pgsql: Recompute allocChunkLimit when reusing an AllocSet context

Recompute allocChunkLimit when reusing an AllocSet context

AllocSet contexts with different maxBlockSize values can have the same
freeListIndex. When reusing a context from one of the context_freelists,
we updated maxBlockSize, but we failed to adjust allocChunkLimit, even
though that ordinarily depends on maxBlockSize. This could result in
some allocations being put on an external block when they shouldn't have
been, resulting in excessive malloc() calls, or if the allocChunkLimit was
left set higher than it should be, allocations could have resulted in
memory not being free()'d after pfree() due to the chunk having been put
on a multi-chunk block (these would still later be cleaned up when the
context is reset, so no memory leaks).

The repercussions of this are only inefficient use of memory and perhaps
some performance instability depending on which context happened to be
top of the freelist when a new AllocSet context was created.

To fix, we move the allocChunkLimit calculation code into a helper
function and call that to reset the allocChunkLimit when reusing a
context.

Author: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Donghang Lin <donghanglin(at)gmail(dot)com>
Reviewed-by: David Rowley <dgrowleyml(at)gmail(dot)com>
Discussion: https://postgr.es/m/1D54961E-6BEF-411A-BFAD-610C3B18D7E7@gmail.com
Backpatch-through: 14

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/440ee5bb1ddc8b23794200720efba17b8a4a44d1

Modified Files
--------------
src/backend/utils/mmgr/aset.c | 77 +++++++++++++++++++++++++++++--------------
1 file changed, 52 insertions(+), 25 deletions(-)