Read the bus release from the nats module's upstream artifact, so the tests' bus is still held to the mesh's once the Dockerfile is gone (issue 1640)

mesh-catalog #434 runs the nats server as the upstream image named in module.json (ADR 0497) and
deletes modules/nats/Dockerfile, so the check failed opening it. The manifest's upstream nats
artifact is read first; the Dockerfile only for a catalogue from before the move (the captured copy).

internal/testbus/testbus_test.go

@@ -19,18 +19,10 @@ import (
 // bus upgrade moves the catalogue's pin; this then fails in the merge check until the tests' pin moves
 // with it, so the controller is never tested against a bus the mesh no longer runs, or not yet.
 func TestTheBusTheTestsRunIsTheBusTheMeshRuns(t *testing.T) {
-	dockerfile := filepath.Join(beside.Catalogue(t), "nats", "Dockerfile")
-	raw, err := os.ReadFile(dockerfile)
-	if err != nil {
-		t.Fatal(err)
-	}
-	pinned := regexp.MustCompile(`upstream: nats ([0-9.]+)-alpine`).FindSubmatch(raw)
-	if pinned == nil {
-		t.Fatalf("%s says no release its digest is", dockerfile)
-	}
-	if string(pinned[1]) != Version {
-		t.Errorf("the tests run bus %s and the catalogue's bus image is %s: move go.mod's nats-server pin "+
-			"(and `go mod vendor`) with the image", Version, pinned[1])
+	release, from := catalogueBusRelease(t, filepath.Join(beside.Catalogue(t), "nats"))
+	if release != Version {
+		t.Errorf("the tests run bus %s and the catalogue's bus image is %s (%s): move go.mod's nats-server pin "+
+			"(and `go mod vendor`) with the image", Version, release, from)
 	}
 	path := os.Getenv("MESH_FACTS")
 	if path == "" {
@@ -57,3 +49,51 @@ func TestTheBusTheTestsRunIsTheBusTheMeshRuns(t *testing.T) {
 }
 
 // A test's bus is its own: what one test makes, the next never sees.
+
+// catalogueBusRelease is the server release the catalogue's nats module runs, and where it was read. Since
+// ADR 0497 the server is an upstream artifact in the module's manifest (`nats:<release>-alpine@sha256:…`);
+// before it, an image built from the module's Dockerfile, whose `upstream: nats <release>-alpine` line said
+// the release its digest is. The manifest is read first; the Dockerfile only where the manifest names no
+// upstream nats image (a catalogue from before that move, such as the captured copy). Neither is a failure.
+func catalogueBusRelease(t *testing.T, module string) (release, from string) {
+	t.Helper()
+	manifest := filepath.Join(module, "module.json")
+	raw, err := os.ReadFile(manifest)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var m struct {
+		Build struct {
+			Artifacts []struct {
+				Name string `json:"name"`
+				Kind string `json:"kind"`
+				From string `json:"from"`
+			} `json:"artifacts"`
+		} `json:"build"`
+	}
+	if err := json.Unmarshal(raw, &m); err != nil {
+		t.Fatalf("%s: %v", manifest, err)
+	}
+	upstream := regexp.MustCompile(`^(?:[^/]+/)*nats:([0-9]+\.[0-9]+\.[0-9]+)-alpine(?:@sha256:[0-9a-f]{64})?$`)
+	for _, a := range m.Build.Artifacts {
+		if a.Kind != "upstream" || !strings.HasPrefix(a.From[strings.LastIndex(a.From, "/")+1:], "nats:") {
+			continue
+		}
+		pinned := upstream.FindStringSubmatch(a.From)
+		if pinned == nil {
+			t.Fatalf("%s: the upstream artifact %q is %q, which says no release in the form nats:<x.y.z>-alpine",
+				manifest, a.Name, a.From)
+		}
+		return pinned[1], manifest
+	}
+	dockerfile := filepath.Join(module, "Dockerfile")
+	raw, err = os.ReadFile(dockerfile)
+	if err != nil {
+		t.Fatalf("%s names no upstream nats image, and the image built before it is not there either: %v", manifest, err)
+	}
+	pinned := regexp.MustCompile(`upstream: nats ([0-9.]+)-alpine`).FindSubmatch(raw)
+	if pinned == nil {
+		t.Fatalf("%s says no release its digest is", dockerfile)
+	}
+	return string(pinned[1]), dockerfile
+}