linux-kernel - Re: Finding out socket/pipe connectivity status

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Date:	Sun, 12 Aug 2007 18:45:38 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: Finding out socket/pipe connectivity status

On Aug 12 2007 17:00, Alan Cox wrote:
>> when a pipe/socket is broken, the process trying to read/write to it 
>> gets SIGPIPE. Is there a way to detect whether the next read/write will 
>> trigger a SIGPIPE? select() does not seem helpful here.

>Processes that are network aware normally set SIGPIPE to SIG_IGN.
>The default behaviour comes from a desire that non aware programs
>shouldn't get stuck spinning on a network error but go away.
>
>If you ignore the signal you'll get a event from select, and then an
>error code.
>
Yes, SIGPIPE is ignored, because it just occurs at the same time I
can check read() for -1 [=too late].

>
>select considers the broken pipe an error that needs reporting
>so it goes ready.

I am in a quite awkward situation. It is about a fuse mount program
operating in xinetd mode (reading/writing to stdin/stdout). When any
of my code is called, I am already involved in a filesystem operation
(e.g. the user called `df`), so there is no point in trying to
handle a broken pipe here. For reference, my_getattr() is,
simplified:

	write(stdout, request);
	/* reference point [A] */
	read(stdin, response);

So my idea had been to launch another thread that monitors stdin for
'breakage' and unmount the fs before a user can start an operation on
myfs. So I've been trying to complete the idea to code. You say a
broken socket raises the read flag, so:

	while(1) {
		select(STDIN_FILENO+1, &read_set, NULL, NULL, NULL);
		/* check if broken */
	}

but the read flag may also raised during [A] above. So I'd have to
use locking ...

	pthread_mutex_lock(&lk);
	write(stdout, request);
	read(stdin, response);
	pthread_mutex_unlock(&lk);

and

	while(1) {
		pthread_mutex_lock(&lk);
		select(STDIN_FILENO+1, &read_set, NULL, NULL,
			&(struct timeval){0, 0});
		/* check if broken */
		pthread_mutex_unlock(&lk);
	}

this has the disadvantage that I have to use {0,0} so as to unlock
&lk ASAP, otherwise I'd be blocking the write(). Additionally, this
makes it busy-loop, which is not so good. Adding a sleep(60) is a
workaround, but I'd prefer some infinite timeout that cancels on
condition, like e.g. select with a NULL timeout.

Maybe there is a thought bug somewhere and it can be done better.
Your advice is highly appreciated,
thanks,
	Jan
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