Run the AS senders as background processes (#4189)

This should fix some "Starting db connection from sentinel context" warnings,
and will mean we get metrics for these processes.

changelog.d/4189.misc

@@ -0,0 +1,2 @@
+Run the AS senders as background processes to fix warnings
+

synapse/appservice/scheduler.py

@@ -53,8 +53,8 @@ import logging
 from twisted.internet import defer
 
 from synapse.appservice import ApplicationServiceState
+from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.util.logcontext import run_in_background
-from synapse.util.metrics import Measure
 
 logger = logging.getLogger(__name__)
 
@@ -104,14 +104,23 @@ class _ServiceQueuer(object):
         self.clock = clock
 
     def enqueue(self, service, event):
-        # if this service isn't being sent something
         self.queued_events.setdefault(service.id, []).append(event)
-        run_in_background(self._send_request, service)
+
+        # start a sender for this appservice if we don't already have one
+
+        if service.id in self.requests_in_flight:
+            return
+
+        run_as_background_process(
+            "as-sender-%s" % (service.id, ),
+            self._send_request, service,
+        )
 
     @defer.inlineCallbacks
     def _send_request(self, service):
-        if service.id in self.requests_in_flight:
-            return
+        # sanity-check: we shouldn't get here if this service already has a sender
+        # running.
+        assert(service.id not in self.requests_in_flight)
 
         self.requests_in_flight.add(service.id)
         try:
@@ -119,8 +128,6 @@ class _ServiceQueuer(object):
                 events = self.queued_events.pop(service.id, [])
                 if not events:
                     return
-
-                with Measure(self.clock, "servicequeuer.send"):
                 try:
                     yield self.txn_ctrl.send(service, events)
                 except Exception:
@@ -223,7 +230,12 @@ class _Recoverer(object):
         self.backoff_counter = 1
 
     def recover(self):
-        self.clock.call_later((2 ** self.backoff_counter), self.retry)
+        def _retry():
+            run_as_background_process(
+                "as-recoverer-%s" % (self.service.id,),
+                self.retry,
+            )
+        self.clock.call_later((2 ** self.backoff_counter), _retry)
 
     def _backoff(self):
         # cap the backoff to be around 8.5min => (2^9) = 512 secs