Implement RedirectException (#6687)

Allow REST endpoint implemnentations to raise a RedirectException, which will
redirect the user's browser to a given location.

synapse/api/errors.py

@@ -17,13 +17,15 @@
 """Contains exceptions and error codes."""
 
 import logging
-from typing import Dict
+from typing import Dict, List
 
 from six import iteritems
 from six.moves import http_client
 
 from canonicaljson import json
 
+from twisted.web import http
+
 logger = logging.getLogger(__name__)
 
 
@@ -80,6 +82,29 @@ class CodeMessageException(RuntimeError):
         self.msg = msg
 
 
+class RedirectException(CodeMessageException):
+    """A pseudo-error indicating that we want to redirect the client to a different
+    location
+
+    Attributes:
+        cookies: a list of set-cookies values to add to the response. For example:
+           b"sessionId=a3fWa; Expires=Wed, 21 Oct 2015 07:28:00 GMT"
+    """
+
+    def __init__(self, location: bytes, http_code: int = http.FOUND):
+        """
+
+        Args:
+            location: the URI to redirect to
+            http_code: the HTTP response code
+        """
+        msg = "Redirect to %s" % (location.decode("utf-8"),)
+        super().__init__(code=http_code, msg=msg)
+        self.location = location
+
+        self.cookies = []  # type: List[bytes]
+
+
 class SynapseError(CodeMessageException):
     """A base exception type for matrix errors which have an errcode and error
     message (as well as an HTTP status code).