anchor_value: Fix parsing of large anchor values.

This handles the conditions when anchor values are larger than
LARGER_THAN_MAX_MESSAGE_ID by clamping them down to it.  Also added
tests for the function parse_anchor_value.

Fixes #16768.

zerver/tests/test_message_fetch.py

@@ -2668,6 +2668,90 @@ class GetOldMessagesTest(ZulipTestCase):
             {unsub_message_id, muted_message_id, first_message_id, extra_message_id},
         )
 
+    def test_parse_anchor_value(self) -> None:
+        hamlet = self.example_user('hamlet')
+        cordelia = self.example_user('cordelia')
+
+        # Send the first message to Hamlet
+        first_message_id = self.send_personal_message(cordelia, hamlet)
+
+        # Send another message
+        self.send_personal_message(cordelia, hamlet)
+
+        user_profile = hamlet
+
+        # Check if the anchor value in response is correct for different
+        # values of anchor parameter in request
+
+        # With anchor input as first_unread, see if response anchor
+        # value is same as the id of first unread message of Hamlet
+        query_params = dict(
+            anchor="first_unread",
+            num_before=10,
+            num_after=10,
+            narrow='[]',
+        )
+        request = POSTRequestMock(query_params, user_profile)
+
+        payload = get_messages_backend(request, user_profile)
+        result = orjson.loads(payload.content)
+        self.assertEqual(result['anchor'], first_message_id)
+
+        # With anchor input as oldest, see if response anchor value is 0
+        query_params = dict(
+            anchor="oldest",
+            num_before=10,
+            num_after=10,
+            narrow='[]',
+        )
+        request = POSTRequestMock(query_params, user_profile)
+
+        payload = get_messages_backend(request, user_profile)
+        result = orjson.loads(payload.content)
+        self.assertEqual(result['anchor'], 0)
+
+        # With anchor input as newest, see if response
+        # anchor value is LARGER_THAN_MAX_MESSAGE_ID
+        query_params = dict(
+            anchor="newest",
+            num_before=10,
+            num_after=10,
+            narrow='[]',
+        )
+        request = POSTRequestMock(query_params, user_profile)
+
+        payload = get_messages_backend(request, user_profile)
+        result = orjson.loads(payload.content)
+        self.assertEqual(result['anchor'], LARGER_THAN_MAX_MESSAGE_ID)
+
+        # With anchor input negative, see if
+        # response anchor value is clamped to 0
+        query_params = dict(
+            anchor="-1",
+            num_before=10,
+            num_after=10,
+            narrow='[]',
+        )
+        request = POSTRequestMock(query_params, user_profile)
+
+        payload = get_messages_backend(request, user_profile)
+        result = orjson.loads(payload.content)
+        self.assertEqual(result['anchor'], 0)
+
+        # With anchor input more than LARGER_THAN_MAX_MESSAGE_ID,
+        # see if response anchor value is clamped down to LARGER_THAN_MAX_MESSAGE_ID
+        query_params = dict(
+            anchor="10000000000000001",
+            num_before=10,
+            num_after=10,
+            narrow='[]',
+        )
+        request = POSTRequestMock(query_params, user_profile)
+
+        payload = get_messages_backend(request, user_profile)
+        result = orjson.loads(payload.content)
+        self.assertEqual(result['anchor'], LARGER_THAN_MAX_MESSAGE_ID)
+
     def test_use_first_unread_anchor_with_some_unread_messages(self) -> None:
         user_profile = self.example_user('hamlet')
 

zerver/views/message_fetch.py

@@ -839,6 +839,8 @@ def parse_anchor_value(anchor_val: Optional[str],
         anchor = int(anchor_val)
         if anchor < 0:
             return 0
+        elif anchor > LARGER_THAN_MAX_MESSAGE_ID:
+            return LARGER_THAN_MAX_MESSAGE_ID
         return anchor
     except ValueError:
         raise JsonableError(_("Invalid anchor"))