Compare a build with the one the mesh registered, not the newest, which may be one put back (review of #336, hq issue 721)

cmd/mesh-controller/build.go

@@ -899,6 +899,34 @@ type answers struct {
 //
 // Routed through the artifact store as the network reaches it now (novox/hq 04-ISSUES/102): a
 // base is recorded by digest and path, and a build machine needs something it can pull.
+// runningBy is, keyed "<module>/<artifact>", each artifact of the build the mesh has registered for its module, as a
+// builder reaches the artifact store (novox/hq issue 721): what a check compares its builds with. Empty when it cannot
+// be read, said, and then nothing is said unchanged.
+func runningBy(ctx context.Context) map[string]string {
+	open, err := openStores(ctx)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "could not read what this mesh runs, so the check compares nothing: %v\n", err)
+		// empty-on-error: said above; a check given nothing to compare with says no module unchanged, and all move
+		return nil
+	}
+	defer open.Close()
+	running, err := open.inventory.RegisteredHeld(ctx)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "could not read what this mesh runs, so the check compares nothing: %v\n", err)
+		// empty-on-error: said above; a check given nothing to compare with says no module unchanged, and all move
+		return nil
+	}
+	address, err := whereABuilderReachesTheStore(ctx, open.inventory)
+	if err != nil || address == "" {
+		return running
+	}
+	routed := make(map[string]string, len(running))
+	for key, reference := range running {
+		routed[key] = catalogue.Rerouted(reference, address)
+	}
+	return routed
+}
+
 func heldBy(ctx context.Context) map[string]string {
 	open, err := openStores(ctx)
 	if err != nil {