GDBus: Add G_DBUS_METHOD_INVOCATION_HANDLED, _UNHANDLED

Like G_SOURCE_REMOVE and G_SOURCE_CONTINUE, these make it clearer what
it means to return TRUE or FALSE.

In particular, in GDBus methods that fail, the failure case still needs
to return TRUE (unlike the typical GError pattern), leading to comments
like this:

    g_dbus_method_invocation_return_error (invocation, ...);
    return TRUE;    /* handled */

which can now be replaced by:

    g_dbus_method_invocation_return_error (invocation, ...);
    return G_DUS_METHOD_INVOCATION_HANDLED;

G_DBUS_METHOD_INVOCATION_UNHANDLED is added for symmetry, but is very
rarely (perhaps never?) useful in practice.

Signed-off-by: Simon McVittie <smcv@collabora.com>

docs/reference/gio/gio-sections-common.txt

@@ -3026,6 +3026,8 @@ g_dbus_method_invocation_return_gerror
 g_dbus_method_invocation_return_dbus_error
 g_dbus_method_invocation_take_error
 g_dbus_method_invocation_return_value_with_unix_fd_list
+G_DBUS_METHOD_INVOCATION_HANDLED
+G_DBUS_METHOD_INVOCATION_UNHANDLED
 <SUBSECTION Standard>
 G_DBUS_METHOD_INVOCATION
 G_IS_DBUS_METHOD_INVOCATION

gio/gdbus-2.0/codegen/codegen.py

@@ -1522,7 +1522,7 @@ class CodeGenerator:
                         '   *\n'
                         '   * If a signal handler returns %%TRUE, it means the signal handler will handle the invocation (e.g. take a reference to @invocation and eventually call %s_complete_%s() or e.g. g_dbus_method_invocation_return_error() on it) and no order signal handlers will run. If no signal handler handles the invocation, the %%G_DBUS_ERROR_UNKNOWN_METHOD error is returned.\n'
                         '   *\n'
-                        '   * Returns: %%TRUE if the invocation was handled, %%FALSE to let other signal handlers run.\n'
+                        '   * Returns: %%G_DBUS_METHOD_INVOCATION_HANDLED or %%TRUE if the invocation was handled, %%G_DBUS_METHOD_INVOCATION_UNHANDLED or %%FALSE to let other signal handlers run.\n'
                         %(i.name, m.name, i.name_lower, m.name_lower), False))
                 self.write_gtkdoc_deprecated_and_since_and_close(m, self.outfile, 2)
                 if m.unix_fd:

gio/gdbusmethodinvocation.h

@@ -33,6 +33,41 @@ G_BEGIN_DECLS
 #define G_DBUS_METHOD_INVOCATION(o)           (G_TYPE_CHECK_INSTANCE_CAST ((o), G_TYPE_DBUS_METHOD_INVOCATION, GDBusMethodInvocation))
 #define G_IS_DBUS_METHOD_INVOCATION(o)        (G_TYPE_CHECK_INSTANCE_TYPE ((o), G_TYPE_DBUS_METHOD_INVOCATION))
 
+/**
+ * G_DBUS_METHOD_INVOCATION_HANDLED:
+ *
+ * The value returned by handlers of the signals generated by
+ * the `gdbus-codegen` tool to indicate that a method call has been
+ * handled by an implementation. It is equal to %TRUE, but using
+ * this macro is sometimes more readable.
+ *
+ * In code that needs to be backwards-compatible with older GLib,
+ * use %TRUE instead, often written like this:
+ *
+ * |[
+ *   g_dbus_method_invocation_return_error (invocation, ...);
+ *   return TRUE;    // handled
+ * ]|
+ *
+ * Since: 2.68
+ */
+#define G_DBUS_METHOD_INVOCATION_HANDLED TRUE GLIB_AVAILABLE_MACRO_IN_2_68
+
+/**
+ * G_DBUS_METHOD_INVOCATION_UNHANDLED:
+ *
+ * The value returned by handlers of the signals generated by
+ * the `gdbus-codegen` tool to indicate that a method call has not been
+ * handled by an implementation. It is equal to %FALSE, but using
+ * this macro is sometimes more readable.
+ *
+ * In code that needs to be backwards-compatible with older GLib,
+ * use %FALSE instead.
+ *
+ * Since: 2.68
+ */
+#define G_DBUS_METHOD_INVOCATION_UNHANDLED FALSE GLIB_AVAILABLE_MACRO_IN_2_68
+
 GLIB_AVAILABLE_IN_ALL
 GType                  g_dbus_method_invocation_get_type             (void) G_GNUC_CONST;
 GLIB_AVAILABLE_IN_ALL