopWhy = b.Tier, "it stands after "+b.Module+", which was not built for the merge"
+			}
+			continue
+		}
+		built.Made = result.Built
+		for _, m := range result.Built {
+			held[b.Module+"/"+m.Name] = m.Reference
+		}
+		say("build", "%s built for the merge under %s (%s)", b.Module, twelve(result.ReuseKey[3:]), built.Took.Round(time.Second))
+		out = append(out, built)
+		if len(result.Built) > 0 {
+			kept = append(kept, CheckBuild{Key: result.ReuseKey, Module: b.Module, Parts: result.ReuseParts,
+				Made: result.Built, Pull: PullRef{Owner: spec.Owner, Repo: spec.Repo, Number: spec.Number},
+				Head: judged.Head, Trunk: judged.Trunk, Check: spec.ID, At: time.Now().UTC()})
+		}
+	}
+	_ = os.RemoveAll(filepath.Join(workspace, "for-the-merge"))
+	if len(kept) > 0 {
+		if err := with.Scratch.Keep(ctx, PullRef{Owner: spec.Owner, Repo: spec.Repo, Number: spec.Number}, spec.ID,
+			time.Now().UTC(), kept); err != nil {
+			// Built and not findable: the walk builds, and says no check built it. Said, never the check's verdict.
+			say("build", "the builds for the merge could not be kept, and the walk will build: %v", err)
+			if err := with.Scratch.Discard(ctx, kept); err != nil {
+				say("build", "and what they made could not all be let go of: %v", err)
+			}
+			for i := range out {
+				if out[i].NotBuilt == "" {
+					out[i].NotBuilt = fmt.Sprintf("built, and could not be kept: %v", err)
+				}
+			}
+		}
+	}
+	return out
+}

