Keep a check build whose key is unchanged instead of building it again

A fresh check asked when only the trunk moved rebuilt every image, though
the reuse key said nothing it is built from changed: a controller check
took about 12 minutes instead of 5.5. The check now keeps a build kept
under its key when a verdict of the build seat names that key and every
digest, names it as the build's own check so a walk takes it, and the
cleanup keeps it while any check names it (novox/hq issue 703).

cmd/mesh-builder/main.go

@@ -319,7 +319,7 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 		// 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}
+				Seats: request.Seats, Named: namedOf(request.Check.Named)}
 		}
 		var v builder.CheckVerdict
 		v, err = builder.Check(building, builder.Command, spec, workspace, registry, forgeFrom(), say)
@@ -352,15 +352,7 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 				reuse.Pulls = append(reuse.Pulls, builder.PullRef{Owner: p.Owner, Repo: p.Repo, Number: p.Number})
 			}
 			// Only what a verdict names is taken (ADR 0350 §2).
-			for _, b := range request.Reuse.Builds {
+			reuse.Named = namedOf(request.Reuse.Builds)
-				named := builder.NamedBuild{Pull: builder.PullRef{Owner: b.Pull.Owner, Repo: b.Pull.Repo,
-					Number: b.Pull.Number}, Check: b.Check, Module: b.Built.Module, Key: b.Built.Key,
-					Digests: map[string]string{}}
-				for _, m := range b.Built.Made {
-					named.Digests[m.Name] = builder.DigestOf(m.Reference)
-				}
-				reuse.Named = append(reuse.Named, named)
-			}
 			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)
@@ -496,11 +488,28 @@ func triedOf(v builder.CheckVerdict) link.Tried {
 	return t
 }
 
+// namedOf are the builds the seat's verdicts name, as the builder believes a scratch record by them (novox/hq ADR
+// 0350 §2, issue 703): each module's key with every artifact's digest.
+func namedOf(builds []link.CheckedBuild) []builder.NamedBuild {
+	var out []builder.NamedBuild
+	for _, b := range builds {
+		named := builder.NamedBuild{Pull: builder.PullRef{Owner: b.Pull.Owner, Repo: b.Pull.Repo,
+			Number: b.Pull.Number}, Check: b.Check, Module: b.Built.Module, Key: b.Built.Key,
+			Digests: map[string]string{}}
+		for _, m := range b.Built.Made {
+			named.Digests[m.Name] = builder.DigestOf(m.Reference)
+		}
+		out = append(out, named)
+	}
+	return out
+}
+
 // 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()}
+		c := link.CheckBuilt{Module: b.Module, Key: b.Key, NotBuilt: b.NotBuilt, Took: b.Took.Round(time.Second).String(),
+			Kept: b.Kept}
 		for _, m := range b.Made {
 			c.Made = append(c.Made, link.MadeArtifact{Name: m.Name, Kind: m.Kind, Reference: m.Reference})
 		}

cmd/mesh-controller/check_builds_test.go

@@ -214,3 +214,28 @@ func TestAWalksLeaveNamesTheBuildsItsPullRequestsVerdictsMade(t *testing.T) {
 		t.Fatalf("verdicts that could not be read named %+v", got.Builds)
 	}
 }
+
+// A check is handed the builds the seat's verdicts name for the modules it builds, read once, and none when they
+// cannot be read (novox/hq issue 703).
+func TestACheckIsHandedTheBuildsTheVerdictsNameForItsModules(t *testing.T) {
+	was := checkedBuildsOfModules
+	t.Cleanup(func() { checkedBuildsOfModules = was })
+	var asked [][]string
+	named := []link.CheckedBuild{{Pull: link.PullRef{Owner: "novox", Repo: "shop", Number: 7}, Check: "build-1",
+		Built: link.CheckBuilt{Module: "app", Key: "r1:1"}}}
+	checkedBuildsOfModules = func(_ context.Context, modules []string) ([]link.CheckedBuild, error) {
+		asked = append(asked, modules)
+		return named, nil
+	}
+	builds := []link.CheckBuild{{Module: "app"}, {Module: "web", Skip: "the merge deletes it"}}
+	if got := namedForTheCheck(t.Context(), builds); !reflect.DeepEqual(got, named) ||
+		!reflect.DeepEqual(asked, [][]string{{"app"}}) {
+		t.Fatalf("the check is handed %+v, read %v", got, asked)
+	}
+	checkedBuildsOfModules = func(context.Context, []string) ([]link.CheckedBuild, error) {
+		return nil, errors.New("no bus")
+	}
+	if got := namedForTheCheck(t.Context(), builds); got != nil {
+		t.Fatalf("verdicts that could not be read named %+v", got)
+	}
+}

