GVariant: Add a G_VARIANT_DICT_INIT macro

The macro could be used at initialization time to avoid having an
unitialized dict, especially with g_auto variables.

The macro tries to be a bit more type-safe by making sure that the asv
parameter is actually "GVariant *".

https://bugzilla.gnome.org/show_bug.cgi?id=766370

glib/gvariant.c

@@ -3796,11 +3796,36 @@ struct heap_dict
 #define GVSD(d)                 ((struct stack_dict *) (d))
 #define GVHD(d)                 ((struct heap_dict *) (d))
 #define GVSD_MAGIC              ((gsize) 2579507750u)
+#define GVSD_MAGIC_PARTIAL      ((gsize) 3488698669u)
 #define GVHD_MAGIC              ((gsize) 2450270775u)
 #define is_valid_dict(d)        (d != NULL && \
                                  GVSD(d)->magic == GVSD_MAGIC)
 #define is_valid_heap_dict(d)   (GVHD(d)->magic == GVHD_MAGIC)
 
+/* Just to make sure that by adding a union to GVariantDict, we didn't
+ * accidentally change ABI. */
+G_STATIC_ASSERT (sizeof (GVariantDict) == sizeof (gsize[16]));
+
+static gboolean
+ensure_valid_dict (GVariantDict *dict)
+{
+  if (is_valid_dict (dict))
+    return TRUE;
+  if (dict->partial_magic == GVSD_MAGIC_PARTIAL)
+    {
+      static GVariantDict cleared_dict;
+
+      /* Make sure that only first two fields were set and the rest is
+       * zeroed to avoid messing up the builder that had parent
+       * address equal to GVSB_MAGIC_PARTIAL. */
+      if (memcmp (cleared_dict.y, dict->y, sizeof cleared_dict.y))
+        return FALSE;
+
+      g_variant_dict_init (dict, dict->asv);
+    }
+  return is_valid_dict (dict);
+}
+
 /**
  * g_variant_dict_new:
  * @from_asv: (allow-none): the #GVariant with which to initialise the
@@ -3908,7 +3933,7 @@ g_variant_dict_lookup (GVariantDict *dict,
   GVariant *value;
   va_list ap;
 
-  g_return_val_if_fail (is_valid_dict (dict), FALSE);
+  g_return_val_if_fail (ensure_valid_dict (dict), FALSE);
   g_return_val_if_fail (key != NULL, FALSE);
   g_return_val_if_fail (format_string != NULL, FALSE);
 
@@ -3953,7 +3978,7 @@ g_variant_dict_lookup_value (GVariantDict       *dict,
 {
   GVariant *result;
 
-  g_return_val_if_fail (is_valid_dict (dict), NULL);
+  g_return_val_if_fail (ensure_valid_dict (dict), NULL);
   g_return_val_if_fail (key != NULL, NULL);
 
   result = g_hash_table_lookup (GVSD(dict)->values, key);
@@ -3979,7 +4004,7 @@ gboolean
 g_variant_dict_contains (GVariantDict *dict,
                          const gchar  *key)
 {
-  g_return_val_if_fail (is_valid_dict (dict), FALSE);
+  g_return_val_if_fail (ensure_valid_dict (dict), FALSE);
   g_return_val_if_fail (key != NULL, FALSE);
 
   return g_hash_table_contains (GVSD(dict)->values, key);
@@ -4007,7 +4032,7 @@ g_variant_dict_insert (GVariantDict *dict,
 {
   va_list ap;
 
-  g_return_if_fail (is_valid_dict (dict));
+  g_return_if_fail (ensure_valid_dict (dict));
   g_return_if_fail (key != NULL);
   g_return_if_fail (format_string != NULL);
 
@@ -4033,7 +4058,7 @@ g_variant_dict_insert_value (GVariantDict *dict,
                              const gchar  *key,
                              GVariant     *value)
 {
-  g_return_if_fail (is_valid_dict (dict));
+  g_return_if_fail (ensure_valid_dict (dict));
   g_return_if_fail (key != NULL);
   g_return_if_fail (value != NULL);
 
@@ -4055,7 +4080,7 @@ gboolean
 g_variant_dict_remove (GVariantDict *dict,
                        const gchar  *key)
 {
-  g_return_val_if_fail (is_valid_dict (dict), FALSE);
+  g_return_val_if_fail (ensure_valid_dict (dict), FALSE);
   g_return_val_if_fail (key != NULL, FALSE);
 
   return g_hash_table_remove (GVSD(dict)->values, key);
@@ -4089,7 +4114,7 @@ g_variant_dict_clear (GVariantDict *dict)
     /* all-zeros case */
     return;
 
