Allow external processes to mark a user as syncing. (#812)

* Add infrastructure to the presence handler to track sync requests in external processes

* Expire stale entries for dead external processes

* Add an http endpoint for making users as syncing

Add some docstrings and comments.

* Fixes

synapse/replication/presence_resource.py

@@ -0,0 +1,59 @@
+# Copyright 2016 OpenMarket Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from synapse.http.server import respond_with_json_bytes, request_handler
+from synapse.http.servlet import parse_json_object_from_request
+
+from twisted.web.resource import Resource
+from twisted.web.server import NOT_DONE_YET
+from twisted.internet import defer
+
+
+class PresenceResource(Resource):
+    """
+    HTTP endpoint for marking users as syncing.
+
+    POST /_synapse/replication/presence HTTP/1.1
+    Content-Type: application/json
+
+    {
+        "process_id": "<process_id>",
+        "syncing_users": ["<user_id>"]
+    }
+    """
+
+    def __init__(self, hs):
+        Resource.__init__(self)  # Resource is old-style, so no super()
+
+        self.version_string = hs.version_string
+        self.clock = hs.get_clock()
+        self.presence_handler = hs.get_presence_handler()
+
+    def render_POST(self, request):
+        self._async_render_POST(request)
+        return NOT_DONE_YET
+
+    @request_handler()
+    @defer.inlineCallbacks
+    def _async_render_POST(self, request):
+        content = parse_json_object_from_request(request)
+
+        process_id = content["process_id"]
+        syncing_user_ids = content["syncing_users"]
+
+        yield self.presence_handler.update_external_syncs(
+            process_id, set(syncing_user_ids)
+        )
+
+        respond_with_json_bytes(request, 200, "{}")