cmd/mesh-controller/checks.go

@@ -273,6 +273,7 @@ func checkRequestFor(ctx context.Context, open *stores, p link.PullUpdated, scop
 			}
 		}
 	}
+	builds := checkBuildsOf(p, scope, entries, clone)
 	return link.BuildRequest{
 		ID:         link.NewBuildID(time.Now()),
 		Repository: repository,
@@ -282,10 +283,33 @@ 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, Builds: checkBuildsOf(p, scope, entries, clone)},
+			Manifests: scope.Manifests, Judge: scope.Judge, Builds: builds, Named: namedForTheCheck(ctx, builds)},
 	}, nil
 }
 
+// namedForTheCheck are the builds the seat's verdicts name for the modules a check builds, read once for the check
+// (novox/hq issue 703): what it may keep instead of building again. None when they cannot be read, and the check
+// builds, as before.
+func namedForTheCheck(ctx context.Context, builds []link.CheckBuild) []link.CheckedBuild {
+	var modules []string
+	for _, b := range builds {
+		if b.Skip == "" {
+			modules = append(modules, b.Module)
+		}
+	}
+	if len(modules) == 0 {
+		return nil
+	}
+	named, err := checkedBuildsOfModules(ctx, modules)
+	if err != nil {
+		fmt.Printf("  the checks' builds of %d module(s) could not be read, so the check builds them: %v\n",
+			len(modules), err)
+		// empty-on-error: said above; a check handed no named build builds every module, as before issue 703
+		return nil
+	}
+	return named
+}
+
 // 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

cmd/mesh-controller/release_plan.go

@@ -386,6 +386,21 @@ var checkedBuildsOf = func(ctx context.Context, pulls []link.PullRef) ([]link.Ch
 	return link.ReadCheckBuilds(ctx, js, buildSeatHeld(ctx), pulls, time.Now().Add(-builder.CheckBuildsKept))
 }
 
+// checkedBuildsOfModules are the builds the build seat's checks of any pull request said they made, or kept, of
+// those modules within the week check builds are kept (novox/hq issue 703): what a check may keep instead of
+// building. A variable so a test needs no bus.
+var checkedBuildsOfModules = func(ctx context.Context, modules []string) ([]link.CheckedBuild, error) {
+	if len(modules) == 0 {
+		return nil, nil
+	}
+	js, err := aBus()
+	if err != nil {
+		return nil, err
+	}
+	defer js.Close()
+	return link.ReadCheckBuildsOf(ctx, js, buildSeatHeld(ctx), modules, time.Now().Add(-builder.CheckBuildsKept))
+}
+
 // reuseWithVerdicts is a walk's leave with the builds its pull requests' verdicts name: the only builds the builder
 // may take (ADR 0350 §2). When they cannot be read the leave names none, and the walk builds, saying why.
 func reuseWithVerdicts(ctx context.Context, r *link.ReuseAsk) *link.ReuseAsk {

internal/builder/builder.go

@@ -374,7 +374,7 @@ func build(ctx context.Context, run Runner, publish Publisher,
 		// 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,
+			keyed, 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)
@@ -383,6 +383,11 @@ func build(ctx context.Context, run Runner, publish Publisher,
 				say("reuse", "not built for the merge: %v", err)
 				return Result{Commit: commit, NoKey: err.Error()}, err
 			}
+			// **A check keeps a build of the same key a verdict names** (novox/hq issue 703): asked again when only
+			// the trunk moved, it builds nothing the earlier check built of an unchanged source.
+			if kept := keptCheckBuild(ctx, *reuse, manifest.Module, key.Key, len(artifacts), say); kept != nil {
+				built, src, reused = kept.Made, keyed, kept
+			}
 		} 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)
@@ -492,6 +497,32 @@ func takeACheckBuild(ctx context.Context, run Runner, publish Publisher, reuse R
 	return taken, keyed, ""
 }
 
