pgsql: Improve replication lag interpolation after idle period

Improve replication lag interpolation after idle period

After sitting idle and fully replayed for a while and then encountering
a new burst of WAL activity, we interpolate between an ancient sample and the
not-yet-reached one for the new traffic. That produced a corner case report
of lag after receiving first new reply from standby, which might sometimes
be a large spike.

Correct this by resetting last_read time and handle that new case.

Author: Thomas Munro

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9ea3c64124af039219aa5030d7af675dce5daa60

Modified Files
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src/backend/replication/walsender.c | 29 +++++++++++++++++++++++++----
1 file changed, 25 insertions(+), 4 deletions(-)