Fix "add user" admin api error when request contains a "msisdn" threepid (#13263)

Co-authored-by: Thomas Weston <thomas.weston@clearspancloud.com>
Co-authored-by: David Robertson <david.m.robertson1@gmail.com>

changelog.d/13263.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in Synapse 1.15.0 where adding a user through the Synapse Admin API with a phone number would fail if the "enable_email_notifs" and "email_notifs_for_new_users" options were enabled. Contributed by @thomasweston12.

synapse/rest/admin/users.py

@@ -373,6 +373,7 @@ class UserRestServletV2(RestServlet):
                     if (
                         self.hs.config.email.email_enable_notifs
                         and self.hs.config.email.email_notif_for_new_users
+                        and medium == "email"
                     ):
                         await self.pusher_pool.add_pusher(
                             user_id=user_id,

tests/rest/admin/test_user.py

@@ -1636,6 +1636,41 @@ class UserRestTestCase(unittest.HomeserverTestCase):
         )
         self.assertEqual(len(pushers), 0)
 
+    @override_config(
+        {
+            "email": {
+                "enable_notifs": True,
+                "notif_for_new_users": True,
+                "notif_from": "test@example.com",
+            },
+            "public_baseurl": "https://example.com",
+        }
+    )
+    def test_create_user_email_notif_for_new_users_with_msisdn_threepid(self) -> None:
+        """
+        Check that a new regular user is created successfully when they have a msisdn
+        threepid and email notif_for_new_users is set to True.
+        """
+        url = self.url_prefix % "@bob:test"
+
+        # Create user
+        body = {
+            "password": "abc123",
+            "threepids": [{"medium": "msisdn", "address": "1234567890"}],
+        }
+
+        channel = self.make_request(
+            "PUT",
+            url,
+            access_token=self.admin_user_tok,
+            content=body,
+        )
+
+        self.assertEqual(201, channel.code, msg=channel.json_body)
+        self.assertEqual("@bob:test", channel.json_body["name"])
+        self.assertEqual("msisdn", channel.json_body["threepids"][0]["medium"])
+        self.assertEqual("1234567890", channel.json_body["threepids"][0]["address"])
+
     def test_set_password(self) -> None:
         """
         Test setting a new password for another user.