internal/builder/reuse.go

@@ -0,0 +1,539 @@
+package builder
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"sort"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/novox/mesh-controller/internal/artifacts"
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// One build per change (novox/hq ADR 0350).
+//
+// **The merge check builds what the merge would move** (§1): every artifact of every module the delivery plan
+// moves, from the merged tree it checks, into the artifact store's scratch namespace — never a module's own —
+// kept under a **reuse key**: the source fingerprint as the build takes it (the module's tree, its contexts' trees,
+// its bases and toolchains by digest; issue 280) with the builder's own version and, for an upstream artifact, the
+// image it copies. **The walk of a trunk commit takes that build** (§2): before building, it takes the reuse key
+// of the trunk commit's tree the same way, and where a check build holds the same key it copies it by digest into
+// the module's namespace and answers with that, building nothing. Otherwise it builds and says why (§3): no check
+// built it, the key differs (naming the part), or the check build is gone; a copy that fails is said and built.
+//
+// **What the key names is what a fresh build could differ by.** The tree is every file the recipe and the
+// compiler see; the contexts are the other repositories it reads, by tree; the bases and toolchains are what it
+// stands on, by digest; the builder's version is how it is built. A build whose source does not pin it — one that
+// resolves packages from the mesh's registry when it is built, or copies an upstream image named by tag — has no
+// key, and is always built.
+
+// Version is this builder's own version, as the reuse key takes it: the digest of the running executable, so any
+// change to how it builds is a change of key (§1). Set once by the build seat's holder (SelfVersion); empty — no
+// key, every build its own — when it cannot be said.
+var Version string
+
+// SelfVersion is the version the reuse key takes: the running executable's digest, or the version stamped at build
+// when the executable cannot be read. Empty when neither says anything ("development" says nothing: every unstamped
+// builder would read as one).
+func SelfVersion(stamped string) string {
+	if exe, err := os.Executable(); err == nil {
+		if f, err := os.Open(exe); err == nil {
+			defer f.Close()
+			sum := sha256.New()
+			if _, err := io.Copy(sum, f); err == nil {
+				return "exe:sha256:" + hex.EncodeToString(sum.Sum(nil))
+			}
+		}
+	}
+	if stamped == "" || stamped == "development" {
+		return ""
+	}
+	return stamped
+}
+
+// ScratchPrefix is the namespace a check builds into (novox/hq ADR 0239 §9, ADR 0350 §1): `scratch/<module>/<artifact>`.
+// Registration refuses a reference into it; only the walk's copy, in the module's own namespace, is registered.
+const ScratchPrefix = "scratch/"
+
+// checksRepository holds one document per check that built: which builds it made, of which pull request, when.
+// What the holder lets go of by (§5).
+const checksRepository = ScratchPrefix + "checks"
+
+// reuseTagPrefix names a check build's record under its module's scratch repository: `r1-<hex>`.
+const reuseTagPrefix = "r1-"
+
+// CheckBuildsKept is how long a check build is kept in the scratch namespace (§5).
+const CheckBuildsKept = 7 * 24 * time.Hour
+
+// The parts of a reuse key, as the walk names the one that differs (§3).
+const (
+	partModule   = "the module"
+	partTree     = "the build source"
+	partBases    = "a base image"
+	partBuilder  = "the builder's version"
+	partContext  = "a context"
+	partTool     = "a toolchain"
+	partUpstream = "an upstream image"
+)
+
+// reuseKey is a build's reuse key and the parts it was made from; Key empty, with why, when it has none.
+type reuseKey struct {
+	Key   string
+	Parts map[string]string
+	Why   string
+}
+
+// keyOf is the reuse key of what src says the build was made from, built by version (ADR 0350 §1).
+func keyOf(src *sourceInputs, manifest catalogue.Manifest, version string) reuseKey {
+	if src == nil || src.unpinned != "" || src.tree == "" {
+		why := "its source does not pin the build"
+		if src != nil && src.unpinned != "" {
+			why = src.unpinned
+		} else if src != nil && src.tree == "" {
+			why = "its tree could not be named"
+		}
+		return reuseKey{Why: why}
+	}
+	if version == "" {
+		return reuseKey{Why: "the builder's version cannot be said"}
+	}
+	parts := map[string]string{partModule: src.module, partTree: src.tree, partBuilder: version}
+	var bases []string
+	seen := map[string]bool{}
+	for _, b := range src.bases {
+		if d := referenceDigest(b); !seen[d] {
+			seen[d] = true
+			bases = append(bases, d)
+		}
+	}
+	sort.Strings(bases)
+	parts[partBases] = strings.Join(bases, " ")
+	for a, t := range src.contexts {
+		parts[partContext+" "+a] = t
+	}
+	for a, t := range src.toolchains {
+		parts[partTool+" "+a] = t
+	}
+	// An upstream artifact is copied, not built: what it copies is in module.json, and so in the tree — but a tag
+	// names whatever its registry says today, so only an image pinned by digest is the same image tomorrow.
+	if manifest.Build != nil {
+		for _, a := range manifest.Build.Artifacts {
+			if a.Kind != catalogue.ArtifactUpstream {
+				continue
+			}
+			if !strings.Contains(a.From, "@sha256:") {
+				return reuseKey{Why: fmt.Sprintf("%s copies %s, named by tag, which can move", a.Name, a.From)}
+			}
+			parts[partUpstream+" "+a.Name] = a.From
+		}
+	}
+	lines := make([]string, 0, len(parts))
+	for k, v := range parts {
+		lines = append(lines, k+"="+v)
+	}
+	sort.Strings(lines)
+	sum := sha256.Sum256([]byte(strings.Join(lines, "\n")))
+	return reuseKey{Key: "r1:" + hex.EncodeToString(sum[:]), Parts: parts}
+}
+
+// tagOf is the tag a key's record is kept under.
+func tagOf(key string) string { return reuseTagPrefix + strings.TrimPrefix(key, "r1:") }
+
+// differing names the parts of two keys that differ, in a fixed order: what the walk says when it builds (§3).
+func differing(was, now map[string]string) []string {
+	names := map[string]bool{}
+	for k := range was {
+		names[k] = true
+	}
+	for k := range now {
+		names[k] = true
+	}
+	var out []string
+	for k := range names {
+		if was[k] != now[k] {
+			out = append(out, k)
+		}
+	}
+	sort.Strings(out)
+	return out
+}
+
+// PullRef names a pull request.
+type PullRef struct {
+	Owner  string `json:"owner"`
+	Repo   string `json:"repo"`
+	Number int    `json:"number"`
+}
+
+func (p PullRef) String() string { return fmt.Sprintf("%s/%s#%d", p.Owner, p.Repo, p.Number) }
+
+// slug is the pull request as a tag's first part: owner.repo.number, every other character a dash.
+func (p PullRef) slug() string {
+	clean := func(s string) string {
+		return strings.Map(func(r rune) rune {
+			if r >= 'a' && r <= 'z' || r >= 'A' && r <= 'Z' || r >= '0' && r <= '9' || r == '_' || r == '-' {
+				return r
+			}
+			return '-'
+		}, s)
+	}
+	return clean(p.Owner) + "." + clean(p.Repo) + "." + strconv.Itoa(p.Number)
+}
+
+// CheckBuild is one module a check built into the scratch namespace, kept under its reuse key (§1).
+type CheckBuild struct {
+	Key    string            `json:"key"`
+	Module string            `json:"module"`
+	Parts  map[string]string `json:"parts"`
+	// Made are its artifacts, in the scratch namespace.
+	Made []catalogue.Built `json:"made"`
+	// Pull is the pull request checked, Head its head and Trunk the trunk commit it was merged into; Check the
+	// check's ask; At when it was built.
+	Pull  PullRef   `json:"pull"`
+	Head  string    `json:"head,omitempty"`
+	Trunk string    `json:"trunk,omitempty"`
+	Check string    `json:"check"`
+	At    time.Time `json:"at"`
+}
+
+// Said is the check build as the walk names it: "the check of owner/repo#n at head H (trunk T)".
+func (c CheckBuild) Said() string {
+	s := "the check of " + c.Pull.String()
+	if c.Head != "" {
+		s += " at head " + twelve(c.Head)
+	}
+	if c.Trunk != "" {
+		s += " (merged into trunk " + twelve(c.Trunk) + ")"
+	}
+	return s
+}
+
+// checkIndex is one check's document under checksRepository: every build it made.
+type checkIndex struct {
+	Pull   PullRef      `json:"pull"`
+	Check  string       `json:"check"`
+	At     time.Time    `json:"at"`
+	Builds []CheckBuild `json:"builds"`
+}
+
+// Scratch is where check builds are kept and taken from: the artifact store's scratch namespace (ADR 0350). The
+// Registry is one; a test's is a map.
+type Scratch interface {
+	// Keep records a check's builds: each under its module and key, and the check's own document.
+	Keep(ctx context.Context, pull PullRef, check string, at time.Time, builds []CheckBuild) error
+	// Find is the check build kept under a module's key; false when none is.
+	Find(ctx context.Context, module, key string) (CheckBuild, bool, error)
+	// OfPulls are the check builds kept for those pull requests, newest first: what the walk names a differing
+	// key from.
+	OfPulls(ctx context.Context, pulls []PullRef) ([]CheckBuild, error)
+	// Take copies one artifact of a check build by digest into the module's own namespace, and answers it there.
+	Take(ctx context.Context, module string, made catalogue.Built) (catalogue.Built, error)
+	// Forget lets go of every check build of a pull request, and Expire of every check older than the bound;
+	// each answers how many checks it let go of.
+	Forget(ctx context.Context, pull PullRef) (int, error)
+	Expire(ctx context.Context, older time.Duration, now time.Time) (int, error)
+	// Discard lets go of what builds a check could not keep made: nothing would ever find them.
+	Discard(ctx context.Context, builds []CheckBuild) error
+}
+
+// scratchPublisher is a Publisher into the scratch namespace: what a check builds is never under a module's
+// own name (ADR 0239 §9). A base the build stands on is copied where every build copies it.
+type scratchPublisher struct{ to Publisher }
+
+func (s scratchPublisher) PublishImage(ctx context.Context, localTag, repository string) (string, error) {
+	return s.to.PublishImage(ctx, localTag, ScratchPrefix+repository)
+}
+
