pgsql: Only clear VACUUM's read stream strategy once in failsafe mode

Only clear VACUUM's read stream strategy once in failsafe mode

112c2683807b4d690 restored failsafe vacuum's abandonment of a buffer
access strategy by clearing the ReadBuffersOperations' strategy
references. But it did so in lazy_scan_heap()'s main loop, meaning it
looped through all the ReadBuffersOperations once per block after
failsafe was engaged. Track it with a local flag and clear the strategy
only once.

Reported-by: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAAKRu_Zse14nSNeCgtnE1LUAH8Of7OmYR%2BCc3O_DAzxt3m6T-g%40mail.gmail.com
Backpatch-through: 18

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/69800557ca32c493853b937916789f0f3da88efe

Modified Files
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src/backend/access/heap/vacuumlazy.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)