+// keptCheckBuild is the build a check keeps instead of building (novox/hq issue 703): the one kept under the
+// module's key, when a verdict of the build seat names its key and every digest — the record alone is never trusted
+// (ADR 0350 §2) — and it holds every artifact the module declares. Nil, and the check builds, otherwise.
+func keptCheckBuild(ctx context.Context, reuse Reuse, module, key string, artifacts int,
+	say func(step, format string, args ...any)) *Reused {
+	if reuse.Scratch == nil || len(reuse.Named) == 0 {
+		return nil
+	}
+	found, ok, err := reuse.Scratch.Find(ctx, module, key)
+	switch {
+	case err != nil:
+		say("reuse", "the scratch namespace could not be read, so the check builds: %v", err)
+		return nil
+	case !ok:
+		return nil
+	case unnamed(reuse.Named, module, key, found) != "":
+		say("reuse", "a build is kept under %s and no verdict names it with its digests: built", twelve(key[3:]))
+		return nil
+	case len(found.Made) != artifacts:
+		return nil
+	}
+	kept := &Reused{From: found, Made: found.Made}
+	say("reuse", "kept, not built: %s", strings.Replace(kept.Words(), "reused from", "the build of", 1))
+	return kept
+}
+
 // 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,

internal/builder/check_builds.go

@@ -37,6 +37,10 @@ type CheckBuilding struct {
 	Held    map[string]string
 	Npmrc   Npmrc
 	Seats   map[string]string
+	// Named are the builds the build seat's verdicts name for the modules this check builds, of any pull request,
+	// within the week check builds are kept (novox/hq issue 703): a build kept under a module's key is kept again,
+	// instead of built, only when one of them names its key and every digest. The record alone is never enough.
+	Named []NamedBuild
 }
 
 // CheckBuilt is one module a check built for its merge, as its verdict lists it (§1): its key and its artifacts, or
@@ -48,6 +52,9 @@ type CheckBuilt struct {
 	// NotBuilt says why it was not: no key, a build that failed, or a module after one of those.
 	NotBuilt string
 	Took     time.Duration
+	// Kept is the check whose build this check kept instead of building, its key unchanged (novox/hq issue 703):
+	// the verdict names that build, by key and digest, as that check's. Empty when this check built it.
+	Kept string
 }
 
 // skipsTheBuilds is why a check builds nothing for its merge; empty when it builds.
@@ -100,8 +107,8 @@ func buildForTheMerge(ctx context.Context, run Runner, spec CheckSpec, with Chec
 		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},
+			filepath.Join(workspace, "for-the-merge"), held, with.Npmrc, forge, log, &mirrored,
-			with.Seats)
+			&Reuse{keyFirst: true, Scratch: with.Scratch, Named: with.Named}, with.Seats)
 		built := CheckBuilt{Module: b.Module, Key: result.ReuseKey, Took: time.Since(began)}
 		switch {
 		case errors.Is(err, errNoKey):
@@ -123,6 +130,17 @@ func buildForTheMerge(ctx context.Context, run Runner, spec CheckSpec, with Chec
 		for _, m := range result.Built {
 			held[b.Module+"/"+m.Name] = m.Reference
 		}
+		if result.Reused != nil {
+			// **Kept, not built** (novox/hq issue 703): the earlier check's build, named as that check's, so the
+			// walk takes it as the verdict names it; this check's document names it too, so it stays while this
+			// check does (ADR 0350 §5).
+			built.Kept = result.Reused.From.Check
+			say("build", "%s kept for the merge under %s: %s", b.Module, twelve(result.ReuseKey[3:]),
+				result.Reused.From.Said())
+			out = append(out, built)
+			kept = append(kept, result.Reused.From)
+			continue
+		}
 		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 {