+func (s scratchPublisher) PublishArchive(ctx context.Context, repository string, body []byte, digest string) (string, error) {
+	return s.to.PublishArchive(ctx, ScratchPrefix+repository, body, digest)
+}
+
+func (s scratchPublisher) MirrorImage(ctx context.Context, from, repository string) (string, error) {
+	m, can := s.to.(Mirrorer)
+	if !can {
+		return "", errors.New("the artifact store here cannot copy an upstream image")
+	}
+	return m.MirrorImage(ctx, from, ScratchPrefix+repository)
+}
+
+func (s scratchPublisher) MirrorBase(ctx context.Context, from, repository, former string) (string, error) {
+	if m, can := s.to.(BaseMirrorer); can {
+		return m.MirrorBase(ctx, from, repository, former)
+	}
+	if m, can := s.to.(Mirrorer); can {
+		return m.MirrorImage(ctx, from, repository)
+	}
+	return "", errors.New("the artifact store here cannot copy a base")
+}
+
+// The registry as the scratch namespace.
+
+func (r Registry) store() artifacts.Store {
+	return artifacts.Store{Address: r.Address, HTTP: r.client()}
+}
+
+const recordMedia = "application/vnd.novox.mesh.check-build.v1+json"
+
+// Keep puts each build's record under its module's scratch repository, by key, and the check's document.
+func (r Registry) Keep(ctx context.Context, pull PullRef, check string, at time.Time, builds []CheckBuild) error {
+	store := r.store()
+	for _, b := range builds {
+		body, err := json.Marshal(b)
+		if err != nil {
+			return err
+		}
+		if _, err := store.PutTagged(ctx, ScratchPrefix+b.Module, tagOf(b.Key), recordMedia, body); err != nil {
+			return fmt.Errorf("keeping %s's check build: %w", b.Module, err)
+		}
+	}
+	body, err := json.Marshal(checkIndex{Pull: pull, Check: check, At: at, Builds: builds})
+	if err != nil {
+		return err
+	}
+	if _, err := store.PutTagged(ctx, checksRepository, pull.slug()+"."+check, recordMedia, body); err != nil {
+		return fmt.Errorf("keeping which builds the check of %s made: %w", pull, err)
+	}
+	return nil
+}
+
+// Find reads the record kept under a module's key.
+func (r Registry) Find(ctx context.Context, module, key string) (CheckBuild, bool, error) {
+	body, _, err := r.store().GetTagged(ctx, ScratchPrefix+module, tagOf(key))
+	if errors.Is(err, artifacts.ErrNoTag) {
+		return CheckBuild{}, false, nil
+	}
+	if err != nil {
+		return CheckBuild{}, false, err
+	}
+	var b CheckBuild
+	if err := json.Unmarshal(body, &b); err != nil {
+		return CheckBuild{}, false, fmt.Errorf("%s's check build under %s is not one: %w", module, tagOf(key), err)
+	}
+	if b.Key != key || b.Module != module {
+		return CheckBuild{}, false, nil
+	}
+	return b, true, nil
+}
+
+// checks are the check documents kept, by tag; only those of the pull requests named when any are.
+func (r Registry) checks(ctx context.Context, pulls []PullRef) (map[string]checkIndex, error) {
+	store := r.store()
+	tags, err := store.Tags(ctx, checksRepository)
+	if err != nil {
+		return nil, err
+	}
+	wanted := map[string]PullRef{}
+	for _, p := range pulls {
+		wanted[p.slug()+"."] = p
+	}
+	out := map[string]checkIndex{}
+	for _, tag := range tags {
+		if len(pulls) > 0 {
+			match := false
+			for prefix := range wanted {
+				match = match || strings.HasPrefix(tag, prefix)
+			}
+			if !match {
+				continue
+			}
+		}
+		body, _, err := store.GetTagged(ctx, checksRepository, tag)
+		if errors.Is(err, artifacts.ErrNoTag) {
+			continue
+		}
+		if err != nil {
+			return nil, err
+		}
+		var c checkIndex
+		if err := json.Unmarshal(body, &c); err != nil {
+			continue // not a document this wrote: left to nobody, said by nothing — it names no build
+		}
+		if len(pulls) > 0 {
+			if p := wanted[c.Pull.slug()+"."]; p != c.Pull {
+				continue // a slug two pull requests share: only the one named
+			}
+		}
+		out[tag] = c
+	}
+	return out, nil
+}
+
+// OfPulls are the check builds of those pull requests, newest first.
+func (r Registry) OfPulls(ctx context.Context, pulls []PullRef) ([]CheckBuild, error) {
+	if len(pulls) == 0 {
+		return nil, nil
+	}
+	checks, err := r.checks(ctx, pulls)
+	if err != nil {
+		return nil, err
+	}
+	var out []CheckBuild
+	for _, c := range checks {
+		out = append(out, c.Builds...)
+	}
+	sort.SliceStable(out, func(i, j int) bool { return out[i].At.After(out[j].At) })
+	return out, nil
+}
+
+// Take copies one artifact of a check build into the module's namespace by digest: an image's manifests as they
+// are, so the digest registered is the digest the check built; an archive's blob mounted across, and held.
+func (r Registry) Take(ctx context.Context, module string, made catalogue.Built) (catalogue.Built, error) {
+	repository := module + "/" + made.Name
+	from := ScratchPrefix + repository
+	if made.Digest != "" || strings.Contains(made.Reference, "/blobs/") {
+		digest := made.Digest
+		if digest == "" {
+			_, digest, _ = strings.Cut(made.Reference, "/blobs/")
+		}
+		where := upstream{base: "http://" + r.Address, repository: from, reference: digest}
+		if err := r.copyBlob(ctx, &source{client: r.client(), mountFrom: from}, where, digest, repository); err != nil {
+			return catalogue.Built{}, err
+		}
+		size, err := r.blobSize(ctx, repository, digest)
+		if err != nil {
+			return catalogue.Built{}, err
+		}
+		if err := r.hold(ctx, repository, digest, int(size)); err != nil {
+			return catalogue.Built{}, err
+		}
+		return catalogue.Built{Name: made.Name, Kind: made.Kind, Digest: digest, Launchers: made.Launchers,
+			Reference: "http://" + r.Address + "/v2/" + repository + "/blobs/" + digest}, nil
+	}
+	_, digest, pinned := strings.Cut(made.Reference, "@")
+	if !pinned || !strings.HasPrefix(digest, "sha256:") {
+		return catalogue.Built{}, fmt.Errorf("%s's check build %s is not pinned by digest", module, made.Reference)
+	}
+	reference, err := r.MirrorBase(ctx, r.Address+"/"+from+"@"+digest, repository, from)
+	if err != nil {
+		return catalogue.Built{}, err
+	}
+	if !strings.HasSuffix(reference, "@"+digest) {
+		return catalogue.Built{}, fmt.Errorf("%s's check build was copied as %s, not as %s", module, reference, digest)
+	}
+	return catalogue.Built{Name: made.Name, Kind: made.Kind, Reference: reference, Launchers: made.Launchers}, nil
+}
+
+// blobSize is the size the store says a blob has.
+func (r Registry) blobSize(ctx context.Context, repository, digest string) (int64, error) {
+	request, err := http.NewRequestWithContext(ctx, http.MethodHead, "http://"+r.Address+"/v2/"+repository+"/blobs/"+digest, nil)
+	if err != nil {
+		return 0, err
+	}
+	response, err := r.client().Do(request)
+	if err != nil {
+		return 0, err
+	}
+	response.Body.Close()
+	if response.StatusCode != http.StatusOK || response.ContentLength < 0 {
+		return 0, fmt.Errorf("%s/blobs/%s: %s", repository, digest, response.Status)
+	}
+	return response.ContentLength, nil
+}
+
+// Forget lets go of every check build of a pull request (§5): what each check built in the scratch namespace, the
+// record under its key when it is still that check's, and the check's document.
+func (r Registry) Forget(ctx context.Context, pull PullRef) (int, error) {
+	checks, err := r.checks(ctx, []PullRef{pull})
+	if err != nil {
+		return 0, err
+	}
+	return r.letGo(ctx, checks)
+}
+
+// Expire lets go of every check older than the bound (§5).
+func (r Registry) Expire(ctx context.Context, older time.Duration, now time.Time) (int, error) {
+	checks, err := r.checks(ctx, nil)
+	if err != nil {
+		return 0, err
+	}
+	old := map[string]checkIndex{}
+	for tag, c := range checks {
+		if now.Sub(c.At) > older {
+			old[tag] = c
+		}
+	}
+	return r.letGo(ctx, old)
+}
+
+// Discard lets go of builds no check document names.
+func (r Registry) Discard(ctx context.Context, builds []CheckBuild) error {
+	_, err := r.letGo(ctx, map[string]checkIndex{"": {Builds: builds}})
+	return err
+}
+
+// letGo lets go of each check's builds, then its document; a check whose builds could not all be let go of keeps
+// its document, so the next sweep asks again.
+func (r Registry) letGo(ctx context.Context, checks map[string]checkIndex) (int, error) {
+	store := r.store()
+	gone := 0
+	var failed []string
+	for tag, c := range checks {
+		ok := true
+		for _, b := range c.Builds {
+			for _, m := range b.Made {
+				reference := catalogue.Recorded(m.Reference)
+				if path, kept := catalogue.InArtifactStore(reference); !kept || !strings.HasPrefix(path, ScratchPrefix) {
+					continue // never anything outside the scratch namespace
+				}
+				if err := store.LetGo(ctx, reference); err != nil && !errors.Is(err, artifacts.Gone) {
+					ok = false
+					failed = append(failed, fmt.Sprintf("%s: %v", m.Reference, err))
+				}
+			}
+			// The record under the key, only while it is still this check's: a later check of the same key
+			// replaced it, and that one is its own check's to let go of.
+			if kept, found, err := r.Find(ctx, b.Module, b.Key); err == nil && found && kept.Check == c.Check {
+				if err := store.DeleteTagged(ctx, ScratchPrefix+b.Module, tagOf(b.Key)); err != nil && !errors.Is(err, artifacts.Gone) {
+					ok = false
+					failed = append(failed, fmt.Sprintf("%s's record: %v", b.Module, err))
+				}
+			}
+		}
+		if !ok {
+			continue
+		}
+		if tag == "" {
+			gone++
+			continue
+		}
+		if err := store.DeleteTagged(ctx, checksRepository, tag); err != nil && !errors.Is(err, artifacts.Gone) {
+			failed = append(failed, fmt.Sprintf("%s: %v", tag, err))
+			continue
+		}
+		gone++
+	}
+	if len(failed) > 0 {
+		sort.Strings(failed)
+		return gone, fmt.Errorf("not let go of: %s", strings.Join(failed, "; "))
+	}
+	return gone, nil
+}
+
+// Reused is what the walk took from a check instead of building (§2, §4).
+type Reused struct {
+	From CheckBuild
+	// Made are the copies in the module's namespace: what is registered.
+	Made []catalogue.Built
+}
+
+// Words is the walk's word for a module it took: "reused from <check build>", with each digest (§4).
+func (r Reused) Words() string {
+	var digests []string
+	for _, m := range r.Made {
+		d := m.Digest
+		if d == "" {
+			_, d, _ = strings.Cut(m.Reference, "@")
+		}
+		digests = append(digests, m.Name+" "+d)
+	}
+	return "reused from " + r.From.Said() + ": " + strings.Join(digests, ", ")
+}

