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Fix inconsistent out-of-memory error reporting in dsa.c. 

Commit 16be2fd1 introduced the flag DSA_ALLOC_NO_OOM to control whether
the DSA allocator would raise an error or return InvalidDsaPointer on
failure to allocate.  One edge case was not handled correctly: if we
fail to allocate an internal "span" object for a large allocation, we
would always return InvalidDsaPointer regardless of the flag; a caller
not expecting that could then dereference a null pointer.

This is a plausible explanation for a one-off report of a segfault.

Remove a redundant pair of braces so that all three stanzas that handle
DSA_ALLOC_NO_OOM match in style, for visual consistency.

While fixing inconsistencies, if FreePageManagerGet() can't supply the
pages that our book-keeping says it should be able to supply, then we
should always report a FATAL error.  Previously we treated that as a
regular allocation failure in one code path, but as a FATAL condition
in another.

Back-patch to 10, where dsa.c landed.

Author: Thomas Munro
Reported-by: Jakub Glapa
Discussion: https://postgr.es/m/CAEepm=2oPqXxyWQ-1o60tpOLrwkw=VpgNXqqF1VN2EyO9zKGQw@mail.gmail.com
This commit is contained in:
Thomas Munro 2019-02-25 10:54:12 +13:00
parent 9e138a401d
commit 29ddb548f6
1 changed files with 16 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
src/backend/utils/mmgr/dsa.c
View File

@ -693,7 +693,16 @@ dsa_allocate_extended(dsa_area *area, size_t size, int flags)
/* Obtain a span object. */
span_pointer = alloc_object(area, DSA_SCLASS_BLOCK_OF_SPANS);
if (!DsaPointerIsValid(span_pointer))
{
/* Raise error unless asked not to. */
if ((flags & DSA_ALLOC_NO_OOM) == 0)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of memory"),
errdetail("Failed on DSA request of size %zu.",
size)));
return InvalidDsaPointer;
}
LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
@ -790,12 +799,10 @@ dsa_allocate_extended(dsa_area *area, size_t size, int flags)
{
/* Raise error unless asked not to. */
if ((flags & DSA_ALLOC_NO_OOM) == 0)
{
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of memory"),
errdetail("Failed on DSA request of size %zu.", size)));
}
return InvalidDsaPointer;
}
@ -1669,13 +1676,14 @@ ensure_active_superblock(dsa_area *area, dsa_area_pool *pool,
return false;
}
}
/*
* This shouldn't happen: get_best_segment() or make_new_segment()
* promised that we can successfully allocate npages.
*/
if (!FreePageManagerGet(segment_map->fpm, npages, &first_page))
{
LWLockRelease(DSA_AREA_LOCK(area));
if (size_class != DSA_SCLASS_BLOCK_OF_SPANS)
dsa_free(area, span_pointer);
return false;
}
elog(FATAL,
"dsa_allocate could not find %zu free pages for superblock",
npages);
LWLockRelease(DSA_AREA_LOCK(area));
/* Compute the start of the superblock. */