Fix a 500 error resulting from empty room_ids

POST /_matrix/client/api/v1/rooms//send/a.b.c gave a 500 error, because we
assumed that rooms always had at least one character.

synapse/types.py

@@ -47,7 +47,7 @@ class DomainSpecificString(
     @classmethod
     def from_string(cls, s):
         """Parse the string given by 's' into a structure object."""
-        if s[0] != cls.SIGIL:
+        if len(s) < 1 or s[0] != cls.SIGIL:
             raise SynapseError(400, "Expected %s string to start with '%s'" % (
                 cls.__name__, cls.SIGIL,
             ))

tests/test_types.py

@@ -15,13 +15,14 @@
 
 from tests import unittest
 
+from synapse.api.errors import SynapseError
 from synapse.server import BaseHomeServer
 from synapse.types import UserID, RoomAlias
 
 mock_homeserver = BaseHomeServer(hostname="my.domain")
 
-class UserIDTestCase(unittest.TestCase):
 
+class UserIDTestCase(unittest.TestCase):
     def test_parse(self):
         user = UserID.from_string("@1234abcd:my.domain")
 
@@ -29,6 +30,11 @@ class UserIDTestCase(unittest.TestCase):
         self.assertEquals("my.domain", user.domain)
         self.assertEquals(True, mock_homeserver.is_mine(user))
 
+    def test_pase_empty(self):
+        with self.assertRaises(SynapseError):
+            UserID.from_string("")
+
+
     def test_build(self):
         user = UserID("5678efgh", "my.domain")
 
@@ -44,7 +50,6 @@ class UserIDTestCase(unittest.TestCase):
 
 
 class RoomAliasTestCase(unittest.TestCase):
-
     def test_parse(self):
         room = RoomAlias.from_string("#channel:my.domain")