internal/builder/reuse_test.go

@@ -0,0 +1,541 @@
+package builder
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"slices"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// One build per change (novox/hq ADR 0350): the merge check builds what the merge would move into the scratch
+// namespace under its reuse key, and the walk of the trunk commit takes that build by digest instead of building.
+
+// aScratch is the scratch namespace in memory: check builds by module and key, and what the walk took.
+type aScratch struct {
+	builds   map[string]CheckBuild
+	took     []string
+	failTake bool
+}
+
+func newScratch() *aScratch { return &aScratch{builds: map[string]CheckBuild{}} }
+
+func (s *aScratch) Keep(_ context.Context, _ PullRef, _ string, _ time.Time, builds []CheckBuild) error {
+	for _, b := range builds {
+		s.builds[b.Module+" "+b.Key] = b
+	}
+	return nil
+}
+
+func (s *aScratch) Find(_ context.Context, module, key string) (CheckBuild, bool, error) {
+	b, ok := s.builds[module+" "+key]
+	return b, ok, nil
+}
+
+func (s *aScratch) OfPulls(_ context.Context, pulls []PullRef) ([]CheckBuild, error) {
+	var out []CheckBuild
+	for _, b := range s.builds {
+		if slices.Contains(pulls, b.Pull) {
+			out = append(out, b)
+		}
+	}
+	return out, nil
+}
+
+func (s *aScratch) Take(_ context.Context, module string, made catalogue.Built) (catalogue.Built, error) {
+	if s.failTake {
+		return catalogue.Built{}, errors.New("the store refused the mount")
+	}
+	s.took = append(s.took, made.Reference)
+	made.Reference = strings.Replace(made.Reference, ScratchPrefix, "", 1)
+	return made, nil
+}
+
+func (s *aScratch) Forget(context.Context, PullRef) (int, error)                  { return 0, nil }
+func (s *aScratch) Expire(context.Context, time.Duration, time.Time) (int, error) { return 0, nil }
+func (s *aScratch) Discard(context.Context, []CheckBuild) error                   { return nil }
+
+const reusedModule = `{"module":"meshboard","version":"1",
+  "build":{"artifacts":[
+    {"name":"server","kind":"image","from":"Dockerfile"},
+    {"name":"look","kind":"archive","from":"files"}]},
+  "resources":[
+    {"id":"svc","type":"container","name":"meshboard","artifact":"server"},
+    {"id":"theme","type":"archive","path":"/opt/meshboard","artifact":"look"}]}`
+
+// aModuleAt is a repository holding the module under mod/, whose directory git names as tree, on its trunk.
+func aModuleAt(t *testing.T, tree string) (*recorded, Runner, string) {
+	t.Helper()
+	r := &recorded{tree: tree, contents: map[string]string{"mod/" + ManifestName: reusedModule,
+		"mod/Dockerfile": "FROM scratch", "mod/files/theme.conf": "dark"}}
+	run := func(ctx context.Context, dir, name string, args ...string) (string, error) {
+		if name == "git" && len(args) > 0 && args[0] == "symbolic-ref" {
+			r.ran = append(r.ran, name+" "+strings.Join(args, " "))
+			return "origin/main\n", nil
+		}
+		return r.run(ctx, dir, name, args...)
+	}
+	return r, run, t.TempDir()
+}
+
+func dockerBuilt(r *recorded) bool {
+	for _, line := range r.ran {
+		if strings.HasPrefix(line, "docker build") {
+			return true
+		}
+	}
+	return false
+}
+
+var aPull = PullRef{Owner: "novox", Repo: "meshboard", Number: 7}
+
+// aCheckBuilt is the check of aPull building the module from a tree, into the scratch namespace.
+func aCheckBuilt(t *testing.T, scratch *aScratch, tree string) CheckBuilt {
+	t.Helper()
+	r, run, workspace := aModuleAt(t, tree)
+	spec := CheckSpec{ID: "build-1", Owner: aPull.Owner, Repo: aPull.Repo, Number: aPull.Number,
+		Builds: []CheckBuildSpec{{Module: "meshboard", Repository: "https://forge.invalid/meshboard.git", Path: "mod",
+			Ref: "main"}}}
+	built := buildForTheMerge(t.Context(), run, spec, CheckBuilding{Publish: r, Scratch: scratch}, workspace, "", "",
+		CheckVerdict{Head: "abcdefabcdef", Trunk: "123456123456"}, GitCredential{}, nil, logging(nil))
+	if len(built) != 1 || built[0].NotBuilt != "" || built[0].Key == "" {
+		t.Fatalf("the check did not build the module for its merge: %+v", built)
+	}
+	if !dockerBuilt(r) {
+		t.Fatal("the check built no image")
+	}
+	for repository := range r.images {
+		if !strings.HasPrefix(repository, ScratchPrefix) {
+			t.Fatalf("the check published %s outside the scratch namespace", repository)
+		}
+	}
+	for repository := range r.archives {
+		if !strings.HasPrefix(repository, ScratchPrefix) {
+			t.Fatalf("the check published %s outside the scratch namespace", repository)
+		}
+	}
+	return built[0]
+}
+
+func withVersion(t *testing.T, v string) {
+	t.Helper()
+	was := Version
+	Version = v
+	t.Cleanup(func() { Version = was })
+}
+
+// A merge whose reuse key equals its check's builds nothing again: the walk copies the check's build by digest
+// and registers that digest.
+func TestAWalkWhoseKeyIsItsChecksBuildsNothingAndRegistersTheChecksDigest(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	checked := aCheckBuilt(t, scratch, "tree-one")
+
+	r, run, workspace := aModuleAt(t, "tree-one")
+	got, err := BuildReusing(t.Context(), run, r, Reuse{Scratch: scratch, Pulls: []PullRef{aPull}},
+		"https://forge.invalid/meshboard.git", "mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if dockerBuilt(r) || len(r.images) > 0 || len(r.archives) > 0 {
+		t.Fatalf("the walk built again: ran %v, published %v %v", r.ran, r.images, r.archives)
+	}
+	if got.Reused == nil || got.Why != "" {
+		t.Fatalf("the walk did not say it reused the check's build: reused %+v, why %q", got.Reused, got.Why)
+	}
+	if got.ReuseKey != checked.Key {
+		t.Fatalf("the walk's key %s is not the check's %s", got.ReuseKey, checked.Key)
+	}
+	if len(scratch.took) != 2 {
+		t.Fatalf("the walk copied %v, not the check's two artifacts", scratch.took)
+	}
+	for _, made := range checked.Made {
+		registered := strings.Replace(made.Reference, ScratchPrefix, "", 1)
+		found := false
+		for _, b := range got.Built {
+			found = found || (b.Reference == registered && b.Digest == made.Digest)
+		}
+		if !found {
+			t.Fatalf("the walk registers %+v, not the check's %s copied", got.Built, made.Reference)
+		}
+	}
+	if image, _ := got.Manifest.Resources[0]["image"].(string); image != "registry.invalid/meshboard/server@sha256:"+strings.Repeat("a", 64) {
+		t.Fatalf("the manifest pins %q, not the check's digest in the module's namespace", image)
+	}
+	words := got.Reused.Words()
+	if !strings.HasPrefix(words, "reused from the check of novox/meshboard#7 at head abcdefabcdef (merged into trunk 123456123456)") ||
+		!strings.Contains(words, "sha256:") {
+		t.Fatalf("the walk says %q", words)
+	}
+}
+
+// A differing key builds, and says which part differs.
+func TestAWalkWhoseKeyDiffersBuildsAndSaysWhy(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	aCheckBuilt(t, scratch, "tree-one")
+
+	r, run, workspace := aModuleAt(t, "tree-two")
+	got, err := BuildReusing(t.Context(), run, r, Reuse{Scratch: scratch, Pulls: []PullRef{aPull}},
+		"https://forge.invalid/meshboard.git", "mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !dockerBuilt(r) || got.Reused != nil {
+		t.Fatalf("a walk whose source differs did not build: reused %+v", got.Reused)
+	}
+	if !strings.Contains(got.Why, "the key differs") || !strings.Contains(got.Why, partTree) {
