1. when evaluating whether there are healthy nodes for a leader race before demoting we need to take into account quorum requirements. Without it the former leader may end up in recovery surrounded by asynchronous nodes.
2. QuorumStateResolver wasn't correctly handling the case when the replica node quickly joined and disconnected, what was resulting in the following errors:
```
File "/home/akukushkin/git/patroni/patroni/quorum.py", line 427, in _generate_transitions
yield from self.__remove_gone_nodes()
File "/home/akukushkin/git/patroni/patroni/quorum.py", line 327, in __remove_gone_nodes
yield from self.sync_update(numsync, sync)
File "/home/akukushkin/git/patroni/patroni/quorum.py", line 227, in sync_update
raise QuorumError(f'Sync {numsync} > N of ({sync})')
patroni.quorum.QuorumError: Sync 2 > N of ({'postgresql2'})
2025-02-14 10:18:07,058 INFO: Unexpected exception raised, please report it as a BUG
File "/home/akukushkin/git/patroni/patroni/quorum.py", line 246, in __iter__
transitions = list(self._generate_transitions())
File "/home/akukushkin/git/patroni/patroni/quorum.py", line 423, in _generate_transitions
yield from self.__handle_non_steady_cases()
File "/home/akukushkin/git/patroni/patroni/quorum.py", line 281, in __handle_non_steady_cases
yield from self.quorum_update(len(voters) - self.numsync, voters)
File "/home/akukushkin/git/patroni/patroni/quorum.py", line 184, in quorum_update
raise QuorumError(f'Quorum {quorum} < 0 of ({voters})')
patroni.quorum.QuorumError: Quorum -1 < 0 of ({'postgresql1'})
2025-02-18 15:50:48,243 INFO: Unexpected exception raised, please report it as a BUG
```
To enable quorum commit:
```diff
$ patronictl.py edit-config
---
+++
@@ -5,3 +5,4 @@
use_pg_rewind: true
retry_timeout: 10
ttl: 30
+synchronous_mode: quorum
Apply these changes? [y/N]: y
Configuration changed
```
By default Patroni will use `ANY 1(list,of,stanbys)` in `synchronous_standby_names`. That is, only one node out of listed replicas will be used for quorum.
If you want to increase the number of quorum nodes it is possible to do it with:
```diff
$ patronictl edit-config
---
+++
@@ -6,3 +6,4 @@
retry_timeout: 10
synchronous_mode: quorum
ttl: 30
+synchronous_node_count: 2
Apply these changes? [y/N]: y
Configuration changed
```
Good old `synchronous_mode: on` is still supported.
Close https://github.com/patroni/patroni/issues/664
Close https://github.com/zalando/patroni/pull/672
