add the register_mxid_from_3pid setting (untested)

synapse/config/registration.py

@@ -34,6 +34,7 @@ class RegistrationConfig(Config):
         self.registrations_require_3pid = config.get("registrations_require_3pid", [])
         self.allowed_local_3pids = config.get("allowed_local_3pids", [])
         self.registration_shared_secret = config.get("registration_shared_secret")
+        self.register_mxid_from_3pid = config.get("register_mxid_from_3pid")
 
         self.bcrypt_rounds = config.get("bcrypt_rounds", 12)
         self.trusted_third_party_id_servers = config["trusted_third_party_id_servers"]
@@ -60,6 +61,13 @@ class RegistrationConfig(Config):
         #     - email
         #     - msisdn
 
+        # Derive the user's matrix ID from a type of 3PID used when registering.
+        # This overrides any matrix ID the user proposes when calling /register
+        # The 3PID type should be present in registrations_require_3pid to avoid
+        # users failing to register if they don't specify the right kind of 3pid.
+        #
+        # register_mxid_from_3pid: email
+
         # Mandate that users are only allowed to associate certain formats of
         # 3PIDs with accounts on this server.
         #

synapse/rest/client/v2_alpha/register.py

@@ -297,13 +297,6 @@ class RegisterRestServlet(RestServlet):
                 session_id, "registered_user_id", None
             )
 
-        if desired_username is not None:
-            yield self.registration_handler.check_username(
-                desired_username,
-                guest_access_token=guest_access_token,
-                assigned_user_id=registered_user_id,
-            )
-
         # Only give msisdn flows if the x_show_msisdn flag is given:
         # this is a hack to work around the fact that clients were shipped
         # that use fallback registration if they see any flows that they don't
@@ -376,6 +369,25 @@ class RegisterRestServlet(RestServlet):
                             Codes.THREEPID_DENIED,
                         )
 
+        if self.hs.config.register_mxid_from_3pid:
+            # override the desired_username based on the 3PID if any.
+            # reset it first to avoid folks picking their own username.
+            desired_username = None
+
+            # we should always have an auth_result if we're going to progress
+            # to register the user (i.e. we haven't picked up a registered_user_id)
+            # from our session store
+            if auth_result and self.hs.config.register_mxid_from_3pid in auth_result:
+                address = auth_result[login_type]['address']
+                desired_username = address.lower()
+
+        if desired_username is not None:
+            yield self.registration_handler.check_username(
+                desired_username,
+                guest_access_token=guest_access_token,
+                assigned_user_id=registered_user_id,
+            )
+
         if registered_user_id is not None:
             logger.info(
                 "Already registered user ID %r for this session",
@@ -390,10 +402,18 @@ class RegisterRestServlet(RestServlet):
                 raise SynapseError(400, "Missing password.",
                                    Codes.MISSING_PARAM)
 
+            if not self.hs.config.register_mxid_from_3pid:
                 desired_username = params.get("username", None)
+            else:
+                # we keep the original desired_username derived from the 3pid above
+                pass
+
             new_password = params.get("password", None)
             guest_access_token = params.get("guest_access_token", None)
 
+            # XXX: don't we need to validate these for length etc like we did on
+            # the ones from the JSON body earlier on in the method?
+
             if desired_username is not None:
                 desired_username = desired_username.lower()