Some problems about cascading replication

Hi,

When I tested the PITR on git master with max_wal_senders > 0,
I found that the following inappropriate log meesage was always
output even though cascading replication is not in progress. Attached
patch fixes this problem.

LOG: terminating all walsender processes to force cascaded
standby(s) to update timeline and reconnect

When making the patch, I found another problem about cascading
replication; When promoting a cascading standby, postmaster sends
SIGUSR2 to any cascading walsenders to kill them. But there is a
orner-case where such walsender fails to receive SIGUSR2 and
survives a standby promotion unexpectedly. This happens when
postmaster sends SIGUSR2 before the walsender marks itself as
a WAL sender, because postmaster sends SIGUSR2 to only the
processes marked as a WAL sender.

To avoid the corner-case, I changed walsender so that it checks
whether recovery is in progress or not again after marking itself
as a WAL sender. If recovery is not in progress even though the
walsender is cascading one, it does the same thing as SIGUSR2
signal handler does, and then exits later. Attached patch also includes
this fix.

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center