Fix missing MCXT_ALLOC_NO_OOM handling in MemoryContextAllocAligned

Fix missing NULL check in MemoryContextAllocAligned().  The underlying
call to MemoryContextAllocExtended() could return NULL when
flags contains MCXT_ALLOC_NO_OOM and the underlying malloc fails.

There are no current callers using MemoryContextAllocAligned() that pass
the MCXT_ALLOC_NO_OOM in core, so no live bug fix in core here. However,
an extension might use this pattern, so we'd better fix.

Fix this so we correctly pass the NULL to the caller rather than trying
to write to a NULL memory address.

This also fixes the same bug in AlignedAllocRealloc(), which is also
unused in core.

Backpatch to v16, where these functions first appeared.

Author: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/07DAC4C3-120D-4F3C-8FEE-BA236F7E9C1D@gmail.com
Backpatch-through: 16

diff --git a/src/backend/utils/mmgr/mcxt.c b/src/backend/utils/mmgr/mcxt.c
index 930fc4573281188bd46299f981f80be4520fa2ae..594c7a93bbad5d605418853b854e3b5dc17db4c8 100644 (file)
--- a/src/backend/utils/mmgr/mcxt.c
+++ b/src/backend/utils/mmgr/mcxt.c
@@ -1541,6 +1541,13 @@ MemoryContextAllocAligned(MemoryContext context,
        unaligned = MemoryContextAllocExtended(context, alloc_size,
                                                                                   flags & ~MCXT_ALLOC_ZERO);
 
+       if (unlikely(unaligned == NULL))
+       {
+               /* NULL can be returned only when using MCXT_ALLOC_NO_OOM */
+               Assert(flags & MCXT_ALLOC_NO_OOM);
+               return NULL;
+       }
+
        /* compute the aligned pointer */
        aligned = (void *) TYPEALIGN(alignto, (char *) unaligned +
                                                                 sizeof(MemoryChunk));