Refuse any change through a verb to a module with a trusted mergeable file

A mergeable file takes any key, not only those its content names, so an
empty runtime configuration a provider reads took a url of the caller's
through the settings verb (hq issue 340 review).

cmd/mesh-controller/modules.go

@@ -1081,6 +1081,16 @@ func throughAVerb() (string, bool) {
 	return verb, verb != ""
 }
 
+// sameLayer says whether two layers hold the same values, an absent layer and an empty one alike.
+func sameLayer(a, b map[string]any) bool {
+	if len(a) == 0 && len(b) == 0 {
+		return true
+	}
+	ra, _ := json.Marshal(a)
+	rb, _ := json.Marshal(b)
+	return string(ra) == string(rb)
+}
+
 // refuseTerminalSettingsThroughAVerb refuses a layer change through a verb that would add, change or remove
 // places or accesses; a change that leaves both as they were is not refused.
 func refuseTerminalSettingsThroughAVerb(ctx context.Context, inv *inventory.Inventory, before, after map[string]any,
@@ -1095,6 +1105,14 @@ func refuseTerminalSettingsThroughAVerb(ctx context.Context, inv *inventory.Inve
 	if err != nil {
 		return fmt.Errorf("which settings of %s are the terminal's cannot be read, so nothing was changed: %w", module, err)
 	}
+	// A trusted mergeable file takes any key, so its module's whole layer is the terminal's (novox/hq issue 340).
+	if files := catalogue.TrustedMergeable(shelf[module]); len(files) > 0 && !sameLayer(before, after) {
+		return fmt.Errorf("the settings of %s on %s are set at the controller's terminal only, never through a verb (this "+
+			"line came through %q): %s merges whatever key a layer sets into a file root or a consumer trusts, so "+
+			"any key could point the module at a listener of the caller's, and whoever may call a verb includes "+
+			"agents (novox/hq issue 340; a file nothing trusts says \"trusted\": false). Nothing was changed",
+			module, where, verb, strings.Join(files, ", "))
+	}
 	for _, key := range catalogue.TerminalKeys(shelf[module]) {
 		was, _ := json.Marshal(before[key])
 		now, _ := json.Marshal(after[key])

cmd/mesh-controller/terminal_settings_test.go

@@ -282,3 +282,65 @@ func TestTheAgentsManagedSettingsAreRefusedThroughAVerb(t *testing.T) {
 		t.Fatalf("reading through the verb: %v", err)
 	}
 }
+
+// A mergeable file takes any key, not only those its content names (novox/hq issue 340): an empty runtime
+// configuration that a provider reads would take a `url` of the caller's, and the provider would send its admin
+// login there. So a module with a mergeable file not marked `"trusted": false` has its whole layer set at the
+// terminal: through a verb, any change is refused, whatever the key.
+func TestAnyKeyOfAModuleWithATrustedMergeableFileIsRefusedThroughAVerb(t *testing.T) {
+	open := aMesh(t)
+	ctx := t.Context()
+	register(t, open, catalogue.Manifest{Module: "keycloak", Version: "1",
+		Resources: []map[string]any{{"id": "runtime-config", "type": "file", "path": "/var/lib/kc/runtime.json",
+			"mode": "0600", "merge": "json", "content": "{}"}}})
+	register(t, open, catalogue.Manifest{Module: "notifier", Version: "1",
+		Resources: []map[string]any{{"id": "look", "type": "file", "path": "/var/lib/notifier/look.json",
+			"mode": "0644", "merge": "json", "trusted": false, "content": "{}"}}})
+	for _, m := range []string{"keycloak", "notifier"} {
+		if _, err := assign(ctx, open, "anchor", m); err != nil {
+			t.Fatal(err)
+		}
+	}
+	layer := func(module string) string {
+		t.Helper()
+		values, _, err := open.inventory.Layer(ctx, "anchor", module)
+		if err != nil {
+			t.Fatal(err)
+		}
+		raw, _ := json.Marshal(values)
+		return string(raw)
+	}
+	refused := func(what string, err error) {
+		t.Helper()
+		if err == nil || !strings.Contains(err.Error(), "controller's terminal") || !strings.Contains(err.Error(), "issue 340") {
+			t.Fatalf("%s: %v", what, err)
+		}
+	}
+	refused("a new url through the verb", throughVerb(t, "settings", map[string]any{"module": "keycloak", "node": "anchor",
+		"values": `{"url":"http://listener.example:8080"}`}))
+	refused("a new url for the mesh through the verb", throughVerb(t, "settings", map[string]any{"module": "keycloak",
+		"values": `{"url":"http://listener.example:8080"}`}))
+	if err := throughVerb(t, "command", map[string]any{"command": `settings set keycloak '{"url":"http://x"}' --node anchor`}); err == nil {
+		t.Fatal("the command verb set a key of a trusted mergeable file")
+	}
+	if got := layer("keycloak"); got != "null" && got != "{}" {
+		t.Fatalf("a refused call kept a layer: %s", got)
+	}
+	if err := atTheTerminal(t, "settings", "set", "keycloak", `{"url":"https://id.example"}`, "--node", "anchor"); err != nil {
+		t.Fatalf("at the terminal: %v", err)
+	}
+	kept := layer("keycloak")
+	refused("changed", throughVerb(t, "settings", map[string]any{"module": "keycloak", "node": "anchor",
+		"values": `{"url":"http://listener.example"}`}))
+	refused("cleared", throughVerb(t, "settings", map[string]any{"module": "keycloak", "node": "anchor", "clear": "true"}))
+	if got := layer("keycloak"); got != kept {
+		t.Fatalf("a refused call changed the layer: %s, was %s", got, kept)
+	}
+	if err := throughVerb(t, "settings", map[string]any{"module": "keycloak", "node": "anchor"}); err != nil {
+		t.Fatalf("reading through the verb: %v", err)
+	}
+	// A mergeable file that says out loud nothing trusts it stays the verb's.
+	if err := throughVerb(t, "settings", map[string]any{"module": "notifier", "node": "anchor", "values": `{"font":13}`}); err != nil {
+		t.Fatalf("a file marked untrusted through the verb: %v", err)
+	}
+}