Run push_receipts_to_remotes as background job (#4707)

I suspect the CPU usage metrics for this are going to /dev/null at the moment.

changelog.d/4707.misc

@@ -0,0 +1 @@
+Run push_receipts_to_remotes as background job.

synapse/handlers/receipts.py

@@ -16,8 +16,8 @@
 
 from twisted.internet import defer
 
+from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.types import get_domain_from_id
-from synapse.util import logcontext
 
 from ._base import BaseHandler
 
@@ -59,7 +59,9 @@ def received_client_receipt(self, room_id, receipt_type, user_id,
         if is_new:
             # fire off a process in the background to send the receipt to
             # remote servers
-            self._push_remotes([receipt])
+            run_as_background_process(
+                'push_receipts_to_remotes', self._push_remotes, receipt
+            )
 
     @defer.inlineCallbacks
     def _received_remote_receipt(self, origin, content):
@@ -125,44 +127,42 @@ def _handle_new_receipts(self, receipts):
 
         defer.returnValue(True)
 
-    @logcontext.preserve_fn   # caller should not yield on this
     @defer.inlineCallbacks
-    def _push_remotes(self, receipts):
-        """Given a list of receipts, works out which remote servers should be
+    def _push_remotes(self, receipt):
+        """Given a receipt, works out which remote servers should be
         poked and pokes them.
         """
         try:
-            # TODO: Some of this stuff should be coallesced.
-            for receipt in receipts:
-                room_id = receipt["room_id"]
-                receipt_type = receipt["receipt_type"]
-                user_id = receipt["user_id"]
-                event_ids = receipt["event_ids"]
-                data = receipt["data"]
-
-                users = yield self.state.get_current_user_in_room(room_id)
-                remotedomains = set(get_domain_from_id(u) for u in users)
-                remotedomains = remotedomains.copy()
-                remotedomains.discard(self.server_name)
-
-                logger.debug("Sending receipt to: %r", remotedomains)
-
-                for domain in remotedomains:
-                    self.federation.send_edu(
-                        destination=domain,
-                        edu_type="m.receipt",
-                        content={
-                            room_id: {
-                                receipt_type: {
-                                    user_id: {
-                                        "event_ids": event_ids,
-                                        "data": data,
-                                    }
+            # TODO: optimise this to move some of the work to the workers.
+            room_id = receipt["room_id"]
+            receipt_type = receipt["receipt_type"]
+            user_id = receipt["user_id"]
+            event_ids = receipt["event_ids"]
+            data = receipt["data"]
+
+            users = yield self.state.get_current_user_in_room(room_id)
+            remotedomains = set(get_domain_from_id(u) for u in users)
+            remotedomains = remotedomains.copy()
+            remotedomains.discard(self.server_name)
+
+            logger.debug("Sending receipt to: %r", remotedomains)
+
+            for domain in remotedomains:
+                self.federation.send_edu(
+                    destination=domain,
+                    edu_type="m.receipt",
+                    content={
+                        room_id: {
+                            receipt_type: {
+                                user_id: {
+                                    "event_ids": event_ids,
+                                    "data": data,
                                 }
-                            },
+                            }
                         },
-                        key=(room_id, receipt_type, user_id),
-                    )
+                    },
+                    key=(room_id, receipt_type, user_id),
+                )
         except Exception:
             logger.exception("Error pushing receipts to remote servers")