pgsql: Remove redundant unlink() in KeepFileRestoredFromArchive() for n

Remove redundant unlink() in KeepFileRestoredFromArchive() for non-WIN32

KeepFileRestoredFromArchive() called unlink() on an existing WAL segment
before durable_rename() replaced it. rename(2) atomically replaces a
target on POSIX systems, and the extra unlink() created a small window
where it would be possible for a concurrent WAL sender to see a segment
as missing.

Note that the window still exists on WIN32, as we lack a safe concurrent
alternative. Perhaps something like ReplaceFile() could be looked at
for this purpose.

This problem is hard to reach in practice, so no backpatch is done. I
also tend to be conservative regarding recovery changes, even small, in
stable branches.

Author: Stepan Neretin <slpmcf(at)gmail(dot)com>
Reviewed-by: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
Reviewed-by: Neil Chen <carpenter(dot)nail(dot)cz(at)gmail(dot)com>
Discussion: https://postgr.es/m/CA+Yyo5S0C25jS_pRWQiGgoiy+X=GyJS7ZJkEq=_SW1WWpkL3Sg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1200dfd60c367e97a5e1f31100d82072eb0178b6

Modified Files
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src/backend/access/transam/xlogarchive.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)