+		t.Fatalf("the walk says %q, not which part of the key differs", got.Why)
+	}
+
+	// And a builder of another version builds too, saying so.
+	withVersion(t, "builder-2")
+	r, run, workspace = aModuleAt(t, "tree-one")
+	got, err = BuildReusing(t.Context(), run, r, Reuse{Scratch: scratch, Pulls: []PullRef{aPull}},
+		"https://forge.invalid/meshboard.git", "mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !dockerBuilt(r) || !strings.Contains(got.Why, partBuilder) {
+		t.Fatalf("a newer builder took the older one's build, or did not say why not: %q", got.Why)
+	}
+
+	// No check of the walk's pull requests built it: said.
+	r, run, workspace = aModuleAt(t, "tree-one")
+	got, _ = BuildReusing(t.Context(), run, r, Reuse{Scratch: newScratch(), Pulls: []PullRef{aPull}},
+		"https://forge.invalid/meshboard.git", "mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if !dockerBuilt(r) || !strings.Contains(got.Why, "no check of novox/meshboard#7 built it") {
+		t.Fatalf("the walk says %q", got.Why)
+	}
+}
+
+// A copy that fails is said and falls back to a build; it never fails the walk.
+func TestAFailedCopyOfAChecksBuildFallsBackToBuilding(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	aCheckBuilt(t, scratch, "tree-one")
+	scratch.failTake = true
+
+	r, run, workspace := aModuleAt(t, "tree-one")
+	got, err := BuildReusing(t.Context(), run, r, Reuse{Scratch: scratch, Pulls: []PullRef{aPull}},
+		"https://forge.invalid/meshboard.git", "mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatalf("a failed copy failed the walk: %v", err)
+	}
+	if !dockerBuilt(r) || got.Reused != nil || len(r.images) == 0 {
+		t.Fatal("a failed copy did not fall back to a build")
+	}
+	if !strings.Contains(got.Why, "copying meshboard's") || !strings.Contains(got.Why, "refused the mount") {
+		t.Fatalf("the walk says %q", got.Why)
+	}
+	for repository := range r.images {
+		if strings.HasPrefix(repository, ScratchPrefix) {
+			t.Fatalf("the walk's own build went into the scratch namespace: %s", repository)
+		}
+	}
+}
+
+// Only a walk takes a check's build: a plain build — a check's, a dry run's, `build --ref`'s — builds, and a commit
+// off the trunk never takes one.
+func TestOnlyAWalkOfATrunkCommitTakesAChecksBuild(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	aCheckBuilt(t, scratch, "tree-one")
+
+	r, run, workspace := aModuleAt(t, "tree-one")
+	if _, err := Build(t.Context(), run, r, "https://forge.invalid/meshboard.git", "mod", "main", workspace, nil,
+		Npmrc{}, GitCredential{}, nil); err != nil {
+		t.Fatal(err)
+	}
+	if !dockerBuilt(r) || len(scratch.took) > 0 {
+		t.Fatal("a build not asked to reuse took a check's build")
+	}
+
+	r, _, workspace = aModuleAt(t, "tree-one") // no trunk said: off it
+	got, err := BuildReusing(t.Context(), r.run, r, Reuse{Scratch: scratch}, "https://forge.invalid/meshboard.git",
+		"mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !dockerBuilt(r) || got.Reused != nil || !strings.Contains(got.Why, "not on its trunk") {
+		t.Fatalf("a commit off the trunk took a check's build: %q", got.Why)
+	}
+}
+
+// A check that did not pass builds nothing for its merge, and one asked to build nothing builds nothing.
+func TestACheckThatFailedBuildsNothingForItsMerge(t *testing.T) {
+	if skipsTheBuilds(CheckVerdict{Gate: &Layer{Verdict: "pass"}, Repo: &Layer{Verdict: "fail"}}) == "" {
+		t.Fatal("a check whose repository layer failed builds for its merge")
+	}
+	if skipsTheBuilds(CheckVerdict{Gate: &Layer{Verdict: "pass"}, Repo: &Layer{Verdict: "warning"}}) != "" {
+		t.Fatal("a check that passed with a warning builds nothing for its merge")
+	}
+	r, run, workspace := aModuleAt(t, "tree-one")
+	if built := buildForTheMerge(t.Context(), run, CheckSpec{ID: "build-1"}, CheckBuilding{Publish: r, Scratch: newScratch()},
+		workspace, "", "", CheckVerdict{}, GitCredential{}, nil, logging(nil)); built != nil || len(r.ran) > 0 {
+		t.Fatalf("a check moving no module built %v, ran %v", built, r.ran)
+	}
+}
+
+// A module whose source does not pin its build has no key and is not built by the check; what stands after it
+// is not built either.
+func TestACheckBuildsNoModuleWithoutAKeyNorWhatStandsAfterIt(t *testing.T) {
+	withVersion(t, "builder-1")
+	r, run, workspace := aModuleAt(t, "tree-one")
+	r.contents["mod/"+ManifestName] = `{"module":"meshboard","version":"1","build":{"artifacts":[
+		{"name":"mirror","kind":"upstream","from":"docker.io/library/alpine:3"}]}}`
+	spec := CheckSpec{ID: "build-1", Owner: "novox", Repo: "meshboard", Number: 7, Builds: []CheckBuildSpec{
+		{Module: "meshboard", Repository: "https://forge.invalid/meshboard.git", Path: "mod", Tier: 0},
+		{Module: "after", Repository: "https://forge.invalid/after.git", Tier: 1}}}
+	built := buildForTheMerge(t.Context(), run, spec, CheckBuilding{Publish: r, Scratch: newScratch()}, workspace, "", "",
+		CheckVerdict{}, GitCredential{}, nil, logging(nil))
+	if len(built) != 2 || !strings.Contains(built[0].NotBuilt, "named by tag") ||
+		!strings.Contains(built[1].NotBuilt, "stands after meshboard") {
+		t.Fatalf("built %+v", built)
+	}
+	if len(r.images) > 0 {
+		t.Fatalf("a module without a key was published: %v", r.images)
+	}
+}
+
+// What the key names: every input a fresh build could differ by.
+func TestTheReuseKeyNamesEveryInputOfTheBuild(t *testing.T) {
+	src := newSourceInputs("m")
+	src.tree = "t1"
+	src.bases = []string{"store:5000/b/x@sha256:" + strings.Repeat("b", 64)}
+	src.contexts["server"] = "ctx1"
+	src.toolchains["tools"] = "go sha256:c 1234"
+	manifest := catalogue.Manifest{Module: "m"}
+	base := keyOf(src, manifest, "v1")
+	if base.Key == "" {
+		t.Fatalf("no key: %s", base.Why)
+	}
+	for name, change := range map[string]func(*sourceInputs) string{
+		partTree: func(s *sourceInputs) string { s.tree = "t2"; return "v1" },
+		partBases: func(s *sourceInputs) string {
+			s.bases = []string{"other@sha256:" + strings.Repeat("c", 64)}
+			return "v1"
+		},
+		partContext: func(s *sourceInputs) string { s.contexts["server"] = "ctx2"; return "v1" },
+		partTool:    func(s *sourceInputs) string { s.toolchains["tools"] = "go sha256:d 1234"; return "v1" },
+		partBuilder: func(*sourceInputs) string { return "v2" },
+	} {
+		s := src.copy()
+		version := change(s)
+		if k := keyOf(s, manifest, version); k.Key == base.Key {
+			t.Errorf("a change of %s left the key the same", name)
+		}
+	}
+	// A base copied to another address is the same base: the key names it by digest.
+	moved := src.copy()
+	moved.bases = []string{"elsewhere:5000/scratch/b/x@sha256:" + strings.Repeat("b", 64)}
+	if keyOf(moved, manifest, "v1").Key != base.Key {
+		t.Error("the same base at another address changed the key")
+	}
+	if k := keyOf(src, manifest, ""); k.Key != "" {
+		t.Error("a builder whose version cannot be said keyed its build")
+	}
+	unpinned := src.copy()
+	unpinned.notPinned("it resolves packages from the mesh's registry at build time")
+	if k := keyOf(unpinned, manifest, "v1"); k.Key != "" || !strings.Contains(k.Why, "registry") {
+		t.Errorf("a build its source does not pin was keyed: %+v", k)
+	}
+}
+
+// aTaggingRegistry is the in-memory registry with tags and their listing, as the store's scratch namespace needs.
+type aTaggingRegistry struct {
+	*aRegistryOfRepositories
