[mutter] background-content: Fix pipeline cache size

[Marge Bot <marge-bot src gnome org>]

commit 3bbfaa03b3944a856540eaca00f76f3975e09cfd
Author: Jonas Ådahl <jadahl gmail com>
Date:   Wed Feb 24 16:39:42 2021 +0100

    background-content: Fix pipeline cache size
    
    The cache had the size 9, which was "big enough" in the past, but when
    more ways pipelines could be constructed, the size was not enough. The
    need to increase the cache size was hard to spot though, since adding
    pipeline flag didn't give any hints about the cache being directly tied
    to these flag values.
    
    So, when enough flag bits were set when attempting to retrieve and put a
    pipeline in the cache, it'd instead overwrite some arbitrary stack
    memory, which would sooner or later result in a memory corruption
    induced crash. Valgrind could not detect this particular memory
    corruption, as it messed up stack memory, not e.g. freed heap memory, so
    it instead got confused and thought plain stack values were unreadable.
    
    Fix these two issues by making the cache size the combination of all
    pipeline flags + 1, so that we can safely put any flag combination in
    the cache.
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1747>

 src/compositor/meta-background-content.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)
---
diff --git a/src/compositor/meta-background-content.c b/src/compositor/meta-background-content.c
index 5031c89694..73db454407 100644
--- a/src/compositor/meta-background-content.c
+++ b/src/compositor/meta-background-content.c
@@ -218,6 +218,11 @@ typedef enum
   PIPELINE_BLEND = (1 << 1),
   PIPELINE_GRADIENT = (1 << 2),
   PIPELINE_ROUNDED_CLIP = (1 << 3),
+
+  PIPELINE_ALL = (PIPELINE_VIGNETTE |
+                  PIPELINE_BLEND |
+                  PIPELINE_GRADIENT |
+                  PIPELINE_ROUNDED_CLIP)
 } PipelineFlags;
 
 struct _MetaBackgroundContent
@@ -324,9 +329,11 @@ on_background_changed (MetaBackground        *background,
 static CoglPipeline *
 make_pipeline (PipelineFlags pipeline_flags)
 {
-  static CoglPipeline *templates[9];
+  static CoglPipeline *templates[PIPELINE_ALL + 1];
   CoglPipeline **templatep;
 
+  g_assert (pipeline_flags < G_N_ELEMENTS (templates));
+
   templatep = &templates[pipeline_flags];
   if (*templatep == NULL)
     {