Improve TURN documentation. (#7167)

Fixes #7179.

changelog.d/6639.bugfix

@@ -0,0 +1 @@
+Fix missing field `default` when fetching user-defined push rules.

changelog.d/7167.doc

@@ -0,0 +1 @@
+Improve README.md by being explicit about public IP recommendation for TURN relaying.

changelog.d/7181.misc

@@ -0,0 +1 @@
+Clean up some LoggingContext code.

docs/turn-howto.md

@@ -11,6 +11,13 @@ TURN server.
 
 The following sections describe how to install [coturn](<https://github.com/coturn/coturn>) (which implements the TURN REST API) and integrate it with synapse.
 
+## Requirements
+
+For TURN relaying with `coturn` to work, it must be hosted on a server/endpoint with a public IP.
+
+Hosting TURN behind a NAT (even with appropriate port forwarding) is known to cause issues
+and to often not work.
+
 ## `coturn` Setup
 
 ### Initial installation

synapse/http/site.py

@@ -193,6 +193,12 @@ class SynapseRequest(Request):
         self.finish_time = time.time()
         Request.connectionLost(self, reason)
 
+        if self.logcontext is None:
+            logger.info(
+                "Connection from %s lost before request headers were read", self.client
+            )
+            return
+
         # we only get here if the connection to the client drops before we send
         # the response.
         #
@@ -236,13 +242,6 @@ class SynapseRequest(Request):
     def _finished_processing(self):
         """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/logging/context.py

@@ -539,6 +539,11 @@ def set_current_context(context: LoggingContextOrSentinel) -> LoggingContextOrSe
     Returns:
         The context that was previously active
     """
+    # everything blows up if we allow current_context to be set to None, so sanity-check
+    # that now.
+    if context is None:
+        raise TypeError("'context' argument may not be None")
+
     current = current_context()
 
     if current is not context:

synapse/storage/data_stores/main/push_rule.py

@@ -41,6 +41,7 @@ def _load_rules(rawrules, enabled_map):
         rule = dict(rawrule)
         rule["conditions"] = json.loads(rawrule["conditions"])
         rule["actions"] = json.loads(rawrule["actions"])
+        rule["default"] = False
         ruleslist.append(rule)
 
     # We're going to be mutating this a lot, so do a deep copy