+	tags map[string]map[string]string // repository → tag → manifest digest
+}
+
+func (m *aTaggingRegistry) serve() http.Handler {
+	under := m.handler()
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if repository, ok := strings.CutSuffix(strings.TrimPrefix(r.URL.Path, "/v2/"), "/tags/list"); ok {
+			m.mu.Lock()
+			var tags []string
+			for tag := range m.tags[repository] {
+				tags = append(tags, tag)
+			}
+			m.mu.Unlock()
+			if tags == nil {
+				w.WriteHeader(http.StatusNotFound)
+				return
+			}
+			_ = json.NewEncoder(w).Encode(map[string]any{"name": repository, "tags": tags})
+			return
+		}
+		repository, kind, rest := splitPath(r.URL.Path)
+		if kind == "blobs" && r.Method == http.MethodHead {
+			m.mu.Lock()
+			body, linked := m.blobs[rest], m.links[repository][rest]
+			m.mu.Unlock()
+			if !linked {
+				w.WriteHeader(http.StatusNotFound)
+				return
+			}
+			w.Header().Set("Content-Length", strconv.Itoa(len(body)))
+			w.WriteHeader(http.StatusOK)
+			return
+		}
+		if kind != "manifests" {
+			under.ServeHTTP(w, r)
+			return
+		}
+		if !strings.HasPrefix(rest, "sha256:") {
+			m.mu.Lock()
+			switch r.Method {
+			case http.MethodPut:
+				body, _ := readAll(r)
+				if m.tags[repository] == nil {
+					m.tags[repository] = map[string]string{}
+				}
+				m.tags[repository][rest] = digestOf(body)
+				m.manifests[repository+"@"+digestOf(body)] = body
+				m.mu.Unlock()
+				w.WriteHeader(http.StatusCreated)
+				return
+			default:
+				digest, ok := m.tags[repository][rest]
+				body, held := m.manifests[repository+"@"+digest]
+				m.mu.Unlock()
+				if !ok || !held {
+					w.WriteHeader(http.StatusNotFound)
+					return
+				}
+				w.Header().Set("Docker-Content-Digest", digest)
+				w.Header().Set("Content-Type", mediaTypeOf(body))
+				if r.Method == http.MethodGet {
+					_, _ = w.Write(body)
+					return
+				}
+				w.WriteHeader(http.StatusOK)
+				return
+			}
+		}
+		if r.Method == http.MethodDelete {
+			m.mu.Lock()
+			for _, tags := range m.tags {
+				for tag, digest := range tags {
+					if digest == rest {
+						delete(tags, tag)
+					}
+				}
+			}
+			m.mu.Unlock()
+		}
+		under.ServeHTTP(w, r)
+	})
+}
+
+// A check's builds go after seven days, and when its pull request closes or is merged; the walk's copy in the
+// module's own namespace stays.
+func TestCheckBuildsExpireAfterSevenDaysAndGoWhenTheirPullRequestCloses(t *testing.T) {
+	m := &aTaggingRegistry{aRegistryOfRepositories: newRegistryOfRepositories(), tags: map[string]map[string]string{}}
+	server := httptest.NewServer(m.serve())
+	defer server.Close()
+	address := strings.TrimPrefix(server.URL, "http://")
+	registry := Registry{Address: address}
+	ctx := t.Context()
+	now := time.Date(2026, 10, 13, 20, 0, 0, 0, time.UTC)
+
+	image := func(repository, body string) string {
+		digest := digestOf([]byte(body))
+		m.manifests[repository+"@"+digest] = []byte(body)
+		return address + "/" + repository + "@" + digest
+	}
+	check := func(pull PullRef, id string, at time.Time, key string) string {
+		reference := image("scratch/meshboard/server", `{"mediaType":"`+mediaManifestOCI+`","layers":[],"n":"`+id+`"}`)
+		build := CheckBuild{Key: key, Module: "meshboard", Made: []catalogue.Built{{Name: "server", Kind: "image",
+			Reference: reference}}, Pull: pull, Check: id, At: at}
+		if err := registry.Keep(ctx, pull, id, at, []CheckBuild{build}); err != nil {
+			t.Fatal(err)
+		}
+		return reference
+	}
+	held := func(reference string) bool {
+		_, digest, _ := strings.Cut(reference, "@")
+		_, ok := m.manifests["scratch/meshboard/server@"+digest]
+		return ok
+	}
+	other := PullRef{Owner: "novox", Repo: "meshboard", Number: 8}
+	old := check(aPull, "build-1", now.Add(-8*24*time.Hour), "r1:"+strings.Repeat("1", 64))
+	fresh := check(aPull, "build-2", now.Add(-time.Hour), "r1:"+strings.Repeat("2", 64))
+	others := check(other, "build-3", now.Add(-time.Hour), "r1:"+strings.Repeat("3", 64))
+	walked := image("meshboard/server", `{"mediaType":"`+mediaManifestOCI+`","layers":[],"n":"walk"}`)
+
+	if _, found, err := registry.Find(ctx, "meshboard", "r1:"+strings.Repeat("2", 64)); err != nil || !found {
+		t.Fatalf("a check build kept is not found by its key: %v %v", found, err)
+	}
+	n, err := registry.Expire(ctx, CheckBuildsKept, now)
+	if err != nil || n != 1 {
+		t.Fatalf("expired %d checks (%v), want the one eight days old", n, err)
+	}
+	if held(old) || !held(fresh) || !held(others) {
+		t.Fatalf("after the week's sweep: old %v, fresh %v, another's %v", held(old), held(fresh), held(others))
+	}
+	if _, found, _ := registry.Find(ctx, "meshboard", "r1:"+strings.Repeat("1", 64)); found {
+		t.Fatal("the expired check build's record is still found")
+	}
+
+	n, err = registry.Forget(ctx, aPull)
+	if err != nil || n != 1 {
+		t.Fatalf("forgot %d checks of %s (%v)", n, aPull, err)
+	}
+	if held(fresh) || !held(others) {
+		t.Fatalf("after %s closed: its build held %v, another pull request's %v", aPull, held(fresh), held(others))
+	}
+	if _, ok := m.manifests["meshboard/server@"+strings.SplitN(walked, "@", 2)[1]]; !ok {
+		t.Fatal("letting go of check builds took the walk's copy in the module's namespace")
+	}
+	if left, _ := registry.OfPulls(ctx, []PullRef{aPull}); len(left) != 0 {
+		t.Fatalf("%s still has check builds: %+v", aPull, left)
+	}
+}
+
+// The walk copies a check build by digest: the digest registered is the digest the check built, and the copy is
+// in the module's own namespace.
+func TestAChecksBuildIsCopiedByDigestIntoTheModulesNamespace(t *testing.T) {
+	m := &aTaggingRegistry{aRegistryOfRepositories: newRegistryOfRepositories(), tags: map[string]map[string]string{}}
+	server := httptest.NewServer(m.serve())
+	defer server.Close()
+	address := strings.TrimPrefix(server.URL, "http://")
+	registry := Registry{Address: address}
+
+	layer := []byte("layer")
+	m.blobs[digestOf(layer)] = layer
+	m.link("scratch/meshboard/server", digestOf(layer))
+	manifest := []byte(`{"mediaType":"` + mediaManifestOCI + `","layers":[{"digest":"` + digestOf(layer) + `","size":5}]}`)
+	m.manifests["scratch/meshboard/server@"+digestOf(manifest)] = manifest
+	archive := []byte("archive")
+	m.blobs[digestOf(archive)] = archive
+	m.link("scratch/meshboard/look", digestOf(archive))
+
+	got, err := registry.Take(t.Context(), "meshboard", catalogue.Built{Name: "server", Kind: "image",
+		Reference: address + "/scratch/meshboard/server@" + digestOf(manifest)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.Reference != address+"/meshboard/server@"+digestOf(manifest) {
+		t.Fatalf("copied as %s", got.Reference)
+	}
+	if m.uploads != 0 || m.mounts != 1 {
+		t.Fatalf("the image's layer was moved (%d uploads, %d mounts), not mounted", m.uploads, m.mounts)
+	}
+	got, err = registry.Take(t.Context(), "meshboard", catalogue.Built{Name: "look", Kind: "archive",
+		Digest: digestOf(archive), Reference: "http://" + address + "/v2/scratch/meshboard/look/blobs/" + digestOf(archive)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.Digest != digestOf(archive) || got.Reference != "http://"+address+"/v2/meshboard/look/blobs/"+digestOf(archive) ||
+		!m.links["meshboard/look"][digestOf(archive)] {
+		t.Fatalf("the archive was copied as %+v", got)
+	}
+}