internal/builder/reuse.go

@@ -494,11 +494,15 @@ func (r Registry) Discard(ctx context.Context, builds []CheckBuild) error {
 func (r Registry) letGo(ctx context.Context, checks, all map[string]checkIndex) (int, error) {
 	store := r.store()
 	stays := map[string]bool{}
+	// recorded are the records under a key another check still names (novox/hq issue 703): a later check that kept
+	// the build names it as the check's that built it, and the walk finds it by that record.
+	recorded := map[string]bool{}
 	for tag, c := range all {
 		if _, going := checks[tag]; going {
 			continue
 		}
 		for _, b := range c.Builds {
+			recorded[b.Module+" "+b.Key+" "+b.Check] = true
 			for _, m := range b.Made {
 				stays[catalogue.Recorded(m.Reference)] = true
 			}
@@ -523,8 +527,10 @@ func (r Registry) letGo(ctx context.Context, checks, all map[string]checkIndex)
 				}
 			}
 			// 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.
+			// replaced it, and that one is its own check's to let go of. A build a check kept is that build's
-			if kept, found, err := r.Find(ctx, b.Module, b.Key); err == nil && found && kept.Check == c.Check {
+			// check's record, and goes with the last check naming it (novox/hq issue 703).
+			if kept, found, err := r.Find(ctx, b.Module, b.Key); err == nil && found &&
+				(kept.Check == c.Check || kept.Check == b.Check) && !recorded[b.Module+" "+b.Key+" "+kept.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))

internal/builder/reuse_test.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"net/http"
 	"net/http/httptest"
+	"reflect"
 	"slices"
 	"strconv"
 	"strings"
@@ -625,3 +626,122 @@ func TestABuildTwoChecksShareGoesWithTheLastOfThem(t *testing.T) {
 		t.Fatal("the build stayed after the last check naming it went")
 	}
 }
+
+// aCheckAgain is a later check (build-2) of aPull building the module from a tree, with the builds the build seat's
+// verdicts name for it (novox/hq issue 703).
+func aCheckAgain(t *testing.T, scratch *aScratch, tree string, leave []NamedBuild) (CheckBuilt, *recorded) {
+	t.Helper()
+	r, run, workspace := aModuleAt(t, tree)
+	spec := CheckSpec{ID: "build-2", 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, Named: leave},
+		workspace, "", "", CheckVerdict{Head: "fedcbafedcba", Trunk: "654321654321"}, GitCredential{}, nil, logging(nil))
+	if len(built) != 1 || built[0].NotBuilt != "" || built[0].Key == "" {
+		t.Fatalf("the second check did not answer the module for its merge: %+v", built)
+	}
+	return built[0], r
+}
+
+// A check asked again when only the trunk moved, whose key is unchanged, builds nothing: it keeps the build an
+// earlier verdict names and names it by key and digest, so the walk can take it (novox/hq issue 703).
+func TestASecondCheckWhoseKeyIsUnchangedBuildsNothingAndNamesTheEarlierBuild(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	first := aCheckBuilt(t, scratch, "tree-one")
+
+	again, r := aCheckAgain(t, scratch, "tree-one", named(first))
+	if dockerBuilt(r) || len(r.images) > 0 || len(r.archives) > 0 {
+		t.Fatalf("the second check built again: ran %v, published %v %v", r.ran, r.images, r.archives)
+	}
+	if again.Key != first.Key || again.Kept != "build-1" || !reflect.DeepEqual(again.Made, first.Made) {
+		t.Fatalf("the second check names %+v, not the first check's build %+v", again, first)
+	}
+	// The walk takes it by what the second verdict names.
+	leave := named(again)
+	leave[0].Check = again.Kept
+	w, run, workspace := aModuleAt(t, "tree-one")