-  g_return_if_fail (is_valid_dict (dict));
+  g_return_if_fail (ensure_valid_dict (dict));
 
   g_hash_table_unref (GVSD(dict)->values);
   GVSD(dict)->values = NULL;
@@ -4120,7 +4145,7 @@ g_variant_dict_end (GVariantDict *dict)
   GHashTableIter iter;
   gpointer key, value;
 
-  g_return_val_if_fail (is_valid_dict (dict), NULL);
+  g_return_val_if_fail (ensure_valid_dict (dict), NULL);
 
   g_variant_builder_init (&builder, G_VARIANT_TYPE_VARDICT);
 

glib/gvariant.h

@@ -436,9 +436,46 @@ gint                            g_variant_compare                       (gconstp
 typedef struct _GVariantDict GVariantDict;
 struct _GVariantDict {
   /*< private >*/
-  gsize x[16];
+  union
+  {
+    struct {
+      GVariant *asv;
+      gsize partial_magic;
+      gsize y[14];
+    };
+    gsize x[16];
+  };
 };
 
+/**
+ * G_VARIANT_DICT_INIT:
+ * @asv: (nullable): a GVariant*
+ *
+ * A stack-allocated #GVariantDict must be initialized if it is used
+ * together with g_auto() to avoid warnings or crashes if function
+ * returns before g_variant_dict_init() is called on the builder.
+ * This macro can be used as initializer instead of an explicit
+ * zeroing a variable when declaring it and a following
+ * g_variant_dict_init(), but it cannot be assigned to a variable.
+ *
+ * The passed @asv has to live long enough for #GVariantDict to gather
+ * the entries from, as the gathering does not happen in the
+ * G_VARIANT_DICT_INIT() call, but rather in functions that make sure
+ * that #GVariantDict is valid.  In context where the initialization
+ * value has to be a constant expression, the only possible value of
+ * @asv is %NULL.  It is still possible to call g_variant_dict_init()
+ * safely with a different @asv right after the variable was
+ * initialized with G_VARIANT_DICT_INIT().
+ *
+ * |[
+ *   g_autoptr(GVariant) variant = get_asv_variant ();
+ *   g_auto(GVariantDict) dict = G_VARIANT_DICT_INIT (variant);
+ * ]|
+ *
+ * Since: 2.50
+ */
+#define G_VARIANT_DICT_INIT(asv) { { { asv, 3488698669u, { 0, } } } }
+
 GLIB_AVAILABLE_IN_2_40
 GVariantDict *                  g_variant_dict_new                      (GVariant             *from_asv);
 

glib/tests/gvariant.c

@@ -4554,6 +4554,51 @@ test_stack_builder_init (void)
   g_variant_unref (variant);
 }
 
+static GVariant *
+get_asv (void)
+{
+  GVariantBuilder builder = G_VARIANT_BUILDER_INIT (G_VARIANT_TYPE_VARDICT);
+
+  g_variant_builder_add (&builder, "{s@v}", "foo", g_variant_new_variant (g_variant_new_string ("FOO")));
+  g_variant_builder_add (&builder, "{s@v}", "bar", g_variant_new_variant (g_variant_new_string ("BAR")));
+
+  return g_variant_ref_sink (g_variant_builder_end (&builder));
+}
+
+static void
+test_stack_dict_init (void)
+{
+  GVariant *asv = get_asv ();
+  GVariantDict dict = G_VARIANT_DICT_INIT (asv);
+  GVariant *variant;
+  GVariantIter iter;
+  gchar *key;
+  GVariant *value;
+
+  g_variant_dict_insert_value (&dict, "baz", g_variant_new_string ("BAZ"));
+  g_variant_dict_insert_value (&dict, "quux", g_variant_new_string ("QUUX"));
+
+  variant = g_variant_ref_sink (g_variant_dict_end (&dict));
+  g_assert_nonnull (variant);
+  g_assert (g_variant_type_equal (g_variant_get_type (variant),
+                                  G_VARIANT_TYPE_VARDICT));
+  g_assert_cmpuint (g_variant_n_children (variant), ==, 4);
+
+  g_variant_iter_init (&iter, variant);
+  while (g_variant_iter_next (&iter, "{sv}", &key, &value))
+    {
+      gchar *strup = g_ascii_strup (key, -1);
+
+      g_assert_cmpstr (strup, ==, g_variant_get_string (value, NULL));
+      g_free (key);
+      g_free (strup);
+      g_variant_unref (value);
+    }
+
+  g_variant_unref (asv);
+  g_variant_unref (variant);
+}
+
 int
 main (int argc, char **argv)
 {
@@ -4614,5 +4659,6 @@ main (int argc, char **argv)
   g_test_add_func ("/gvariant/error-quark", test_error_quark);
 
   g_test_add_func ("/gvariant/stack-builder-init", test_stack_builder_init);
+  g_test_add_func ("/gvariant/stack-dict-init", test_stack_dict_init);
   return g_test_run ();
 }