[glib/mcatanzaro/#2076: 15/16] gatomicarray: suppress valgrind memory leak warnings

[Michael Catanzaro <mcatanzaro src gnome org>]

commit a412ffe4d3b3775c00703cf31dd302b1200fd0e5
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Mon Nov 23 15:51:16 2020 -0600

    gatomicarray: suppress valgrind memory leak warnings
    
    The problem occurs because we keep a pointer inside the allocated block,
    instead of a pointer to the start of the block:
    
    ```
    ==180238== 16 bytes in 1 blocks are possibly lost in loss record 3,086 of 16,075
    ==180238==    at 0x483980B: malloc (vg_replace_malloc.c:309)
    ==180238==    by 0x548942C: g_malloc (gmem.c:102)
    ==180238==    by 0x54A4748: g_slice_alloc (gslice.c:1025)
    ==180238==    by 0x53D0AAF: freelist_alloc (gatomicarray.c:77)
    ==180238==    by 0x53D0B85: _g_atomic_array_copy (gatomicarray.c:133)
    ==180238==    by 0x53F8E6D: iface_node_set_offset_L (gtype.c:1347)
    ==180238==    by 0x53F91F1: type_node_add_iface_entry_W (gtype.c:1444)
    ==180238==    by 0x53F93DF: type_add_interface_Wm (gtype.c:1477)
    ==180238==    by 0x53FC946: g_type_add_interface_static (gtype.c:2852)
    ==180238==    by 0x4A3D53A: gtk_menu_shell_accessible_get_type_once (gtkmenushellaccessible.c:26)
    ==180238==    by 0x4A3D495: gtk_menu_shell_accessible_get_type (gtkmenushellaccessible.c:26)
    ==180238==    by 0x4C8AC44: gtk_menu_shell_class_init (gtkmenushell.c:424)
    ```
    
    Note we cannot use VALGRIND_FREELIKE_BLOCK() in freelist_free() because we
    have not actually freed the FreeListNode and need to dereference it in
    freelist_alloc() to decide whether to reuse the block. That would result
    in a use-after-free warning before we would get a chance to call
    VALGRIND_MALLOCLIKE_BLOCK() in the reuse path.
    
    Also note that this free list only ever grows: it never shrinks for the
    lifetime of the application, so nothing here will ever be truely freed,
    although unused elements are eligible for reuse.
    
    Fix suggested by Philip Withnall
    
    Related: #2076

 gobject/gatomicarray.c | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/gobject/gatomicarray.c b/gobject/gatomicarray.c
index 5ebf6b89b..43111e8c7 100644
--- a/gobject/gatomicarray.c
+++ b/gobject/gatomicarray.c
@@ -17,6 +17,7 @@
 
 #include "config.h"
 
+#include "../glib/gvalgrind.h"
 #include <string.h>
 
 #include "gatomicarray.h"
@@ -77,6 +78,11 @@ freelist_alloc (gsize size, gboolean reuse)
   mem = g_slice_alloc (real_size);
   mem = ((char *) mem) + sizeof (gsize);
   G_ATOMIC_ARRAY_DATA_SIZE (mem) = size;
+
+#if ENABLE_VALGRIND
+  VALGRIND_MALLOCLIKE_BLOCK (mem, real_size - sizeof (gsize), FALSE, FALSE);
+#endif
+
   return mem;
 }