Upon start postmaster process performs various safety checks if there is a postmaster.pid file in the data directory. Although Patroni already detected that the running process corresponding to the postmaster.pid is not a postmaster, the new postmaster might fail to start, because it thinks that postmaster.pid is already locked.
Important!!! Unlink of postmaster.pid isn't an option in this case, because it has a lot of nasty race conditions.
Luckily there is a workaround to this problem, we can pass the pid from postmaster.pid in the `PG_GRANDPARENT_PID` environment variable and postmaster will ignore it.
More likely to hit such problem if you run Patroni and postgres in the docker container.
* Show information about scheduled failover and maintenance mode when showing list of cluster members. Fixes https://github.com/zalando/patroni/issues/557
* Fix postgres version check functions (postgres 10 and above compatibility) and apply pep8 formatting to the tests.
* Bump some configuration parameters to match with postgres 10 defaults.
* Fix name of contributor in release notes.
Introduces a PostmasterProcess object that identifies a running process via pid and start time.
When pid file is parsed and the correct process identified this object is passed around.
When the process goes away we try to find a new one in case somebody restarted postgres behind our back.
