add some logging for the keyring queue

why is it so damn slow?

synapse/crypto/keyring.py

@@ -40,6 +40,7 @@ from synapse.api.errors import Codes, SynapseError
 from synapse.crypto.keyclient import fetch_server_key
 from synapse.util import logcontext, unwrapFirstError
 from synapse.util.logcontext import (
+    LoggingContext,
     PreserveLoggingContext,
     preserve_fn,
     run_in_background,
@@ -216,23 +217,34 @@ class Keyring(object):
             servers have completed. Follows the synapse rules of logcontext
             preservation.
         """
+        loop_count = 1
         while True:
             wait_on = [
-                self.key_downloads[server_name]
+                (server_name, self.key_downloads[server_name])
                 for server_name in server_names
                 if server_name in self.key_downloads
             ]
-            if wait_on:
-                with PreserveLoggingContext():
-                    yield defer.DeferredList(wait_on)
-            else:
+            if not wait_on:
                 break
+            logger.info(
+                "Waiting for existing lookups for %s to complete [loop %i]",
+                [w[0] for w in wait_on], loop_count,
+            )
+            with PreserveLoggingContext():
+                yield defer.DeferredList((w[1] for w in wait_on))
+
+            loop_count += 1
+
+        ctx = LoggingContext.current_context()
 
         def rm(r, server_name_):
-            self.key_downloads.pop(server_name_, None)
+            with PreserveLoggingContext(ctx):
+                logger.debug("Releasing key lookup lock on %s", server_name_)
+                self.key_downloads.pop(server_name_, None)
             return r
 
         for server_name, deferred in server_to_deferred.items():
+            logger.debug("Got key lookup lock on %s", server_name)
             self.key_downloads[server_name] = deferred
             deferred.addBoth(rm, server_name)