pgsql: Fix read_relmap_file() concurrency on Windows.

Fix read_relmap_file() concurrency on Windows.

Commit d8cd0c6c95c0120168df93aae095df4e0682a08a introduced a file
rename that could fail on Windows, probably due to other backends
having an open file handle to the old file of the same name.
Re-arrange the locking slightly to prevent that, by making sure the
open() and close() run while we hold the lock.

Thomas Munro. I added an explanatory comment.

Discussion: https://postgr.es/m/CA%2BhUKGLZtCTgp4NTWV-wGbR2Nyag71%3DEfYTKjDKnk%2BfkhuFMHw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a2e97cb2b6fb64c3ca3198f5c5f31190bc14c703

Modified Files
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src/backend/utils/cache/relmapper.c | 34 +++++++++++++++++++++-------------
1 file changed, 21 insertions(+), 13 deletions(-)