Fix up logcontext handling in (federation) TransactionQueue

Avoid using preserve_context_over_function, which has problems with respect to
logcontexts.

synapse/federation/transaction_queue.py

@@ -20,8 +20,8 @@ from .persistence import TransactionActions
 from .units import Transaction, Edu
 
 from synapse.api.errors import HttpResponseException
+from synapse.util import logcontext
 from synapse.util.async import run_on_reactor
-from synapse.util.logcontext import preserve_context_over_fn, preserve_fn
 from synapse.util.retryutils import NotRetryingDestination, get_retry_limiter
 from synapse.util.metrics import measure_func
 from synapse.handlers.presence import format_user_presence_state, get_interested_remotes
@@ -231,11 +231,9 @@ class TransactionQueue(object):
                 (pdu, order)
             )
 
-            preserve_context_over_fn(
-                self._attempt_new_transaction, destination
-            )
+            self._attempt_new_transaction(destination)
 
-    @preserve_fn  # the caller should not yield on this
+    @logcontext.preserve_fn  # the caller should not yield on this
     @defer.inlineCallbacks
     def send_presence(self, states):
         """Send the new presence states to the appropriate destinations.
@@ -299,7 +297,7 @@ class TransactionQueue(object):
                     state.user_id: state for state in states
                 })
 
-                preserve_fn(self._attempt_new_transaction)(destination)
+                self._attempt_new_transaction(destination)
 
     def send_edu(self, destination, edu_type, content, key=None):
         edu = Edu(
@@ -321,9 +319,7 @@ class TransactionQueue(object):
         else:
             self.pending_edus_by_dest.setdefault(destination, []).append(edu)
 
-        preserve_context_over_fn(
-            self._attempt_new_transaction, destination
-        )
+        self._attempt_new_transaction(destination)
 
     def send_failure(self, failure, destination):
         if destination == self.server_name or destination == "localhost":
@@ -336,9 +332,7 @@ class TransactionQueue(object):
             destination, []
         ).append(failure)
 
-        preserve_context_over_fn(
-            self._attempt_new_transaction, destination
-        )
+        self._attempt_new_transaction(destination)
 
     def send_device_messages(self, destination):
         if destination == self.server_name or destination == "localhost":
@@ -347,15 +341,24 @@ class TransactionQueue(object):
         if not self.can_send_to(destination):
             return
 
-        preserve_context_over_fn(
-            self._attempt_new_transaction, destination
-        )
+        self._attempt_new_transaction(destination)
 
     def get_current_token(self):
         return 0
 
-    @defer.inlineCallbacks
     def _attempt_new_transaction(self, destination):
+        """Try to start a new transaction to this destination
+
+        If there is already a transaction in progress to this destination,
+        returns immediately. Otherwise kicks off the process of sending a
+        transaction in the background.
+
+        Args:
+            destination (str):
+
+        Returns:
+            None
+        """
         # list of (pending_pdu, deferred, order)
         if destination in self.pending_transactions:
             # XXX: pending_transactions can get stuck on by a never-ending
@@ -368,6 +371,19 @@ class TransactionQueue(object):
             )
             return
 
+        logger.debug("TX [%s] Starting transaction loop", destination)
+
+        # Drop the logcontext before starting the transaction. It doesn't
+        # really make sense to log all the outbound transactions against
+        # whatever path led us to this point: that's pretty arbitrary really.
+        #
+        # (this also means we can fire off _perform_transaction without
+        # yielding)
+        with logcontext.PreserveLoggingContext():
+            self._transaction_transmission_loop(destination)
+
+    @defer.inlineCallbacks
+    def _transaction_transmission_loop(self, destination):
         pending_pdus = []
         try:
             self.pending_transactions[destination] = 1