A number of C functions were added in releases 4.2 to 4.4; however
these were renamed in 5.3 to prevent naming clashes with other
extensions.
This does however mean that when upgrading from one of the above
versions, the intermediate upgrade steps will attempt to create
SQL functions referencing C functions which no longer exist in
repmgr.so, and hence cause the upgrade to fail.
We can work around this by providing empty upgrade scripts
from these versions to 4.4, which skip the problematic CREATE
FUNCTION commands. The functions will be correctly created in
the 5.2--5.3 upgrade script.
This is intended to ensure that all nodes have a constant LSN while
making the failover decision.
This feature is experimental and needs to be explicitly enabled with the
configuration file option "standby_disconnect_on_failover".
Note enabling this option will result in a delay in the failover decision
until the WAL receiver is disconnected on all nodes.