Put back on a failed gate only a build of the module's own repository

A build refused for its repository is still recorded, and a fork carries the
commit the module was registered at: the rollback's search for the previous
build would have found it and registered it by the back door.

cmd/mesh-controller/build_source_test.go

@@ -184,3 +184,39 @@ func asTheOperator(t *testing.T, inv *inventory.Inventory, b link.BuildResult) l
 	}
 	return b
 }
+
+// A rollback puts back only a build of the module's own repository: an agent's build of the module's name,
+// recorded and refused, at the very commit the machine ran before (a fork carries it), is never registered by
+// the back door of a failed gate.
+func TestARollbackNeverPutsBackABuildFromAnotherRepository(t *testing.T) {
+	open := theCatalogue(t)
+	ctx := t.Context()
+	inv := open.inventory
+	fork := onTrunk("build-1791500000000000000", "agent/mesh-catalog", "git", "modules/sudo",
+		map[string]any{"module": "sudo", "version": "evil"})
+	fork.Commit = "c0ffee0123456789" // the commit sudo was registered at
+	keptAsked(t, inv, fork.ID, "agent/mesh-catalog", false)
+	if _, _, err := takeIn(ctx, inv, fork); !errors.Is(err, errNotItsSource) {
+		t.Fatalf("the fork's build was taken in: %v", err)
+	}
+	failed := onTrunk("build-1791600000000000000", "novox/mesh-catalog", "git", "modules/sudo",
+		map[string]any{"module": "sudo", "version": "2"})
+	failed.Commit = "badbadbad0123456"
+	if _, _, err := takeIn(ctx, inv, failed); err != nil {
+		t.Fatal(err)
+	}
+	record, _, err := inv.BuildByID(ctx, failed.ID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	previous, found, err := inv.PreviousBuild(ctx, "sudo", "c0ffee0123456789", record)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if found && previous.ID == fork.ID {
+		t.Fatalf("a rollback would put back the fork's build %s", previous.ID)
+	}
+	if !found || previous.ID != "build-sudo" {
+		t.Fatalf("a rollback puts back %q (found %v), want the registered build-sudo", previous.ID, found)
+	}
+}