From b28a7ed5030148139af6c6c03e635b068a6eae15 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Thu, 20 Sep 2018 13:44:20 +0100
Subject: [PATCH 1/2] Fix spurious exceptions when client closes conncetion

If a HTTP handler throws an exception while processing a request we
automatically write a JSON error response. If the handler had already
started writing a response twisted throws an exception.

We should check for this case and simple abort the connection if there
was an error after the response had started being written.
---
 synapse/http/server.py | 49 ++++++++++++++++++++++++++++++------------
 1 file changed, 35 insertions(+), 14 deletions(-)

diff --git a/synapse/http/server.py b/synapse/http/server.py
index 2d5c23e673..b4b25cab19 100644
--- a/synapse/http/server.py
+++ b/synapse/http/server.py
@@ -84,10 +84,21 @@ def wrap_json_request_handler(h):
             logger.info(
                 "%s SynapseError: %s - %s", request, code, e.msg
             )
-            respond_with_json(
-                request, code, e.error_dict(), send_cors=True,
-                pretty_print=_request_user_agent_is_curl(request),
-            )
+
+            # Only respond with an error response if we haven't already started
+            # writing, otherwise lets just kill the connection
+            if request.startedWriting:
+                if request.transport:
+                    try:
+                        request.transport.abortConnection()
+                    except Exception:
+                        # abortConnection throws if the connection is already closed
+                        pass
+            else:
+                respond_with_json(
+                    request, code, e.error_dict(), send_cors=True,
+                    pretty_print=_request_user_agent_is_curl(request),
+                )
 
         except Exception:
             # failure.Failure() fishes the original Failure out
@@ -100,16 +111,26 @@ def wrap_json_request_handler(h):
                 request,
                 f.getTraceback().rstrip(),
             )
-            respond_with_json(
-                request,
-                500,
-                {
-                    "error": "Internal server error",
-                    "errcode": Codes.UNKNOWN,
-                },
-                send_cors=True,
-                pretty_print=_request_user_agent_is_curl(request),
-            )
+            # Only respond with an error response if we haven't already started
+            # writing, otherwise lets just kill the connection
+            if request.startedWriting:
+                if request.transport:
+                    try:
+                        request.transport.abortConnection()
+                    except Exception:
+                        # abortConnection throws if the connection is already closed
+                        pass
+            else:
+                respond_with_json(
+                    request,
+                    500,
+                    {
+                        "error": "Internal server error",
+                        "errcode": Codes.UNKNOWN,
+                    },
+                    send_cors=True,
+                    pretty_print=_request_user_agent_is_curl(request),
+                )
 
     return wrap_async_request_handler(wrapped_request_handler)
 

From 13f6f1624b21aab82fc7f0edf90a61ce635b91be Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Thu, 20 Sep 2018 13:52:09 +0100
Subject: [PATCH 2/2] Newsfile

---
 changelog.d/3925.misc | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/3925.misc

diff --git a/changelog.d/3925.misc b/changelog.d/3925.misc
new file mode 100644
index 0000000000..3e41f78ff5
--- /dev/null
+++ b/changelog.d/3925.misc
@@ -0,0 +1 @@
+Fix spurious exceptions when remote http client closes conncetion