pgsql: Don't ERROR on PreallocXlogFiles() race condition.

Don't ERROR on PreallocXlogFiles() race condition.

Before a restartpoint finishes PreallocXlogFiles(), a startup process
KeepFileRestoredFromArchive() call can unlink the preallocated segment.
If a CHECKPOINT sql command had elicited the restartpoint experiencing
the race condition, that sql command failed.  Moreover, the restartpoint
omitted its log_checkpoints message and some inessential resource
reclamation.  Prevent the ERROR by skipping open() of the segment.
Since these consequences are so minor, no back-patch.

Discussion: https://postgr.es/m/20210202151416.GB3304930@rfd.leadboat.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2b3e4672f7602a6bb46a0735a1b41b635508b290

Modified Files
--------------
src/backend/access/transam/xlog.c     | 79 +++++++++++++++++++++++++----------
src/backend/replication/walreceiver.c |  4 +-
src/include/access/xlog.h             |  2 +-
3 files changed, 58 insertions(+), 27 deletions(-)