+	got, err := BuildReusing(t.Context(), run, w, Reuse{Scratch: scratch, Pulls: []PullRef{aPull}, Named: leave},
+		"https://forge.invalid/meshboard.git", "mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil || got.Reused == nil || dockerBuilt(w) {
+		t.Fatalf("the walk did not take the kept build: %v, why %q", err, got.Why)
+	}
+}
+
+// A check whose key changed builds, as before.
+func TestASecondCheckWhoseKeyChangedBuilds(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	first := aCheckBuilt(t, scratch, "tree-one")
+
+	again, r := aCheckAgain(t, scratch, "tree-two", named(first))
+	if !dockerBuilt(r) || again.Kept != "" || again.Key == first.Key {
+		t.Fatalf("a check whose key changed did not build: %+v", again)
+	}
+}
+
+// A scratch record no verdict names is never kept: the store takes writes from anything on its network, so a
+// check builds rather than name a planted record (novox/hq issue 703, ADR 0350 §2).
+func TestASecondCheckKeepsNoRecordAVerdictDoesNotName(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	first := aCheckBuilt(t, scratch, "tree-one")
+	for name, leave := range map[string][]NamedBuild{
+		"no verdict": nil,
+		"another digest": func() []NamedBuild {
+			n := named(first)
+			n[0].Digests["server"] = "sha256:" + strings.Repeat("e", 64)
+			return n
+		}(),
+		"another key": func() []NamedBuild { n := named(first); n[0].Key = "r1:" + strings.Repeat("f", 64); return n }(),
+	} {
+		t.Run(name, func(t *testing.T) {
+			again, r := aCheckAgain(t, scratch, "tree-one", leave)
+			if !dockerBuilt(r) || again.Kept != "" {
+				t.Fatalf("a record no verdict names was kept: %+v", again)
+			}
+		})
+	}
+}
+
+// A build a later check kept stays when the check that built it goes: its artifacts and its record, which the later
+// check's verdict names (novox/hq issue 703, ADR 0350 §5).
+func TestABuildALaterCheckKeptStaysWhenTheCheckThatBuiltItGoes(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, 14, 9, 0, 0, 0, time.UTC)
+
+	body := `{"mediaType":"` + mediaManifestOCI + `","layers":[]}`
+	digest := digestOf([]byte(body))
+	m.manifests["scratch/meshboard/server@"+digest] = []byte(body)
+	key := "r1:" + strings.Repeat("1", 64)
+	b := CheckBuild{Key: key, Module: "meshboard", Pull: aPull, Check: "build-1", At: now,
+		Made: []catalogue.Built{{Name: "server", Kind: "image", Reference: address + "/scratch/meshboard/server@" + digest}}}
+	if err := registry.Keep(ctx, aPull, "build-1", now, []CheckBuild{b}); err != nil {
+		t.Fatal(err)
+	}
+	// Another pull request's check keeps it: its own document names the build, which stays the first check's.
+	other := PullRef{Owner: "novox", Repo: "meshboard", Number: 8}
+	if err := registry.Keep(ctx, other, "build-2", now, []CheckBuild{b}); err != nil {
+		t.Fatal(err)
+	}
+	held := func() bool { _, ok := m.manifests["scratch/meshboard/server@"+digest]; return ok }
+	found := func() bool { _, ok, _ := registry.Find(ctx, "meshboard", key); return ok }
+
+	if n, err := registry.Forget(ctx, aPull); err != nil || n != 1 {
+		t.Fatalf("forgot %d (%v)", n, err)
+	}
+	if !held() || !found() {
+		t.Fatalf("the build a later check names went with the check that built it: held %v, record %v", held(), found())
+	}
+	if n, err := registry.Forget(ctx, other); err != nil || n != 1 {
+		t.Fatalf("forgot %d (%v)", n, err)
+	}
+	if held() || found() {
+		t.Fatalf("the build stayed after the last check naming it went: held %v, record %v", held(), found())
+	}
+}

