IPC leaks connections (when internally using PubSubSession)

[gnunicorn]

Heyah,

investigating a leak in IPC connections in Parity I was able to pin-point the location to the way that IPC-Server and PubSubSession are interacting leading to an internal circle of never ending futures. Which in turn means we are keeping those connections around even after they've terminated, keep the socket open and - ultimatily - leak those connections as well as memory, causing Parity to be oom-killed (at least I suspect this error to be the cause of that error. Can't rule out there might be another leak).

I've commited a short example triggering that behaviour, together with some code comments of the cause (IMHO) in this branch.

To reproduce this bug, check out the leaking-ipc-connections-example branch, cd into ipc and run

RUST_LOG=ipc=trace cargo run --example with-pubsubsession

Now have something connect to the open socket /tmp/json-ipc-test.ipc and immediately close again or die, like so:

#!/bin/sh -e
for i in `seq 0 1000`
do
    echo $i
    nc localhost 30303 </dev/null >/dev/null &
    pid=$!
    sleep 0.1
    kill $pid
done;

You'll see a bunch of messages appear in the example looking something like

    Finished dev [unoptimized + debuginfo] target(s) in 4.23 secs
     Running `/home/ben/dev/parity/jsonrpc/target/debug/examples/with-pubsubsession`
TRACE 2018-06-05T10:33:41Z: jsonrpc_ipc_server::server: Accepted incoming IPC connection: 1
TRACE 2018-06-05T10:33:41Z: with_pubsubsession: ext
TRACE 2018-06-05T10:33:41Z: jsonrpc_ipc_server::server: Accepted incoming IPC connection: 2
TRACE 2018-06-05T10:33:41Z: with_pubsubsession: ext
TRACE 2018-06-05T10:33:41Z: jsonrpc_ipc_server::server: Accepted incoming IPC connection: 3
TRACE 2018-06-05T10:33:41Z: with_pubsubsession: ext

Notice how the closing message Peer: service finished is absent -- connections aren't dropped even though the peer is gone.

You can also watch it grow the open file handlers with lsof (e.g. while true; do echo "`date +%s` : `lsof -c with-pubs | wc -l`"; sleep 5; done)

[dvdplm]

For posterity, here's a repeat-script that worked for me (thanks @debris):

#!/bin/sh -e
for i in `seq 0 1000`
do
    echo $i
    nc -U /tmp/json-ipc-test.ipc </dev/null >/dev/null &
    pid=$!
    sleep 0.1
    kill $pid
done;

(also, om macos, brew uninstall netcat if you have it)