From 6a11bdf01d5f2de758162f345c83c3ae4004e3d8 Mon Sep 17 00:00:00 2001
From: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>
Date: Tue, 13 Aug 2024 17:55:05 +0100
Subject: [PATCH] Add a utility function for generating fake event IDs (#17557)

---
 changelog.d/17557.misc            |  1 +
 synapse/rest/client/room.py       |  9 +++++----
 synapse/util/events.py            | 29 +++++++++++++++++++++++++++++
 tests/handlers/test_federation.py |  6 +-----
 4 files changed, 36 insertions(+), 9 deletions(-)
 create mode 100644 changelog.d/17557.misc
 create mode 100644 synapse/util/events.py

diff --git a/changelog.d/17557.misc b/changelog.d/17557.misc
new file mode 100644
index 000000000..535f4b6e5
--- /dev/null
+++ b/changelog.d/17557.misc
@@ -0,0 +1 @@
+Add a utility function for generating random event IDs.
\ No newline at end of file
diff --git a/synapse/rest/client/room.py b/synapse/rest/client/room.py
index 903c74f6d..7d57904d6 100644
--- a/synapse/rest/client/room.py
+++ b/synapse/rest/client/room.py
@@ -67,7 +67,8 @@ from synapse.streams.config import PaginationConfig
 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 @@ class RoomStateEventRestServlet(RestServlet):
                 )
                 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 @@ class RoomSendEventRestServlet(TransactionRestServlet):
             )
             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 @@ class RoomRedactEventRestServlet(TransactionRestServlet):
 
             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}
diff --git a/synapse/util/events.py b/synapse/util/events.py
new file mode 100644
index 000000000..ad9b94657
--- /dev/null
+++ b/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)
diff --git a/tests/handlers/test_federation.py b/tests/handlers/test_federation.py
index 3fe5b0a1b..9847893fc 100644
--- a/tests/handlers/test_federation.py
+++ b/tests/handlers/test_federation.py
@@ -44,7 +44,7 @@ from synapse.rest.client import login, room
 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
@@ -52,10 +52,6 @@ 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,