It could happen that the WAL segment required for `pg_rewind` doesn't exist in the `pg_wal` anymore and therefore `pg_rewind` can't find the checkpoint location before the diverging point.
Starting from PostgreSQL 13 `pg_rewind` could use `restore_command` for fetching missing WALs, but we can do better than that.
On older PostgreSQL versions Patroni will parse the stdout and stderr of failed rewind attempt, try to fetch the missing WAL by calling the `restore_command`, and repeat an attempt.
We don't need to rewind when:
1. replayed location for the former replica is not ahead of switchpoint
2. end of checkpoint record for the former primary is the same as switchpoint
In order to get the end of checkpoint record we use the `pg_waldump` and parse its output.
Close https://github.com/zalando/patroni/issues/1493
when Patroni is trying to figure out the necessity of pg_rewind it could write the content history file from the primary into the log. The history file is growing with every failover/switchover and eventually starts taking too many lines in the log, most of them are not so much useful.
Instead of showing the raw data, we will show only 3 lines before the current replica timeline and 2 lines after.
Replicas are waiting for checkpoint indication via member key of the leader in DCS. The key is normally updated only one time per HA loop.
Without waking the main thread up replicas will have to wait up to `loop_wait` seconds longer than necessary.
It is safe to call pg_rewind on the replica only when pg_control on the primary contains information about the latest timeline. Postgres is usually doing immediate checkpoint right after promote and in most cases it works just fine. Unfortunately we regularly receive complaints that it takes to long (minutes) until the checkpoint is done and replicas can't perform rewind. At the same time doing the checkpoint manually immediately helped. So Patroni starts doing the same. When the promotion happened and postgres is not running in recovery, we explicitly issue the checkpoint.
We are intentionally not using the AsyncExecutor here, because we want the HA loop continues doing its normal flow.
* Convert postgresql.py into a package
* Factor out cancellable process into a separate class
* Factor out connection handler into a separate class
* Move postmaster into postgresql package
* Factor out pg_rewind into a separate class
* Factor out bootstrap into a separate class
* Factor out slots handler into a separate class
* Factor out postgresql config handler into a separate class
* Move callback_executor into postgresql package
This is just a careful refactoring, without code changes.
