Fix incorrect sentinel byte logic in GenerationRealloc()

This only affects MEMORY_CONTEXT_CHECKING builds.

This fixes an off-by-one issue in GenerationRealloc() where the
fast-path code which tries to reuse the existing allocation if the
existing chunk is >= the new requested size.  The code there thought it
was always ok to use the existing chunk, but when oldsize == size there
isn't enough space to store the sentinel byte.  If both sizes matched
exactly set_sentinel() would overwrite the first byte beyond the chunk
and then subsequent GenerationRealloc() calls could then fail the
Assert(chunk->requested_size < oldsize) check which is trying to ensure
the chunk is large enough to store the sentinel.

The same issue does not exist in aset.c as the sentinel checking code
only adds a sentinel byte if there's enough space in the chunk.

Reported-by: Alexander Lakhin <[email protected]>
Discussion: https://postgr.es/m/49275921-7b39-41af-5eb8-97b50ce3312e@gmail.com
Backpatch-through: 16, where the problem was introduced by 0e480385e

diff --git a/src/backend/utils/mmgr/generation.c b/src/backend/utils/mmgr/generation.c
index b858b8d0f7b8fd71217e9e6f30cd52e8ba077118..0238c111d2fa9d0f10c2a29f59f6ed9c0b834a76 100644 (file)
--- a/src/backend/utils/mmgr/generation.c
+++ b/src/backend/utils/mmgr/generation.c
@@ -846,8 +846,8 @@ GenerationRealloc(void *pointer, Size size, int flags)
 #endif
 
    /*
-    * Maybe the allocated area already is >= the new size.  (In particular,
-    * we always fall out here if the requested size is a decrease.)
+    * Maybe the allocated area already big enough.  (In particular, we always
+    * fall out here if the requested size is a decrease.)
     *
     * This memory context does not use power-of-2 chunk sizing and instead
     * carves the chunks to be as small as possible, so most repalloc() calls
@@ -855,7 +855,12 @@ GenerationRealloc(void *pointer, Size size, int flags)
     *
     * XXX Perhaps we should annotate this condition with unlikely()?
     */
+#ifdef MEMORY_CONTEXT_CHECKING
+   /* With MEMORY_CONTEXT_CHECKING, we need an extra byte for the sentinel */
+   if (oldsize > size)
+#else
    if (oldsize >= size)
+#endif
    {
 #ifdef MEMORY_CONTEXT_CHECKING
        Size        oldrequest = chunk->requested_size;