pgsql: When WalSndCaughtUp, sleep only in WalSndWaitForWal().

When WalSndCaughtUp, sleep only in WalSndWaitForWal().

Before sleeping, WalSndWaitForWal() sends a keepalive if MyWalSnd->write
< sentPtr. That is important in logical replication. When the latest
physical LSN yields no logical replication messages (a common case),
that keepalive elicits a reply, and processing the reply updates
pg_stat_replication.replay_lsn. WalSndLoop() lacks that; when
WalSndLoop() slept, replay_lsn advancement could stall until
wal_receiver_status_interval elapsed. This sometimes stalled
src/test/subscription/t/001_rep_changes.pl for up to 10s.

Discussion: https://postgr.es/m/20200406063649.GA3738151@rfd.leadboat.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/421685812290406daea58b78dfab0346eb683bbb

Modified Files
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src/backend/replication/walsender.c | 21 ++++++++-------------
1 file changed, 8 insertions(+), 13 deletions(-)