Sanity-check database before running upgrades (#6982)

Some of the database deltas rely on `config.server_name` being set correctly,
so we should check that it is before running the deltas.

Fixes #6870.

changelog.d/6982.feature

+Check that server_name is correctly set before running database updates.

synapse/storage/data_stores/main/__init__.py

@@ -20,6 +20,7 @@ import logging
 import time
 
 from synapse.api.constants import PresenceState
+from synapse.config.homeserver import HomeServerConfig
 from synapse.storage.database import Database
 from synapse.storage.engines import PostgresEngine
 from synapse.storage.util.id_generators import (
@@ -117,16 +118,6 @@ class DataStore(
         self._clock = hs.get_clock()
         self.database_engine = database.engine
 
-        all_users_native = are_all_users_on_domain(
-            db_conn.cursor(), database.engine, hs.hostname
-        )
-        if not all_users_native:
-            raise Exception(
-                "Found users in database not native to %s!\n"
-                "You cannot changed a synapse server_name after it's been configured"
-                % (hs.hostname,)
-            )
-
         self._stream_id_gen = StreamIdGenerator(
             db_conn,
             "events",
@@ -567,13 +558,26 @@ class DataStore(
         )
 
 
-def are_all_users_on_domain(txn, database_engine, domain):
+def check_database_before_upgrade(cur, database_engine, config: HomeServerConfig):
+    """Called before upgrading an existing database to check that it is broadly sane
+    compared with the configuration.
+    """
+    domain = config.server_name
+
     sql = database_engine.convert_param_style(
         "SELECT COUNT(*) FROM users WHERE name NOT LIKE ?"
     )
     pat = "%:" + domain
-    txn.execute(sql, (pat,))
-    num_not_matching = txn.fetchall()[0][0]
+    cur.execute(sql, (pat,))
+    num_not_matching = cur.fetchall()[0][0]
     if num_not_matching == 0:
-        return True
-    return False
+        return
+
+    raise Exception(
+        "Found users in database not native to %s!\n"
+        "You cannot changed a synapse server_name after it's been configured"
+        % (domain,)
+    )
+
+
+__all__ = ["DataStore", "check_database_before_upgrade"]

synapse/storage/prepare_database.py

@@ -278,13 +278,17 @@ def _upgrade_existing_database(
             the current_version wasn't generated by applying those delta files.
         database_engine (DatabaseEngine)
         config (synapse.config.homeserver.HomeServerConfig|None):
-            application config, or None if we are connecting to an existing
-            database which we expect to be configured already
+            None if we are initialising a blank database, otherwise the application
+            config
         data_stores (list[str]): The names of the data stores to instantiate
             on the given database.
         is_empty (bool): Is this a blank database? I.e. do we need to run the
             upgrade portions of the delta scripts.
     """
+    if is_empty:
+        assert not applied_delta_files
+    else:
+        assert config
 
     if current_version > SCHEMA_VERSION:
         raise ValueError(
@@ -292,6 +296,13 @@ def _upgrade_existing_database(
             + "new for the server to understand"
         )
 
+    # some of the deltas assume that config.server_name is set correctly, so now
+    # is a good time to run the sanity check.
+    if not is_empty and "main" in data_stores:
+        from synapse.storage.data_stores.main import check_database_before_upgrade
+
+        check_database_before_upgrade(cur, database_engine, config)
+
     start_ver = current_version
     if not upgraded:
         start_ver += 1