Add more logging to debug slow startup (#8264)

I'm hoping this will provide some pointers for debugging
https://github.com/matrix-org/synapse/issues/7968.

changelog.d/8264.misc

@@ -0,0 +1 @@
+Add more logging to debug slow startup.

synapse/storage/databases/__init__.py

@@ -47,9 +47,14 @@ class Databases:
             engine = create_engine(database_config.config)
 
             with make_conn(database_config, engine) as db_conn:
-                logger.info("Preparing database %r...", db_name)
-
+                logger.info("[database config %r]: Checking database server", db_name)
                 engine.check_database(db_conn)
+
+                logger.info(
+                    "[database config %r]: Preparing for databases %r",
+                    db_name,
+                    database_config.databases,
+                )
                 prepare_database(
                     db_conn, engine, hs.config, databases=database_config.databases,
                 )
@@ -57,7 +62,9 @@ class Databases:
                 database = DatabasePool(hs, database_config, engine)
 
                 if "main" in database_config.databases:
-                    logger.info("Starting 'main' data store")
+                    logger.info(
+                        "[database config %r]: Starting 'main' database", db_name
+                    )
 
                     # Sanity check we don't try and configure the main store on
                     # multiple databases.
@@ -72,7 +79,9 @@ class Databases:
                         persist_events = PersistEventsStore(hs, database, main)
 
                 if "state" in database_config.databases:
-                    logger.info("Starting 'state' data store")
+                    logger.info(
+                        "[database config %r]: Starting 'state' database", db_name
+                    )
 
                     # Sanity check we don't try and configure the state store on
                     # multiple databases.
@@ -85,7 +94,7 @@ class Databases:
 
                 self.databases.append(database)
 
-                logger.info("Database %r prepared", db_name)
+                logger.info("[database config %r]: prepared", db_name)
 
             # Closing the context manager doesn't close the connection.
             # psycopg will close the connection when the object gets GCed, but *only*
@@ -98,10 +107,10 @@ class Databases:
 
         # Sanity check that we have actually configured all the required stores.
         if not main:
-            raise Exception("No 'main' data store configured")
+            raise Exception("No 'main' database configured")
 
         if not state:
-            raise Exception("No 'state' data store configured")
+            raise Exception("No 'state' database configured")
 
         # We use local variables here to ensure that the databases do not have
         # optional types.

synapse/storage/databases/main/__init__.py

@@ -591,6 +591,7 @@ 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.
     """
+    logger.info("Running sanity-checks on database...")
     domain = config.server_name
 
     sql = database_engine.convert_param_style(

synapse/storage/prepare_database.py

@@ -82,10 +82,18 @@ def prepare_database(db_conn, database_engine, config, databases=["main", "state
 
     try:
         cur = db_conn.cursor()
+
+        logger.info("%r: Checking existing schema version", databases)
         version_info = _get_or_create_schema_state(cur, database_engine)
 
         if version_info:
             user_version, delta_files, upgraded = version_info
+            logger.info(
+                "%r: Existing schema is %i (+%i deltas)",
+                databases,
+                user_version,
+                len(delta_files),
+            )
 
             # config should only be None when we are preparing an in-memory SQLite db,
             # which should be empty.
@@ -111,6 +119,8 @@ def prepare_database(db_conn, database_engine, config, databases=["main", "state
                 databases=databases,
             )
         else:
+            logger.info("%r: Initialising new database", databases)
+
             # if it's a worker app, refuse to upgrade the database, to avoid multiple
             # workers doing it at once.
             if config and config.worker_app is not None:

synapse/storage/util/id_generators.py

@@ -15,6 +15,7 @@
 
 import contextlib
 import heapq
+import logging
 import threading
 from collections import deque
 from typing import Dict, List, Set
@@ -24,6 +25,8 @@ from typing_extensions import Deque
 from synapse.storage.database import DatabasePool, LoggingTransaction
 from synapse.storage.util.sequence import PostgresSequenceGenerator
 
+logger = logging.getLogger(__name__)
+
 
 class IdGenerator:
     def __init__(self, db_conn, table, column):
@@ -48,6 +51,8 @@ def _load_current_id(db_conn, table, column, step=1):
     Returns:
         int
     """
+    # debug logging for https://github.com/matrix-org/synapse/issues/7968
+    logger.info("initialising stream generator for %s(%s)", table, column)
     cur = db_conn.cursor()
     if step == 1:
         cur.execute("SELECT MAX(%s) FROM %s" % (column, table))