Add very basic domain validation for DomainSpecificString.

changelog.d/9071.bugfix

@@ -0,0 +1 @@
+Fix "Failed to send request" errors when a client provides an invalid room alias.

synapse/types.py

@@ -37,6 +37,7 @@
 from unpaddedbase64 import decode_base64
 
 from synapse.api.errors import Codes, SynapseError
+from synapse.http.endpoint import parse_and_validate_server_name
 
 if TYPE_CHECKING:
     from synapse.appservice.api import ApplicationService
@@ -257,8 +258,13 @@ def to_string(self) -> str:
 
     @classmethod
     def is_valid(cls: Type[DS], s: str) -> bool:
+        """Parses the input string and attempts to ensure it is valid."""
         try:
-            cls.from_string(s)
+            obj = cls.from_string(s)
+            # Apply additional validation to the domain. This is only done
+            # during  is_valid (and not part of from_string) since it is
+            # possible for invalid data to exist in room-state, etc.
+            parse_and_validate_server_name(obj.domain)
             return True
         except Exception:
             return False

tests/test_types.py

@@ -58,6 +58,10 @@ def test_build(self):
 
         self.assertEquals(room.to_string(), "#channel:my.domain")
 
+    def test_validate(self):
+        id_string = "#test:domain,test"
+        self.assertFalse(RoomAlias.is_valid(id_string))
+
 
 class GroupIDTestCase(unittest.TestCase):
     def test_parse(self):