internal/builder/source.go

@@ -64,6 +64,43 @@ type sourceInputs struct {
 	// contextsAtHead is false once a context was cloned at something other than its trunk's head: its
 	// closure is not the one the next merge there meets, and the build says no build source.
 	contextsAtHead bool
+	// onlyKey says the inputs are being taken for a reuse key (novox/hq ADR 0350 §2): each artifact goes as far
+	// as what its source fingerprint reads, and nothing is built or published.
+	onlyKey bool
+}
+
+// keyOnly is whether these inputs are only being taken for a reuse key.
+func (s *sourceInputs) keyOnly() bool { return s != nil && s.onlyKey }
+
+// copy is the inputs taken so far, apart: a walk's key is taken from them without touching the build's own.
+func (s *sourceInputs) copy() *sourceInputs {
+	c := newSourceInputs(s.module)
+	c.tree, c.unpinned, c.prefix, c.ownIsAll, c.contextsAtHead = s.tree, s.unpinned, s.prefix, s.ownIsAll, s.contextsAtHead
+	c.bases = append([]string{}, s.bases...)
+	for k, v := range s.contexts {
+		c.contexts[k] = v
+	}
+	for k, v := range s.toolchains {
+		c.toolchains[k] = v
+	}
+	for k, v := range s.own {
+		c.own[k] = v
+	}
+	for k, v := range s.read {
+		if v == nil {
+			c.read[k] = nil
+			continue
+		}
+		set := map[string]bool{}
+		for e := range v {
+			set[e] = true
+		}
+		c.read[k] = set
+	}
+	for k, v := range s.readAs {
+		c.readAs[k] = v
+	}
+	return c
 }
 
 // contextOffHead says a context was not its trunk's head.

internal/catalogue/verbs.go

@@ -362,6 +362,12 @@ var ControllerVerbs = []Verb{
 			"register": "\"true\" to register what it builds",
 			"older":    "\"true\", with register: even though a newer build of the module is registered",
 		}, []string{"id"}, "register", "older")},
+	// novox/hq ADR 0350 §5: asked by the delivery's owner when the forge says a pull request closed unmerged.
+	{Name: "forget-checks", Description: "Let go of every build a pull request's merge checks made for its merge in " +
+		"the artifact store's scratch namespace (novox/hq ADR 0350 §5): asked when it closed unmerged; a merged one's go " +
+		"when the walk answering its merge ends, and any check build goes after a week.",
+		Input: schema(map[string]string{"owner": "the repository's owner", "repo": "the repository",
+			"number": "the pull request's number"}, []string{"owner", "repo", "number"})},
 	{Name: "kill", Description: "End a build where it runs: the machine that took it stops its commands and containers and " +
 		"announces it failed, killed by hand — settled, never handed to another machine.",
 		Input: schema(map[string]string{"id": "the build's id"}, []string{"id"})},

