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