Fix error in is_mine_id when encountering a malformed ID (#13746)

Previously, `is_mine_id` would raise an exception when passed an ID with
no colons. Return `False` instead.

Fixes #13040.

Signed-off-by: Sean Quah <seanq@matrix.org>

synapse/server.py

@@ -341,7 +341,17 @@ class HomeServer(metaclass=abc.ABCMeta):
         return domain_specific_string.domain == self.hostname
 
     def is_mine_id(self, string: str) -> bool:
-        return string.split(":", 1)[1] == self.hostname
+        """Determines whether a user ID or room alias originates from this homeserver.
+
+        Returns:
+            `True` if the hostname part of the user ID or room alias matches this
+            homeserver.
+            `False` otherwise, or if the user ID or room alias is malformed.
+        """
+        localpart_hostname = string.split(":", 1)
+        if len(localpart_hostname) < 2:
+            return False
+        return localpart_hostname[1] == self.hostname
 
     @cache_in_self
     def get_clock(self) -> Clock: