Factor SSO success handling out of CAS login (#4264)

This is mostly factoring out the post-CAS-login code to somewhere we can reuse
it for other SSO flows, but it also fixes the userid mapping while we're at it.

tests/test_types.py

@@ -14,7 +14,7 @@
 # limitations under the License.
 
 from synapse.api.errors import SynapseError
-from synapse.types import GroupID, RoomAlias, UserID
+from synapse.types import GroupID, RoomAlias, UserID, map_username_to_mxid_localpart
 
 from tests import unittest
 from tests.utils import TestHomeServer
@@ -79,3 +79,32 @@ class GroupIDTestCase(unittest.TestCase):
             except SynapseError as exc:
                 self.assertEqual(400, exc.code)
                 self.assertEqual("M_UNKNOWN", exc.errcode)
+
+
+class MapUsernameTestCase(unittest.TestCase):
+    def testPassThrough(self):
+        self.assertEqual(map_username_to_mxid_localpart("test1234"), "test1234")
+
+    def testUpperCase(self):
+        self.assertEqual(map_username_to_mxid_localpart("tEST_1234"), "test_1234")
+        self.assertEqual(
+            map_username_to_mxid_localpart("tEST_1234", case_sensitive=True),
+            "t_e_s_t__1234",
+        )
+
+    def testSymbols(self):
+        self.assertEqual(
+            map_username_to_mxid_localpart("test=$?_1234"),
+            "test=3d=24=3f_1234",
+        )
+
+    def testLeadingUnderscore(self):
+        self.assertEqual(map_username_to_mxid_localpart("_test_1234"), "=5ftest_1234")
+
+    def testNonAscii(self):
+        # this should work with either a unicode or a bytes
+        self.assertEqual(map_username_to_mxid_localpart(u'têst'), "t=c3=aast")
+        self.assertEqual(
+            map_username_to_mxid_localpart(u'têst'.encode('utf-8')),
+            "t=c3=aast",
+        )