pull out handle_key_deferred to top level

There's no need for this to be a nested definition; pulling it out not only
makes it more efficient, but makes it easier to check that it's not accessing
any local variables it shouldn't be.

synapse/crypto/keyring.py

@@ -122,48 +122,6 @@ class Keyring(object):
 
             verify_requests.append(verify_request)
 
-        @defer.inlineCallbacks
-        def handle_key_deferred(verify_request):
-            server_name = verify_request.server_name
-            try:
-                _, key_id, verify_key = yield verify_request.deferred
-            except IOError as e:
-                logger.warn(
-                    "Got IOError when downloading keys for %s: %s %s",
-                    server_name, type(e).__name__, str(e.message),
-                )
-                raise SynapseError(
-                    502,
-                    "Error downloading keys for %s" % (server_name,),
-                    Codes.UNAUTHORIZED,
-                )
-            except Exception as e:
-                logger.exception(
-                    "Got Exception when downloading keys for %s: %s %s",
-                    server_name, type(e).__name__, str(e.message),
-                )
-                raise SynapseError(
-                    401,
-                    "No key for %s with id %s" % (server_name, verify_request.key_ids),
-                    Codes.UNAUTHORIZED,
-                )
-
-            json_object = verify_request.json_object
-
-            logger.debug("Got key %s %s:%s for server %s, verifying" % (
-                key_id, verify_key.alg, verify_key.version, server_name,
-            ))
-            try:
-                verify_signed_json(json_object, server_name, verify_key)
-            except:
-                raise SynapseError(
-                    401,
-                    "Invalid signature for server %s with key %s:%s" % (
-                        server_name, verify_key.alg, verify_key.version
-                    ),
-                    Codes.UNAUTHORIZED,
-                )
-
         server_to_deferred = {
             server_name: defer.Deferred()
             for server_name, _ in server_and_json
@@ -208,7 +166,7 @@ class Keyring(object):
         # Pass those keys to handle_key_deferred so that the json object
         # signatures can be verified
         return [
-            preserve_context_over_fn(handle_key_deferred, verify_request)
+            preserve_context_over_fn(_handle_key_deferred, verify_request)
             for verify_request in verify_requests
         ]
 
@@ -740,3 +698,46 @@ class Keyring(object):
             ],
             consumeErrors=True,
         ).addErrback(unwrapFirstError))
+
+
+@defer.inlineCallbacks
+def _handle_key_deferred(verify_request):
+    server_name = verify_request.server_name
+    try:
+        _, key_id, verify_key = yield verify_request.deferred
+    except IOError as e:
+        logger.warn(
+            "Got IOError when downloading keys for %s: %s %s",
+            server_name, type(e).__name__, str(e.message),
+        )
+        raise SynapseError(
+            502,
+            "Error downloading keys for %s" % (server_name,),
+            Codes.UNAUTHORIZED,
+        )
+    except Exception as e:
+        logger.exception(
+            "Got Exception when downloading keys for %s: %s %s",
+            server_name, type(e).__name__, str(e.message),
+        )
+        raise SynapseError(
+            401,
+            "No key for %s with id %s" % (server_name, verify_request.key_ids),
+            Codes.UNAUTHORIZED,
+        )
+
+    json_object = verify_request.json_object
+
+    logger.debug("Got key %s %s:%s for server %s, verifying" % (
+        key_id, verify_key.alg, verify_key.version, server_name,
+    ))
+    try:
+        verify_signed_json(json_object, server_name, verify_key)
+    except:
+        raise SynapseError(
+            401,
+            "Invalid signature for server %s with key %s:%s" % (
+                server_name, verify_key.alg, verify_key.version
+            ),
+            Codes.UNAUTHORIZED,
+        )