Fix exceptions when a connection is closed before we read the headers

This fixes bugs introduced in #3700, by making sure that we behave sanely
when an incoming connection is closed before the headers are read.

synapse/http/site.py

@@ -182,7 +182,7 @@ class SynapseRequest(Request):
         # the client disconnects.
         with PreserveLoggingContext(self.logcontext):
             logger.warn(
-                "Error processing request: %s %s", reason.type, reason.value,
+                "Error processing request %r: %s %s", self, reason.type, reason.value,
             )
 
             if not self._is_processing:
@@ -219,6 +219,12 @@ class SynapseRequest(Request):
         """Log the completion of this request and update the metrics
         """
 
+        if self.logcontext is None:
+            # this can happen if the connection closed before we read the
+            # headers (so render was never called). In that case we'll already
+            # have logged a warning, so just bail out.
+            return
+
         usage = self.logcontext.get_resource_usage()
 
         if self._processing_finished_time is None:

synapse/util/logcontext.py

@@ -402,7 +402,9 @@ class PreserveLoggingContext(object):
 
     __slots__ = ["current_context", "new_context", "has_parent"]
 
-    def __init__(self, new_context=LoggingContext.sentinel):
+    def __init__(self, new_context=None):
+        if new_context is None:
+            new_context = LoggingContext.sentinel
         self.new_context = new_context
 
     def __enter__(self):