[glib: 1/2] gmem: Only evaluate pointer argument to g_clear_pointer() once

[Emmanuele Bassi <ebassi src gnome org>]

commit 2e9c31af11b7d2d18052d5bbcdc3611f2f7480f5
Author: Philip Withnall <withnall endlessm com>
Date:   Wed Aug 22 14:47:52 2018 +0100

    gmem: Only evaluate pointer argument to g_clear_pointer() once
    
    The new typeof() macro version of g_clear_pointer() was evaluating its
    pointer argument more than once, meaning any side effects would be
    evaluated multiple times.
    
    The existing (other) macro version of g_clear_pointer() was evaluating
    its argument exactly once. This mismatch could have confused people or
    lead to subtle bugs.
    
    See https://gitlab.gnome.org/GNOME/glib/issues/1494.
    
    Signed-off-by: Philip Withnall <withnall endlessm com>

 glib/gmem.h        |  5 +++--
 glib/tests/utils.c | 27 +++++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)
---
diff --git a/glib/gmem.h b/glib/gmem.h
index 1860d014f..81f8cdde3 100644
--- a/glib/gmem.h
+++ b/glib/gmem.h
@@ -114,8 +114,9 @@ gpointer g_try_realloc_n  (gpointer  mem,
 #define g_clear_pointer(pp, destroy)                                           \
   G_STMT_START {                                                               \
     G_STATIC_ASSERT (sizeof *(pp) == sizeof (gpointer));                       \
-    __typeof__(*(pp)) _ptr = *(pp);                                            \
-    *(pp) = NULL;                                                              \
+    __typeof__((pp)) _pp = (pp);                                               \
+    __typeof__(*(pp)) _ptr = *_pp;                                             \
+    *_pp = NULL;                                                               \
     if (_ptr)                                                                  \
       (destroy) (_ptr);                                                        \
   } G_STMT_END
diff --git a/glib/tests/utils.c b/glib/tests/utils.c
index 95c6138e4..79bc08184 100644
--- a/glib/tests/utils.c
+++ b/glib/tests/utils.c
@@ -533,6 +533,32 @@ test_clear_pointer_cast (void)
   g_assert_null (hash_table);
 }
 
+/* Test that the macro version of g_clear_pointer() only evaluates its argument
+ * once, just like the function version would. */
+static void
+test_clear_pointer_side_effects (void)
+{
+  gchar **my_string_array, **i;
+
+  my_string_array = g_new0 (gchar*, 3);
+  my_string_array[0] = g_strdup ("hello");
+  my_string_array[1] = g_strdup ("there");
+  my_string_array[2] = NULL;
+
+  i = my_string_array;
+
+  g_clear_pointer (i++, g_free);
+
+  g_assert_true (i == &my_string_array[1]);
+  g_assert_null (my_string_array[0]);
+  g_assert_nonnull (my_string_array[1]);
+  g_assert_null (my_string_array[2]);
+
+  g_free (my_string_array[1]);
+  g_free (my_string_array[2]);
+  g_free (my_string_array);
+}
+
 static int obj_count;
 
 static void
@@ -673,6 +699,7 @@ main (int   argc,
   g_test_add_func ("/utils/specialdir/desktop", test_desktop_special_dir);
   g_test_add_func ("/utils/clear-pointer", test_clear_pointer);
   g_test_add_func ("/utils/clear-pointer-cast", test_clear_pointer_cast);
+  g_test_add_func ("/utils/clear-pointer/side-effects", test_clear_pointer_side_effects);
   g_test_add_func ("/utils/take-pointer", test_take_pointer);
   g_test_add_func ("/utils/clear-source", test_clear_source);
   g_test_add_func ("/utils/misc-mem", test_misc_mem);