internal/inventory/plans.go

@@ -286,6 +286,9 @@ type PlanModule struct {
 	FirstAt *time.Time `json:"first_at,omitempty"`
 	Commit  string     `json:"commit,omitempty"`
 	Why     string     `json:"why,omitempty"`
+	// Made is what the walk's build of it did (novox/hq ADR 0350 §4): "reused from <check build>", naming the pull
+	// request's head, the trunk commit and the digests, or "built: <why>". Said in `plan-moved` and kept here.
+	Made string `json:"made,omitempty"`
 	// Build is the id of the build the plan asked for this module (novox/hq ADR 0219), so the plan
 	// matches its outcome by id — the one thing every outcome echoes, a failed one that never learnt
 	// its module's name included. Empty in a plan from before it was kept, which is matched by

internal/link/build.go

@@ -92,6 +92,24 @@ type BuildRequest struct {
 	// facts snapshot, raises the throwaway stores the check needs, runs the repository's own
 	// merge-check.sh and answers its verdict. Nothing is built, published or registered.
 	Check *CheckRequest `json:"check,omitempty"`
+	// Reuse makes this ask a walk's build of a trunk commit (novox/hq ADR 0350 §2): the builder takes the reuse key
+	// of the commit's tree and, where a check build holds the same key, copies it by digest into the module's
+	// namespace instead of building. Pulls are the pull requests the walk answers, whose checks name the part of a
+	// differing key. Nil for every other ask — a check, a dry run, `build --ref`, `rebuild`, `replay` — which never
+	// takes a check's build. A builder from before it ignores it and builds.
+	Reuse *ReuseAsk `json:"reuse,omitempty"`
+}
+
+// ReuseAsk is a walk's leave to take a check's build (novox/hq ADR 0350 §2).
+type ReuseAsk struct {
+	Pulls []PullRef `json:"pulls,omitempty"`
+}
+
+// PullRef names a pull request.
+type PullRef struct {
+	Owner  string `json:"owner"`
+	Repo   string `json:"repo"`
+	Number int    `json:"number"`
 }
 
 // CheckRequest is what a merge check needs beyond the repository and its head.
@@ -128,6 +146,34 @@ type CheckRequest struct {
 	// repository's own check is not run for a group: each member's pull request runs its own.
 	Group   string        `json:"group,omitempty"`
 	Members []GroupMember `json:"members,omitempty"`
+	// Builds are the modules the delivery plan moves, in the walk's order (novox/hq ADR 0350 §1): the check builds
+	// each, from the merge it judged, into the scratch namespace once it has judged and not failed, for the walk to
+	// take. A build seat from before it builds nothing, and the walk builds as before.
+	Builds []CheckBuild `json:"builds,omitempty"`
+}
+
+// CheckBuild is one module a check builds for its merge: from the merge it made when Merged — a module built from
+// the repository checked, at the branch merged into — else from Repository at Ref, as the walk would.
+type CheckBuild struct {
+	Module     string `json:"module"`
+	Repository string `json:"repository,omitempty"`
+	Path       string `json:"path,omitempty"`
+	Ref        string `json:"ref,omitempty"`
+	Merged     bool   `json:"merged,omitempty"`
+	Tier       int    `json:"tier"`
+	// Skip says why the check cannot build it as the walk would — a module that reads the repository checked
+	// through a context, which a check cannot hand the merge — and nothing after it is built either.
+	Skip string `json:"skip,omitempty"`
+}
+
+// CheckBuilt is one build a check made for its merge, as its verdict lists it (novox/hq ADR 0350 §1): the reuse key
+// and the artifacts, in the scratch namespace — or why it was not built.
+type CheckBuilt struct {
+	Module   string         `json:"module"`
+	Key      string         `json:"key,omitempty"`
+	Made     []MadeArtifact `json:"made,omitempty"`
+	NotBuilt string         `json:"not-built,omitempty"`
+	Took     string         `json:"took,omitempty"`
 }
 
 // GroupMember is one other head of a delivery group's composed check.
@@ -175,6 +221,8 @@ type CheckOutcome struct {
 	Trunk   string            `json:"trunk,omitempty"`
 	Beside  map[string]string `json:"beside,omitempty"`
 	Members []CheckedMember   `json:"members,omitempty"`
+	// Built are the builds it made for its merge (novox/hq ADR 0350 §1).
+	Built []CheckBuilt `json:"built,omitempty"`
 }
 
 // SourceOnSeat names a repository by the seat whose holder serves it and its path there.
@@ -264,6 +312,14 @@ type BuildResult struct {
 	// Failed is why, when it did.
 	Failed string `json:"failed,omitempty"`
 
+	// ReuseKey is the build's reuse key (novox/hq ADR 0350 §1). Reused, on a walk's build that took a check's build
+	// instead of building, is "reused from <check build>" with its digests, and ReusedFrom which; BuiltWhy, on one
+	// that built, why it took none (§3, §4). Both empty from a builder that predates it.
+	ReuseKey   string      `json:"reuse-key,omitempty"`
+	Reused     string      `json:"reused,omitempty"`
+	ReusedFrom *ReusedFrom `json:"reused-from,omitempty"`
+	BuiltWhy   string      `json:"built-why,omitempty"`
+
 	// Source is the request's, echoed: the seat form of the repository, for whoever registers.
 	Source *SourceOnSeat `json:"source,omitempty"`
 
@@ -300,3 +356,25 @@ type MadeArtifact struct {
 	Kind      string `json:"kind"`
 	Reference string `json:"reference"`
 }
+
+// ReusedFrom is the check build a walk's build took (novox/hq ADR 0350 §4): its pull request, the head and the trunk
+// commit the check merged it into, the check's ask, and its key.
+type ReusedFrom struct {
+	Pull  PullRef `json:"pull"`
+	Head  string  `json:"head,omitempty"`
+	Trunk string  `json:"trunk,omitempty"`
+	Check string  `json:"check,omitempty"`
+	Key   string  `json:"key"`
+}
+
+// MadeWords is what a walk says a module's build did (novox/hq ADR 0350 §4): "reused from <check build>", or "built:
+// <why>". A build seat from before it says neither, and that is said.
+func (r BuildResult) MadeWords() string {
+	switch {
+	case r.Reused != "":
+		return r.Reused
+	case r.BuiltWhy != "":
+		return "built: " + r.BuiltWhy
+	}
+	return "built: the build seat did not say whether a check's build could be taken (it predates ADR 0350)"
+}

internal/link/events.go

@@ -290,6 +290,9 @@ type Checked struct {
 	Trunk  string            `json:"trunk,omitempty"`
 	Beside map[string]string `json:"beside,omitempty"`
 	Took   string            `json:"took,omitempty"`
+	// Built are the builds the check made for its merge, each with its reuse key and digests, or why not
+	// (novox/hq ADR 0350 §1): what the walk of the merge takes instead of building.
+	Built []CheckBuilt `json:"built,omitempty"`
 }
 
 // CheckedMember is one head a group's composed check judged, and the trunk commit it was merged into (novox/hq

module.json

@@ -76,6 +76,7 @@
     "rebuild",
     "replay",
     "kill",
+    "forget-checks",
     "pause",
     "resume",
     "hand-act",

testdata/store-tests

@@ -59,3 +59,4 @@
 ./cmd/mesh-controller TestACrossingSendSupersedesASettingsGate
 ./cmd/mesh-controller TestASettingsWalkFailsOnARefusalThatCannotClear
 ./cmd/mesh-controller TestAProviderElsewhereServesItsOwnComposedName
+./cmd/mesh-controller TestAReusedCheckBuildIsRegisteredByItsDigestAndGatedAsABuiltOne