Add a utility function for generating fake event IDs (#17557)

changelog.d/17557.misc

@@ -0,0 +1 @@
+Add a utility function for generating random event IDs.

synapse/rest/client/room.py

@@ -67,7 +67,8 @@
 from synapse.types import JsonDict, Requester, StreamToken, ThirdPartyInstanceID, UserID
 from synapse.types.state import StateFilter
 from synapse.util.cancellation import cancellable
-from synapse.util.stringutils import parse_and_validate_server_name, random_string
+from synapse.util.events import generate_fake_event_id
+from synapse.util.stringutils import parse_and_validate_server_name
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -325,7 +326,7 @@ async def on_PUT(
                 )
                 event_id = event.event_id
         except ShadowBanError:
-            event_id = "$" + random_string(43)
+            event_id = generate_fake_event_id()
 
         set_tag("event_id", event_id)
         ret = {"event_id": event_id}
@@ -377,7 +378,7 @@ async def _do(
             )
             event_id = event.event_id
         except ShadowBanError:
-            event_id = "$" + random_string(43)
+            event_id = generate_fake_event_id()
 
         set_tag("event_id", event_id)
         return 200, {"event_id": event_id}
@@ -1193,7 +1194,7 @@ async def _do(
 
             event_id = event.event_id
         except ShadowBanError:
-            event_id = "$" + random_string(43)
+            event_id = generate_fake_event_id()
 
         set_tag("event_id", event_id)
         return 200, {"event_id": event_id}

synapse/util/events.py

@@ -0,0 +1,29 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2024 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+#
+#
+
+from synapse.util.stringutils import random_string
+
+
+def generate_fake_event_id() -> str:
+    """
+    Generate an event ID from random ASCII characters.
+
+    This is primarily useful for generating fake event IDs in response to
+    requests from shadow-banned users.
+
+    Returns:
+        A string intended to look like an event ID, but with no actual meaning.
+    """
+    return "$" + random_string(43)

tests/handlers/test_federation.py

@@ -44,18 +44,14 @@
 from synapse.server import HomeServer
 from synapse.storage.databases.main.events_worker import EventCacheEntry
 from synapse.util import Clock
-from synapse.util.stringutils import random_string
+from synapse.util.events import generate_fake_event_id
 
 from tests import unittest
 from tests.test_utils import event_injection
 
 logger = logging.getLogger(__name__)
 
 
-def generate_fake_event_id() -> str:
-    return "$fake_" + random_string(43)
-
-
 class FederationTestCase(unittest.FederatingHomeserverTestCase):
     servlets = [
         admin.register_servlets,