Warn if postgres database has non-C locale.

changelog.d/6734.bugfix

@@ -0,0 +1 @@
+Warn if postgrse database has a non-C locale, as that can cause issues when upgrading locales (e.g. due to upgrading OS).

synapse/storage/engines/postgres.py

@@ -13,8 +13,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import logging
+
 from ._base import IncorrectDatabaseSetup
 
+logger = logging.getLogger(__name__)
+
 
 class PostgresEngine(object):
     single_threaded = False
@@ -52,6 +56,20 @@ def check_database(self, db_conn, allow_outdated_version: bool = False):
                     "See docs/postgres.rst for more information." % (rows[0][0],)
                 )
 
+            txn.execute(
+                "SELECT datcollate, datctype FROM pg_database WHERE datname = current_database()"
+            )
+            collation, ctype = txn.fetchone()
+            if collation != "C":
+                logger.warning(
+                    "Database has incorrect collation of %r. Should be 'C'", collation
+                )
+
+            if ctype != "C":
+                logger.warning(
+                    "Database has incorrect ctype of %r. Should be 'C'", ctype
+                )
+
     def convert_param_style(self, sql):
         return sql.replace("?", "%s")