From owner-freebsd-bugs@FreeBSD.ORG Mon Oct 1 07:26:07 2007 Return-Path: Delivered-To: freebsd-bugs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id EE75D16A41B; Mon, 1 Oct 2007 07:26:07 +0000 (UTC) (envelope-from brde@optusnet.com.au) Received: from mail18.syd.optusnet.com.au (mail18.syd.optusnet.com.au [211.29.132.199]) by mx1.freebsd.org (Postfix) with ESMTP id 8367C13C478; Mon, 1 Oct 2007 07:26:07 +0000 (UTC) (envelope-from brde@optusnet.com.au) Received: from besplex.bde.org (c220-239-235-248.carlnfd3.nsw.optusnet.com.au [220.239.235.248]) by mail18.syd.optusnet.com.au (8.13.1/8.13.1) with ESMTP id l917Q2hi009530 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Mon, 1 Oct 2007 17:26:04 +1000 Date: Mon, 1 Oct 2007 17:26:02 +1000 (EST) From: Bruce Evans X-X-Sender: bde@besplex.bde.org To: Bo Lindbergh In-Reply-To: <200709302055.l8UKtOUQ071624@igloo.stacken.kth.se> Message-ID: <20071001164720.M1839@besplex.bde.org> References: <200709302055.l8UKtOUQ071624@igloo.stacken.kth.se> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Cc: freebsd-bugs@freebsd.org, FreeBSD-gnats-submit@freebsd.org Subject: Re: kern/116770: Unfortunate fifo/O_NONBLOCK/kevent interaction X-BeenThere: freebsd-bugs@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Bug reports List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 01 Oct 2007 07:26:08 -0000 On Sun, 30 Sep 2007, Bo Lindbergh wrote: >> Description: > When a fifo with no writers is opened nonblockingly for reading and > the resulting file descriptor is added to a kqueue, kevent will > immediately report an EOF condition. This is less than useful. But it is an EOF condition: in this state, read() must return 0 to indicate EOF. select(), poll() and kqueue could have another mechanism for reporting this state, but don't in FreeBSD. Some other OS's have a specially broken select() and/or poll() for fifos but not for other file types so that the polling read condition doesn't actually report the read condition for fifos only. >> Fix: > Don't CANTRCVMORE the socketpair immediately after creating it. Add > code to fifo_read_f to avoid calling soreceive blockingly when there > are zero writers. That would break read(). > Or just add a fi_seen_at_least_one_writer flag to struct fifoinfo... Fixing this, or even implementing bug for bug compatibility with other OS's, is not easy. See PR's 34020, 53447, 76144, 76125, 76525, 94722 and the resulting commits for previous attempts to fix this. PR 94722 does something like this. It only tries to fix poll(). The behaviour of select() on a read descriptor cannot be changed, except to remove old buggy attempts to fix this problem, since select() on a read descriptor has no way to distinguish initial EOF from hangup. select() on an exceptional descriptor could consider hangup as an exception; there is no standard for this, but since exceptional descriptors are rarely used, changing the behaviour for them wouldn't break much. Kqueue has flags so it should be able to use the fix for poll() fairly easily. Bruce