[erlang-bugs] prim_inet:close/1 race condition

[Raimo Niskanen]

On Thu, Oct 18, 2012 at 04:39:10PM +0400, Dmitry Belyaev wrote:
> Is it known bug?
> Can anyone propose a patch for prim_inet?

That seems like a bug (race condition).
We'll have a look at it. The most probable solution for now
will be to do move the unlink to after the close a'la:

    catch erlang:port_close(S),
    unlink(S),
    receive {'EXIT',S,_} -> ok after 0 -> ok end,

Some releases after the current code was written, the guarantees
of unlink/1 were improved, so that after unlink/1 returns - link
related messages _are_ in the process message box making
receive after 0 safe.

> 
> Thank you.
> 
> On 12.10.2012, at 1:06, Dmitry Belyaev wrote:
> 
> > Some days ago we found that we have thousands of leaked sockets in our project.
> > 
> > These sockets were ports with state like this:
> > [{name,"tcp_inet"},
> >  {links,[]},
> >  {connected,<0.54.0>},...]
> > 
> > We made investigation and found the cause of the leaks.
> > 
> > We have inets option {exit_on_close, false} to read statistics from the socket after it was closed by the peer. Process that controls the socket does not trap_exit and is linked with some another process.
> > At the end of connection controller calls gen_tcp:close/1 and sometimes the linked process dies at that the same moment. We found out that the gen_tcp:close/1 calls prim_inet:close/1, the first action of which is unlink from controlling process. So, when controller is unlinked from the port and is killed by the signal, port stays in the system because of exit_on_close feature.
> > 
> > I've made a module that sometimes may reveal the problem. https://gist.github.com/3875485
> > On my system a half of dozen calls to close_bug:start(1000) do find such leaked ports. 
> > I haven't found the right solution for the problem yet, so no patches at the moment.
> > 
> > Thank you for your attention.
> > 
> > -- 
> > Dmitry Belyaev
> 

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-- 

/ Raimo Niskanen, Erlang/OTP, Ericsson AB