internal/link/build.go

@@ -164,6 +164,10 @@ type CheckRequest struct {
 	// 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"`
+	// Named are the builds the seat's verdicts name for those modules, of any pull request, within the week check
+	// builds are kept (novox/hq issue 703): the check keeps a build kept under a module's unchanged key, instead of
+	// building it again, only when one of these names its key and every digest. A build seat from before it builds.
+	Named []CheckedBuild `json:"named,omitempty"`
 	// Try marks a branch tried before its pull request (novox/hq issue 686): the repository's own check for a
 	// pushed branch or commit, asked by a builder through the controller's `try` verb. Its outcome is read from
 	// the build's log (StepTried) and said to nobody: no status on the forge, nothing recorded or registered,
@@ -212,6 +216,9 @@ type CheckBuilt struct {
 	Made     []MadeArtifact `json:"made,omitempty"`
 	NotBuilt string         `json:"not-built,omitempty"`
 	Took     string         `json:"took,omitempty"`
+	// Kept names the check whose build this check kept instead of building, its key unchanged (novox/hq issue 703):
+	// a walk takes it as that check's build. Empty when this check built it.
+	Kept string `json:"kept,omitempty"`
 }
 
 // GroupMember is one other head of a delivery group's composed check.

internal/link/check_builds_test.go

@@ -37,3 +37,33 @@ func TestNatsTheBuildsAWalkMayTakeAreTheChecksOutcomesOfItsPullRequests(t *testi
 		t.Fatalf("read %+v", got)
 	}
 }
+
+// A check reads the builds every pull request's checks made of the modules it builds, and a kept build is named as
+// the check's that built it (novox/hq issue 703).
+func TestNatsACheckReadsTheBuildsOfItsModulesAndAKeptBuildIsTheBuildersCheck(t *testing.T) {
+	js := aBus(t)
+	since := time.Now().Add(-time.Minute)
+	say := func(r BuildResult) {
+		body, _ := json.Marshal(r)
+		if _, err := js.Context().Publish(BuildOutcomeOf(TheBuildMachine), body); err != nil {
+			t.Fatal(err)
+		}
+	}
+	app := CheckBuilt{Module: "app", Key: "r1:1", Made: []MadeArtifact{{Name: "server", Reference: "s/app/server@sha256:a"}}}
+	web := CheckBuilt{Module: "web", Key: "r1:2", Made: []MadeArtifact{{Name: "server", Reference: "s/web/server@sha256:b"}}}
+	say(BuildResult{ID: "build-1", Checked: &CheckRequest{Owner: "novox", Repo: "shop", Number: 7},
+		Check: &CheckOutcome{Verdict: "pass", Built: []CheckBuilt{app, web}}})
+	kept := app
+	kept.Kept = "build-1"
+	say(BuildResult{ID: "build-2", Checked: &CheckRequest{Owner: "novox", Repo: "till", Number: 3},
+		Check: &CheckOutcome{Verdict: "pass", Built: []CheckBuilt{kept}}})
+
+	got, err := ReadCheckBuildsOf(t.Context(), js, TheBuildMachine, []string{"app"}, since)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got) != 2 || got[0].Check != "build-1" || got[0].Pull.Repo != "till" || got[1].Check != "build-1" ||
+		got[1].Built.Module != "app" {
+		t.Fatalf("read %+v", got)
+	}
+}

internal/link/queue.go

@@ -375,6 +375,26 @@ func ReadCheckBuilds(ctx context.Context, js *broker.JetStream, seat string, pul
 	for _, p := range pulls {
 		wanted[p] = true
 	}
+	return readCheckBuilds(ctx, js, seat, since, func(p PullRef, _ CheckBuilt) bool { return wanted[p] })
+}
+
+// ReadCheckBuildsOf is every build the seat's checks of any pull request said they made, or kept, of those modules
+// since a moment (novox/hq issue 703): what a check may keep instead of building. Newest first.
+func ReadCheckBuildsOf(ctx context.Context, js *broker.JetStream, seat string, modules []string, since time.Time) ([]CheckedBuild, error) {
+	if len(modules) == 0 {
+		return nil, nil
+	}
+	wanted := map[string]bool{}
+	for _, m := range modules {
+		wanted[m] = true
+	}
+	return readCheckBuilds(ctx, js, seat, since, func(_ PullRef, b CheckBuilt) bool { return wanted[b.Module] })
+}
+
+// readCheckBuilds is every build the seat's checks said they made, or kept, that wanted takes. A kept build is named
+// as the check's that built it (novox/hq issue 703): the record under its key is that check's.
+func readCheckBuilds(ctx context.Context, js *broker.JetStream, seat string, since time.Time,
+	wanted func(PullRef, CheckBuilt) bool) ([]CheckedBuild, error) {
 	subject := BuildOutcomeOf(seat)
 	sub, err := js.Context().PullSubscribe(subject, "",
 		nats.BindStream(broker.EventsStream), nats.StartTime(since), nats.AckNone())
@@ -402,12 +422,13 @@ func ReadCheckBuilds(ctx context.Context, js *broker.JetStream, seat string, pul
 				continue
 			}
 			pull := PullRef{Owner: r.Checked.Owner, Repo: r.Checked.Repo, Number: r.Checked.Number}
-			if !wanted[pull] {
-				continue
-			}
 			for _, b := range r.Check.Built {
-				if b.NotBuilt == "" && b.Key != "" {
+				if b.NotBuilt == "" && b.Key != "" && wanted(pull, b) {
-					out = append(out, CheckedBuild{Pull: pull, Check: r.ID, Built: b})
+					check := r.ID
+					if b.Kept != "" {
+						check = b.Kept
+					}
+					out = append(out, CheckedBuild{Pull: pull, Check: check, Built: b})
 				}
 			}
 		}