pgsql: Tolerate ENOSYS failure from sync_file_range().

Tolerate ENOSYS failure from sync_file_range().

One unintended consequence of commit 9ccdd7f6 was that Windows WSL
users started getting a panic whenever we tried to initiate data
flushing with sync_file_range(), because WSL does not implement that
system call. Previously, they got a stream of periodic warnings,
which was also undesirable but at least ignorable.

Prevent the panic by handling ENOSYS specially and skipping the panic
promotion with data_sync_elevel(). Also suppress future attempts
after the first such failure so that the pre-existing problem of
noisy warnings is improved.

Back-patch to 9.6 (older branches were not affected in this way by
9ccdd7f6).

Author: Thomas Munro and James Sewell
Tested-by: James Sewell
Reported-by: Bruce Klein
Discussion: https://postgr.es/m/CA+mCpegfOUph2U4ZADtQT16dfbkjjYNJL1bSTWErsazaFjQW9A@mail.gmail.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/64b8c51bccad13462a8876f269c746e24e9068fd

Modified Files
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src/backend/storage/file/fd.c | 21 ++++++++++++++++++++-
1 file changed, 20 insertions(+), 1 deletion(-)