pgsql: Assert correct VM page passed to pruning

Assert correct VM page passed to pruning

Before pruning a heap page, we get the current status of the
corresponding VM page. If the passed in vmbuffer isn't the right one,
visibilitymap_get_status() will silently unpin it and pin the correct
page. Pruning assumes the caller manages the vmbuffer lifecycle, so this
would leave the caller with a stale VM reference and would leak the new
VM pin. To avoid mistakes in development, assert that the correct VM
page is pinned before beginning.

Reported-by: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-by: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
Discussion: https://postgr.es/m/CAAKRu_amj7qLF4c=9ijd=708Fu2G8gg-2EqwBu=aCdAHU2sPHg@mail.gmail.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d8516db39dfa73042e3ba28f9efc78da4dd6792d

Modified Files
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src/backend/access/heap/pruneheap.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)