With the recovery configuration changes in PostgreSQL 12, there will
be situations where we'll need to determine the version number from
a dormant data directory in order to determine whether to write
recovery.conf or not.
Most of the time we can simply get the version number directly from
the connection handle. Previously it was held in a global variable,
which was an icky way of doing things.
In a few special cases we also need the actual version string, which
is obtained directly from the database.
This is a somewhat convoluted attempt to retrieve the current LSN
of any node, regardless of whether in recovery or not, and if in
recovery, independent of whether streaming or recovering from
archive.
Return 0 if the node is not in recovery. In which case it's probably
rather pointless calling this function anyway.
Return -1 if the "last_seen" field has never been set (i.e. repmgrd
hasn't started yet).
As of PostgreSQL commit d9dd406f, variable length arrays are no longer
permitted. As they're not actually required anyway, just define appropriate
constants.
Also noted in GitHub #510.
The connections used to check repmgrd status on all nodes were not being
closed if repmgrd was not running. Normally this wouldn't be a huge
problem as they will go away when repmgr terminates or the PostgreSQL
server restarted. However, if shutdown mode is "smart", the open
connection on the demotion candidate will cause the shutdown operation
to fail until repmgr times out.
If the upstream comes back on line (e.g. after a switchover), and its
status is no longer primary, restart monitoring to ensure the correct
primary (potentially the current node) is being monitored.
While scanning for a new primary following a promotion script failure,
repmgrd was treating a witness server as a potential new primary
and would attempt to "follow" it. Fortunately "repmgr standby follow"
would do the right thing and choose the actual primary, if available,
otherwise do nothing, so the cluster would eventually end up in the
correct state, albeit for the wrong reason.
By skipping the witness server as a potential new primary,
repmgrd will do the right thing if the original primary does come
back online, i.e. resume monitoring as before.
