pgsql: Make dsa_allocate interface more like MemoryContextAlloc.

Make dsa_allocate interface more like MemoryContextAlloc.

A new function dsa_allocate_extended now takes flags which indicate
that huge allocations should be permitted, that out-of-memory
conditions should not throw an error, and/or that the returned memory
should be zero-filled, just like MemoryContextAllocateExtended.

Commit 9acb85597f1223ac26a5b19a9345849c43d0ff54, which added
dsa_allocate0, was broken because it failed to account for the
possibility that dsa_allocate() might return InvalidDsaPointer.
This fixes that problem along the way.

Thomas Munro, with some comment changes by me.

Discussion: http://postgr.es/m/CA+Tgmobt7CcF_uQP2UQwWmu4K9qCHehMJP9_9m1urwP8hbOeHQ@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/16be2fd100199bdf284becfcee02c5eb20d8a11d

Modified Files
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src/backend/utils/mmgr/dsa.c | 76 ++++++++++++++++++++++++++++++++------------
src/include/utils/dsa.h | 16 ++++++++--
2 files changed, 69 insertions(+), 23 deletions(-)