From e980982b59dea38ec10a5c58993d09e02f845d28 Mon Sep 17 00:00:00 2001
From: Sean Quah <8349537+squahtx@users.noreply.github.com>
Date: Mon, 7 Nov 2022 13:49:31 +0000
Subject: [PATCH] Do not reject `/sync` requests with unrecognised filter
 fields (#14369)

For forward compatibility, Synapse needs to ignore fields it does not
recognise instead of raising an error.

Fixes #14365.

Signed-off-by: Sean Quah <seanq@matrix.org>
---
 changelog.d/14369.bugfix    |  1 +
 synapse/api/filtering.py    |  8 ++++----
 tests/api/test_filtering.py | 21 +++++++++++++++++++--
 3 files changed, 24 insertions(+), 6 deletions(-)
 create mode 100644 changelog.d/14369.bugfix

diff --git a/changelog.d/14369.bugfix b/changelog.d/14369.bugfix
new file mode 100644
index 000000000..e6709f4ee
--- /dev/null
+++ b/changelog.d/14369.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug where Synapse would raise an error when encountering an unrecognised field in a `/sync` filter, instead of ignoring it for forward compatibility.
diff --git a/synapse/api/filtering.py b/synapse/api/filtering.py
index 26be377d0..a9888381b 100644
--- a/synapse/api/filtering.py
+++ b/synapse/api/filtering.py
@@ -43,7 +43,7 @@ if TYPE_CHECKING:
     from synapse.server import HomeServer
 
 FILTER_SCHEMA = {
-    "additionalProperties": False,
+    "additionalProperties": True,  # Allow new fields for forward compatibility
     "type": "object",
     "properties": {
         "limit": {"type": "number"},
@@ -63,7 +63,7 @@ FILTER_SCHEMA = {
 }
 
 ROOM_FILTER_SCHEMA = {
-    "additionalProperties": False,
+    "additionalProperties": True,  # Allow new fields for forward compatibility
     "type": "object",
     "properties": {
         "not_rooms": {"$ref": "#/definitions/room_id_array"},
@@ -77,7 +77,7 @@ ROOM_FILTER_SCHEMA = {
 }
 
 ROOM_EVENT_FILTER_SCHEMA = {
-    "additionalProperties": False,
+    "additionalProperties": True,  # Allow new fields for forward compatibility
     "type": "object",
     "properties": {
         "limit": {"type": "number"},
@@ -143,7 +143,7 @@ USER_FILTER_SCHEMA = {
             },
         },
     },
-    "additionalProperties": False,
+    "additionalProperties": True,  # Allow new fields for forward compatibility
 }
 
 
diff --git a/tests/api/test_filtering.py b/tests/api/test_filtering.py
index a82c4eed8..d5524d296 100644
--- a/tests/api/test_filtering.py
+++ b/tests/api/test_filtering.py
@@ -46,19 +46,36 @@ class FilteringTestCase(unittest.HomeserverTestCase):
         self.datastore = hs.get_datastores().main
 
     def test_errors_on_invalid_filters(self):
+        # See USER_FILTER_SCHEMA for the filter schema.
         invalid_filters = [
-            {"boom": {}},
+            # `account_data` must be a dictionary
             {"account_data": "Hello World"},
+            # `event_fields` entries must not contain backslashes
             {"event_fields": [r"\\foo"]},
-            {"room": {"timeline": {"limit": 0}, "state": {"not_bars": ["*"]}}},
+            # `event_format` must be "client" or "federation"
             {"event_format": "other"},
+            # `not_rooms` must contain valid room IDs
             {"room": {"not_rooms": ["#foo:pik-test"]}},
+            # `senders` must contain valid user IDs
             {"presence": {"senders": ["@bar;pik.test.com"]}},
         ]
         for filter in invalid_filters:
             with self.assertRaises(SynapseError):
                 self.filtering.check_valid_filter(filter)
 
+    def test_ignores_unknown_filter_fields(self):
+        # For forward compatibility, we must ignore unknown filter fields.
+        # See USER_FILTER_SCHEMA for the filter schema.
+        filters = [
+            {"org.matrix.msc9999.future_option": True},
+            {"presence": {"org.matrix.msc9999.future_option": True}},
+            {"room": {"org.matrix.msc9999.future_option": True}},
+            {"room": {"timeline": {"org.matrix.msc9999.future_option": True}}},
+        ]
+        for filter in filters:
+            self.filtering.check_valid_filter(filter)
+            # Must not raise.
+
     def test_valid_filters(self):
         valid_filters = [
             {