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 {
-				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)
-			}
+			reuse.Named = namedOf(request.Reuse.Builds)
 			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})
 		}