pgsql: In caught-up logical walsender, sleep only in WalSndWaitForWal()

In caught-up logical walsender, sleep only in WalSndWaitForWal().

Before sleeping, WalSndWaitForWal() sends a keepalive if MyWalSnd->write
< sentPtr. When the latest physical LSN yields no logical replication
messages (a common case), that keepalive elicits a reply. 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.

Reviewed by Fujii Masao and Michael Paquier.

Discussion: https://postgr.es/m/20200418070142.GA1075445@rfd.leadboat.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f246ea3b2a5e0b75e44f0f18157c4b5e10b5547f

Modified Files
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src/backend/replication/walsender.c | 20 +++++++++++---------
1 file changed, 11 insertions(+), 9 deletions(-)