Say that the builder writes references only once each build module answered one (issue 1886, ADR 0522 review)

The references come from the build modules' own pushes, so a new builder beside
an old build module writes none; saying it at start would send the controller
to keys that fail not stored.

modules/mesh-delivery-builder/internal/build/r4e_test.go

@@ -383,3 +383,21 @@ func TestACandidatesFetchNamesTheNamedPathsTheIndexerGaveIt(t *testing.T) { // A
 		t.Errorf("fetch request = %+v, %v; want the candidate's base, the module's directory and its named paths", req, err)
 	}
 }
+
+// 1886/AC5: the builder tells who answered a reference: the build module whose step named one, and none for a step that
+// answered a digest alone (a build module from before R4e).
+func TestTheBuilderTellsWhichBuildModuleAnsweredAReference(t *testing.T) {
+	r := newRig(t, goBundle)
+	var answered []string
+	r.b.Answered = func(module string) { answered = append(answered, module) }
+	r.bm.noRef["ace/tool"] = true
+	r.consider(t, "modules/m")
+	if len(answered) != 0 {
+		t.Fatalf("a step that answered no reference was told as one: %v", answered)
+	}
+	delete(r.bm.noRef, "ace/tool")
+	r.consider(t, "modules/m")
+	if len(answered) != 1 || answered[0] != "mesh-build-go" {
+		t.Fatalf("the build module that answered a reference: %v, want mesh-build-go", answered)
+	}
+}

modules/mesh-delivery-builder/internal/build/run.go

@@ -45,6 +45,9 @@ type Builder struct {
 	// Limit is how many builds run at once; four when 0.
 	Limit int
 	Logf  func(format string, args ...any)
+	// Answered is told the build module whose step answered a reference (issue 1886): what the builder says it writes
+	// rests on it. Nil tells nobody.
+	Answered func(buildModule string)
 
 	once     sync.Once
 	sem      chan struct{}
@@ -749,6 +752,9 @@ func (b *Builder) runLane(ctx context.Context, l *lane, repository string, m del
 		if built.Reference != "" && (stored(t.Kind) || !candidate) {
 			l.references[t.Artifact] = built.Reference
 		}
+		if built.Reference != "" && b.Answered != nil {
+			b.Answered(l.module)
+		}
 	}
 }