Don't call g_variant_store() with NULL

modulemd/meson.build

@@ -365,6 +365,7 @@ c_tests = {
 'service_level'       : [ 'tests/test-modulemd-service-level.c' ],
 'translation'         : [ 'tests/test-modulemd-translation.c' ],
 'translation_entry'   : [ 'tests/test-modulemd-translation-entry.c' ],
+'variant_deep_copy'   : [ 'tests/test-modulemd-variant_deep_copy.c' ],
 'obsoletes'           : [ 'tests/test-modulemd-obsoletes.c' ],
 'quoting'             : [ 'tests/test-modulemd-quoting.c' ],
 }

modulemd/modulemd-util.c

@@ -363,9 +363,13 @@ modulemd_variant_deep_copy (GVariant *variant)
 {
   const GVariantType *data_type = g_variant_get_type (variant);
   gsize data_size = g_variant_get_size (variant);
-  gpointer data = g_malloc0 (data_size);
+  gpointer data = NULL;
 
-  g_variant_store (variant, data);
+  if (data_size > 0)
+    {
+      data = g_malloc0 (data_size);
+      g_variant_store (variant, data);
+    }
 
   return g_variant_ref_sink (g_variant_new_from_data (
     data_type, data, data_size, FALSE, destroy_variant_data, data));

modulemd/tests/test-modulemd-variant_deep_copy.c

@@ -0,0 +1,69 @@
+/*
+ * This file is part of libmodulemd
+ * Copyright (C) 2025 Red Hat, Inc.
+ *
+ * Fedora-License-Identifier: MIT
+ * SPDX-2.0-License-Identifier: MIT
+ * SPDX-3.0-License-Identifier: MIT
+ *
+ * This program is free software.
+ * For more information on the license, see COPYING.
+ * For more information on free software, see <https://www.gnu.org/philosophy/free-sw.en.html>.
+ */
+
+#include <glib.h>
+#include <glib/gprintf.h>
+#include <locale.h>
+#include <stdlib.h>
+
+#include "private/modulemd-util.h"
+
+/*
+ * modulemd_variant_deep_copy() triggered a GLib critical warning
+ * from glib >= 2.84.1 when parsing a /data/xmd modulemd-stream-v2 element
+ * with {} value (an empty flow mapping). This test exhibits that code path
+ * and relies on G_DEBUG=fatal-criticals environment variable to crash the
+ * test.
+ * <https://github.com/fedora-modularity/libmodulemd/issues/623>.
+ */
+static void
+test_empty_a_sv (void)
+{
+  g_autoptr (GVariantDict) dictionary =
+    NULL; /* g_variant_dict_end() does not free */
+  g_autoptr (GVariant) input = NULL;
+  g_autoptr (GVariant) output = NULL;
+
+  /* Build a GVariant with an empty dictionary, results to an "a{sv}" of
+   * zero size. */
+  dictionary = g_variant_dict_new (NULL);
+  input = g_variant_dict_end (dictionary);
+
+  /* Exhibit the library. */
+  output = modulemd_variant_deep_copy (input);
+  g_assert_true (output != NULL);
+
+  /* Compare the content. */
+  g_assert_true (g_variant_get_type (output) == g_variant_get_type (input));
+  g_assert_true (g_variant_get_size (output) == g_variant_get_size (input));
+}
+
+
+int
+main (int argc, char *argv[])
+{
+  setlocale (LC_ALL, "");
+  g_test_init (&argc, &argv, NULL);
+  g_test_set_nonfatal_assertions ();
+
+  if (!g_setenv ("G_DEBUG", "fatal-criticals", TRUE))
+    {
+      g_fprintf (stderr, "Failed to set G_DEBUG environment variable.\n");
+      exit (EXIT_FAILURE);
+    }
+
+  g_test_add_func ("/modulemd/util/variant_deep_copy/empty_a{sv}",
+                   test_empty_a_sv);
+
+  return g_test_run ();
+}