A module may mirror an image it did not write

A module usually runs software somebody else built: a database module
ships configuration and a provisioner and does not build a database. It
could name the upstream reference directly, and then every machine needs
a route to a public registry and the reference is a tag somebody else can
move — which is what pinning exists to prevent.

So an artifact may be `upstream`: pulled by the reference the module
names, pushed into the mesh's own registry, and pinned by the digest that
registry assigns. This is what the bootstrap already does by hand; it is
now something a module can say.

Refused: an upstream reference with no tag or digest, because what gets
mirrored would be whatever `latest` means today and a module pinned to
that is not pinned. And the rule that a build reads only its own
repository does not apply to it — applying it anyway refused every
reference with a registry host in it, which the test caught.

Written by trying to write a real postgres module and finding it could
not be said. It can now: two directories, two containers pinned by
digest, a superuser password sealed to the machine, and the grants
manifest — six resources from one assignment, all accepted by the host's
own parser.

That exercise also found my manifest wrong rather than the host: a
container declared `restart-on`, which is a service field, and the host
refused it by name. It is right to. A container whose own definition
changes is recreated, and a file it mounts is read by the process inside,
which is that image's business.

internal/builder/builder_test.go

@@ -226,3 +226,64 @@ func TestABuildThatCannotPushFails(t *testing.T) {
 		t.Fatal("a build that could publish nothing reported success")
 	}
 }
+
+func TestAnUpstreamImageIsMirroredRatherThanBuilt(t *testing.T) {
+	// A module usually runs software it did not write. Naming the upstream reference directly
+	// would need every machine to reach a public registry, and would pin to a tag somebody else
+	// can move.
+	const mirrors = `{"module":"postgres","version":"1",
+	  "build":{"artifacts":[{"name":"store","kind":"upstream","from":"postgres:17-alpine"}]},
+	  "resources":[{"id":"db","type":"container","name":"mesh-postgres","artifact":"store"}]}`
+
+	r, workspace := aRepository(t, mirrors, nil)
+	got, err := Build(context.Background(), r.run, r, "https://forge.invalid/postgres.git", "", workspace)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Pulled, not built.
+	var pulled, built bool
+	for _, line := range r.ran {
+		if strings.HasPrefix(line, "docker pull postgres:17-alpine") {
+			pulled = true
+		}
+		if strings.HasPrefix(line, "docker build") {
+			built = true
+		}
+	}
+	if !pulled {
+		t.Fatalf("the upstream image was not fetched: %v", r.ran)
+	}
+	if built {
+		t.Fatalf("something was built for an image that is mirrored: %v", r.ran)
+	}
+	// And the resource names what this registry serves, pinned by the digest it assigned.
+	image, _ := got.Manifest.Resources[0]["image"].(string)
+	if !strings.Contains(image, "@sha256:") {
+		t.Fatalf("the mirrored image is not pinned by digest: %q", image)
+	}
+	if strings.Contains(image, "17-alpine") {
+		t.Fatalf("the resource still names the upstream tag: %q", image)
+	}
+}
+
+func TestAnUpstreamImageWithNoTagIsRefused(t *testing.T) {
+	// What gets mirrored would be whatever `latest` means today, and a module pinned to that is
+	// not pinned.
+	_, err := catalogue.ParseManifest([]byte(`{"module":"a","version":"1","build":{"artifacts":[
+		{"name":"x","kind":"upstream","from":"postgres"}]}}`))
+	if err == nil {
+		t.Fatal("an untagged upstream reference was accepted")
+	}
+	if !strings.Contains(err.Error(), "no tag or digest") {
+		t.Fatalf("unhelpful refusal: %v", err)
+	}
+}
+
+func TestAnUpstreamReferenceIsNotAPathInTheRepository(t *testing.T) {
+	// The rule that a build reads only its own repository must not refuse every reference with a
+	// registry host in it.
+	if _, err := catalogue.ParseManifest([]byte(`{"module":"a","version":"1","build":{"artifacts":[
+		{"name":"x","kind":"upstream","from":"registry.example/library/postgres:17"}]}}`)); err != nil {
+		t.Fatalf("a perfectly ordinary upstream reference was refused: %v", err)
+	}
+}