pgsql: Make on-access pruning pin visibility map before locking heap pa

Make on-access pruning pin visibility map before locking heap page

b46e1e54d078 pinned the VM after acquiring a cleanup lock on the heap
page when on-access pruning. This was not correct, as pinning the VM may
require I/O. Pin the VM before acquiring the lock. This could mean
occasional unneeded pinning when the buffer is under contention, but
that should be rare.

Reported-by: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAAKRu_aR6e2mNVo-DNHDUfBE5i0cbE4%3Du%3D_Qi9%2Bv573qiGtBig%40mail.gmail.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/512d3e8919b649d36a6f246c657c8b8df7536e8a

Modified Files
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src/backend/access/heap/pruneheap.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)