Fix pg_rewind on typical Debian/Ubuntu systems (#2225)

On Debian/Ubuntu systems it is common to keep Postgres config files outside of the data directory.
It created a couple of problems for pg_rewind support in Patroni.
1. The `--config_file` argument must be supplied while figuring out the `restore_command` GUC value on Postgres v12+
2. With Postgres v13+ pg_rewind by itself can't find postgresql.conf in order to figure out `restore_command` and therefore we have to use Patroni as a fallback for fetching missing WAL's that are required for rewind.

This commit addresses both problems.

patroni/postgresql/__init__.py

@@ -798,7 +798,8 @@ class Postgresql(object):
         return True
 
     def get_guc_value(self, name):
-        cmd = [self.pgcommand('postgres'), '-D', self._data_dir, '-C', name]
+        cmd = [self.pgcommand('postgres'), '-D', self._data_dir, '-C', name,
+               '--config-file={}'.format(self.config.postgresql_conf)]
         try:
             data = subprocess.check_output(cmd)
             if data:

patroni/postgresql/rewind.py

@@ -311,8 +311,14 @@ class Rewind(object):
         restore_command = self._postgresql.config.get('recovery_conf', {}).get('restore_command') \
             if self._postgresql.major_version < 120000 else self._postgresql.get_guc_value('restore_command')
 
+        # currently, pg_rewind expects postgresql.conf to be inside $PGDATA, which is not the case on e.g. Debian
+        # Fix this logic if e.g. PG15 receives an update for pg_rewind:
+        pg_rewind_can_restore = self._postgresql.major_version >= 130000 \
+            and restore_command \
+            and self._postgresql.config._config_dir == self._postgresql.data_dir
+
         cmd = [self._postgresql.pgcommand('pg_rewind')]
-        if self._postgresql.major_version >= 130000 and restore_command:
+        if pg_rewind_can_restore:
             cmd.append('--restore-target-wal')
         cmd.extend(['-D', self._postgresql.data_dir, '--source-server', dsn])
 
@@ -329,7 +335,7 @@ class Rewind(object):
             if ret == 0:
                 return True
 
-            if not restore_command or self._postgresql.major_version >= 130000:
+            if not restore_command or pg_rewind_can_restore:
                 return False
 
             missing_wal = self._find_missing_wal(results['stderr']) or self._find_missing_wal(results['stdout'])