Implement RedirectException (#6687)

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

2 files changed, 35 insertions, 5 deletions

diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index 9e9844b47c..1c9456e583 100644
--- a/synapse/api/errors.py
+++ b/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).
diff --git a/synapse/http/server.py b/synapse/http/server.py
index 943d12c907..04bc2385a2 100644
--- a/synapse/http/server.py
+++ b/synapse/http/server.py
@@ -14,8 +14,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import cgi
 import collections
+import html
 import http.client
 import logging
 import types
@@ -36,6 +36,7 @@ import synapse.metrics
 from synapse.api.errors import (
     CodeMessageException,
     Codes,
+    RedirectException,
     SynapseError,
     UnrecognizedRequestError,
 )
@@ -153,14 +154,18 @@ def _return_html_error(f, request):
 
     Args:
         f (twisted.python.failure.Failure):
-        request (twisted.web.iweb.IRequest):
+        request (twisted.web.server.Request):
     """
     if f.check(CodeMessageException):
         cme = f.value
         code = cme.code
         msg = cme.msg
 
-        if isinstance(cme, SynapseError):
+        if isinstance(cme, RedirectException):
+            logger.info("%s redirect to %s", request, cme.location)
+            request.setHeader(b"location", cme.location)
+            request.cookies.extend(cme.cookies)
+        elif isinstance(cme, SynapseError):
             logger.info("%s SynapseError: %s - %s", request, code, msg)
         else:
             logger.error(
@@ -178,7 +183,7 @@ def _return_html_error(f, request):
             exc_info=(f.type, f.value, f.getTracebackObject()),
         )
 
-    body = HTML_ERROR_TEMPLATE.format(code=code, msg=cgi.escape(msg)).encode("utf-8")
+    body = HTML_ERROR_TEMPLATE.format(code=code, msg=html.escape(msg)).encode("utf-8")
     request.setResponseCode(code)
     request.setHeader(b"Content-Type", b"text/html; charset=utf-8")
     request.setHeader(b"Content-Length", b"%i" % (len(body),))