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

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

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/406e937d124842b3c268f1d96848b73d8defc05a

Modified Files
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src/backend/utils/mmgr/dsa.c | 24 ++++++++++++++++--------
1 file changed, 16 insertions(+), 8 deletions(-)