pgsql: Avoid leaking memory during large-scale REASSIGN OWNED BY operat

Avoid leaking memory during large-scale REASSIGN OWNED BY operations.

The various ALTER OWNER routines tend to leak memory in
CurrentMemoryContext. That's not a problem when they're only called
once per command; but in this usage where we might be touching many
objects, it can amount to a serious memory leak. Fix that by running
each call in a short-lived context.

(DROP OWNED BY likely has a similar issue, except that you'll probably
run out of lock table space before noticing. REASSIGN is worth fixing
since for most non-table object types, it won't take any lock.)

Back-patch to all supported branches. Unfortunately, in the back
branches this helps to only a limited extent, since the sinval message
queue bloats quite a lot in this usage before commit 3aafc030a,
consuming memory more or less comparable to what's actually leaked.
Still, it's clearly a leak with a simple fix, so we might as well fix it.

Justin Pryzby, per report from Guillaume Lelarge

Discussion: https://postgr.es/m/CAECtzeW2DAoioEGBRjR=CzHP6TdL=yosGku8qZxfX9hhtrBB0Q@mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5cf08b4db79d6e3a82c4b92f8c9cf35e87c70d01

Modified Files
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src/backend/catalog/pg_shdepend.c | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)