linux-kernel - Re: recv() hangs until SIGCHLD ?

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Message-ID: <48F30772.7040207@motion-twin.com>
Date:	Mon, 13 Oct 2008 10:31:46 +0200
From:	Nicolas Cannasse <ncannasse@...ion-twin.com>
To:	Stephen Hemminger <shemminger@...tta.com>
CC:	linux-net@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: recv() hangs until SIGCHLD ?

>> If there is data and the thread didn't wake up then that is a libc or 
>> kernel problem;
>> but if there is no data, then look for cases where earlier interrupted 
>> io actually
>> consumed the data already or blame the sending process not the receiver.
>> Also are the sockets blocking or non-blocking?
> 
> The sockets are non-blocking.

Sorry, I made a spelling mistake here.

I wanted to tell that the sockets ARE blocking (default behavior).

> In a practical case, we have a thread blocked in recv() for more than 12 
> hours, which is way beyond the timeout of the sender connection. The 
> socket has already been closed by the sender so recv() should at least 
> be noticed and returns 0.

To provide more informations :

Doing a lsof on the receiver, we can see that it has several ESTABLISHED 
sockets connected to a given host/sender. Doing a lsof on the host does 
not give any socket connected to the receiver (since they have been 
closed due to a timeout).

Also, the application correctly handles 0.
The pseudo-code is the following :

loop:
ret = recv()
if( ret == -1 ) {
    if( errno == EINTR ) goto loop;
    return -1;
}
return ret;

Then, on the higher level, in case we get an error ( ret <= 0 ) then we 
close the socket.

At first, we were using the libmysqlclient but since we had the bug with 
it we rewrote a mysql client so we can more easily check what's 
occurring. The same bug seems to occur with both implementations.

Best,
Nicolas
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