pgsql: bufmgr: Fix ordering of checks in PinBuffer()

bufmgr: Fix ordering of checks in PinBuffer()

The check for skip_if_not_valid added in 819dc118c0f6 was put at the start of
the loop. A CAS loop in theory does allow to make that check in a race free
manner. However, just after the check, there's a
    old_buf_state = WaitBufHdrUnlocked(buf);
which introduces a race, because it would allow BM_VALID to be cleared, after
the skip_if_not_valid check.

Fix by restarting the loop after WaitBufHdrUnlocked().

Reported-by: Yura Sokolov <y.sokolov@postgrespro.ru>
Discussion: https://postgr.es/m/5bf667f3-5270-4b19-a08f-0facbecdff68@postgrespro.ru

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c0af4eb4e71ecd4826ae6c9985f1d10ace5e1be6

Modified Files
--------------
src/backend/storage/buffer/bufmgr.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)