Re: non-blocking connect and EAGAIN

[Chad MILLER]

Hi, Dmitriy, Vladimir!


On 19 Sep 2007, at 07:40, Vladimir Shebordaev wrote:

Hi, Dmitriy,

would you please specify when you get those reconnects?

The Linux connect() system call on non-blocking AF_UNIX sockets should return immediately with EAGAIN when the peer's backlog queue is full.

        if (skb_queue_len(&other->sk_receive_queue) >
            other->sk_max_ack_backlog) {
                err = -EAGAIN;
                if (!timeo)
                        goto out_unlock;

                timeo = unix_wait_for_peer(other, timeo);

                err = sock_intr_errno(timeo);
                if (signal_pending(current))
                        goto out;
                sock_put(other);
                goto restart;
        }


Notably, the BSDs don't send EAGAIN, as far as I can tell.

Vladimir's right here. The Linux kernel doesn't normally send errno EINPROGRESS, but it does send EAGAIN for this case:

Otherwise connect() will block until there is some room available on receiving end. MySQL client intention is to literally follow that system call when there is no timeout option explicitly specified (see the comments in my_connect() right above the lines you've cited). So, what you get looks like intended behavior from both kernel and MySQL side.

Agreed, for the most part. (I don't know that the kernel sends EAGAIN /only/ for no-timeout/non-blocking connect()ion attempts. I didn't dig wider than the above.) The Linux kernel truly couldn't accept the connect() syscall, and this is a valid problem. The library code behaves correctly because the library /should/ pass errors from the kernel up to the client. This specific case isn't one I think we considered, but client code should handle all errors the OS could generate; the library shouldn't insulate the client from the kernel, but it should from the server.

Please check out the MySQL 5.0 trouble shooting page at <http:// dev.mysql.com/doc/refman/5.0/en/can-not-connect-to-server.html>. You've probably got your server crashed or stalled due to some real bug. If so, you should try to reproduce it and file a bug report. But please upgrade to decent MySQL version first of all.

- chad

It could be a crashed server that's causing the problem, I suppose. More likely, if it's not, please keep us included if there's another bottleneck in connecting that you find.

Dmitriy MiksIr wrote:

Hello!

fcntl64(3, F_SETFL, O_RDWR|O_NONBLOCK)  = 0

Mysql's connect do not detect this error:
  if ((res != 0) && (s_err != EINPROGRESS))
  {
    errno= s_err;                       /* Restore it */
    return(-1);
  }

I got a lot of mysql errors "Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (11)". I trace one of this error and see, what non-blocking connect return EAGAIN. See: connect(3, {sa_family=AF_FILE, path="/var/lib/mysql/mysql.sock"}, 110) = -1 EAGAIN (Resource temporarily unavailable) Is this kernel bug (Linux 2.6.16-std26-smp-alt1)?... which return EAGAIN instead of EINPROGRESS, or some other troubles can force EAGAIN on unix socket connect?

--
Chad Miller, Software Developer                         [EMAIL PROTECTED]
MySQL Inc., www.mysql.com
Orlando, Florida, USA                                13-20z,  UTC-0400
Office: +1 408 213 6740                         sip:[EMAIL PROTECTED]

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