From 4876af06ddb280b0a01903c1da17466b13124eb3 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Date: Thu, 23 Jul 2020 16:52:33 +0100
Subject: [PATCH] Abort federation requests if the client disconnects early
 (#7930)

For inbound federation requests, if a given remote server makes too many
requests at once, we start stacking them up rather than processing them
immediatedly.

However, that means that there is a fair chance that the requesting server will
disconnect before we start processing the request. In that case, if it was a
read-only request (ie, a GET request), there is absolutely no point in
building a response (and some requests are quite expensive to handle).

Even in the case of a POST request, one of two things will happen:

 * Most likely, the requesting server will retry the request and we'll get the
   information anyway.

 * Even if it doesn't, the requesting server has to assume that we didn't get
   the memo, and act accordingly.

In short, we're better off aborting the request at this point rather than
ploughing on with what might be a quite expensive request.
---
 changelog.d/7930.feature               | 1 +
 synapse/federation/transport/server.py | 6 ++++++
 2 files changed, 7 insertions(+)
 create mode 100644 changelog.d/7930.feature

diff --git a/changelog.d/7930.feature b/changelog.d/7930.feature
new file mode 100644
index 000000000..a27e4812d
--- /dev/null
+++ b/changelog.d/7930.feature
@@ -0,0 +1 @@
+Abort federation requests where the client disconnects before the ratelimiter expires.
diff --git a/synapse/federation/transport/server.py b/synapse/federation/transport/server.py
index 24f7d4b3b..5e111aa90 100644
--- a/synapse/federation/transport/server.py
+++ b/synapse/federation/transport/server.py
@@ -338,6 +338,12 @@ class BaseFederationServlet(object):
                 if origin:
                     with ratelimiter.ratelimit(origin) as d:
                         await d
+                        if request._disconnected:
+                            logger.warning(
+                                "client disconnected before we started processing "
+                                "request"
+                            )
+                            return -1, None
                         response = await func(
                             origin, content, request.args, *args, **kwargs
                         )