Optionally use an on-disk sqlite db in tests (#11702)

* Optionally use an on-disk sqlite db in tests

When debugging a test it is sometimes useful to inspect the state of the
DB. This is not easy when the db is in-memory: one cannot attach the
sqlite CLI to another process's DB.

With this change, if SYNAPSE_TEST_PERSIST_SQLITE_DB is set, we use
 `_trial_temp/test.db` as our sqlite database. One can then use 
`sqlite3 _trial_temp/test.db` and query to your heart's content.

The DB is destroyed and recreated between different test cases.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

changelog.d/11702.misc

@@ -0,0 +1 @@
+Add the option to write sqlite test dbs to disk when running tests.

tests/server.py

@@ -14,6 +14,8 @@
 import hashlib
 import json
 import logging
+import os
+import os.path
 import time
 import uuid
 import warnings
@@ -71,6 +73,7 @@ from tests.utils import (
     POSTGRES_HOST,
     POSTGRES_PASSWORD,
     POSTGRES_USER,
+    SQLITE_PERSIST_DB,
     USE_POSTGRES_FOR_TESTS,
     MockClock,
     default_config,
@@ -739,9 +742,23 @@ def setup_test_homeserver(
             },
         }
     else:
+        if SQLITE_PERSIST_DB:
+            # The current working directory is in _trial_temp, so this gets created within that directory.
+            test_db_location = os.path.abspath("test.db")
+            logger.debug("Will persist db to %s", test_db_location)
+            # Ensure each test gets a clean database.
+            try:
+                os.remove(test_db_location)
+            except FileNotFoundError:
+                pass
+            else:
+                logger.debug("Removed existing DB at %s", test_db_location)
+        else:
+            test_db_location = ":memory:"
+
         database_config = {
             "name": "sqlite3",
-            "args": {"database": ":memory:", "cp_min": 1, "cp_max": 1},
+            "args": {"database": test_db_location, "cp_min": 1, "cp_max": 1},
         }
 
     if "db_txn_limit" in kwargs:

tests/utils.py

@@ -42,6 +42,10 @@ POSTGRES_HOST = os.environ.get("SYNAPSE_POSTGRES_HOST", None)
 POSTGRES_PASSWORD = os.environ.get("SYNAPSE_POSTGRES_PASSWORD", None)
 POSTGRES_BASE_DB = "_synapse_unit_tests_base_%s" % (os.getpid(),)
 
+# When debugging a specific test, it's occasionally useful to write the
+# DB to disk and query it with the sqlite CLI.
+SQLITE_PERSIST_DB = os.environ.get("SYNAPSE_TEST_PERSIST_SQLITE_DB") is not None
+
 # the dbname we will connect to in order to create the base database.
 POSTGRES_DBNAME_FOR_INITIAL_CREATE = "postgres"