X-Recipient: archive-cygwin AT delorie DOT com DomainKey-Signature: a=rsa-sha1; c=nofws; d=sourceware.org; h=list-id :list-unsubscribe:list-subscribe:list-archive:list-post :list-help:sender:mime-version:from:date:message-id:subject:to :content-type; q=dns; s=default; b=RWQint1Io1VpkMf769WHbgiKCePSt xaQf5iVTLrHurlIlt1Id6kJgXWnKuVN+gRdjCAy0d5GOGm4L7nxRcOUFeaCQrAWy 2SE82tdjHSoNV3yErAxzfUtgSbdkp6za+bMUD6jdHQVF+bIRMRGUXNAxq+h6bL5A /nA/7UNez9unUY= DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=sourceware.org; h=list-id :list-unsubscribe:list-subscribe:list-archive:list-post :list-help:sender:mime-version:from:date:message-id:subject:to :content-type; s=default; bh=cNHy2rdSsH6Ydk/ceAdu/YrWbEA=; b=qlB Gu30Ro8deqWZWC1cBmTrQB5ee3E+jwhdjZpxmJ1DfSJAG+UdW1swrVGN+YvoC7vb usQy7N8BI273wLZJGmR3ymIK289q+H8gIzD4e5GDxOALxa1iRhp2sPOAli00t3SL uBfIKksplg40CPJlT6+IBswA3Q/tnmt9HNkSojVA= Mailing-List: contact cygwin-help AT cygwin DOT com; run by ezmlm List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: cygwin-owner AT cygwin DOT com Mail-Followup-To: cygwin AT cygwin DOT com Delivered-To: mailing list cygwin AT cygwin DOT com Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-0.3 required=5.0 tests=AWL,BAYES_00,FREEMAIL_FROM,RCVD_IN_DNSWL_NONE,RCVD_IN_SORBS_SPAM,SPF_PASS,UNSUBSCRIBE_BODY autolearn=no version=3.3.2 spammy=conveniently, tricky, Hx-languages-length:2147, states X-HELO: mail-io0-f172.google.com X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:from:date:message-id:subject:to; bh=KTDT68+wtNom7a8RswFCkRNN0gga/jezK7NcJGTiIdo=; b=D7dMPkbPktlXebXHBNG2j1I7xeq6E8LRrDipZT6QZ0CRuKE1R4mc6HeRboaw2LYK0H PnCCNC5xGD38pGz/nVEg5puHQ7nHxnCrWdBS5riPT6ccVSsRJ4zaagWr3M+Tu50seOYe eUAa1kZYcZSyPFg2V8AWstkAuEDORoZq7iF4nbxWpHmDmSLt1GIfe/cWLpgostw4rQJH R3ZSjhOLZ0JzFxTJHBOm4K5wQFqqpXuhhuevOTKIP5tqMwKcZdiprmdEQQ8hXHtKoTZx EfwfvzOfQjrvNaMAVzDZFN+j0HwG6KdmGW+rz+BXvOHfJ5nbS9a/e24d/m5fgMOX9jRT hM3w== X-Gm-Message-State: AMCzsaUuLW2umnMLbwW1UoWdfpGM9qS4tpUH5h1VXMn5nOTxI5lrGq1V iJO1vJQmVmiSykNfqz78w3175qN2Y4wsQMVsB9t328g+ X-Google-Smtp-Source: ABhQp+SPFTW1vMLuXZVNBnDplMRK5SVmCR/6tJqKr/Z5SIwR4fAuSqUDk9nwAj94lWD3X83gCpw0PvAmZ0iPNkIHDRQ= X-Received: by 10.107.20.137 with SMTP id 131mr24261084iou.137.1510072788458; Tue, 07 Nov 2017 08:39:48 -0800 (PST) MIME-Version: 1.0 From: Erik Bray Date: Tue, 7 Nov 2017 17:39:48 +0100 Message-ID: Subject: Bug in poll/select readable state on write end of pipe To: cygwin AT cygwin DOT com Content-Type: text/plain; charset="UTF-8" X-IsSubscribed: yes Hi, I encountered this bug through a hang in Twisted, but narrowed it down to a simple example (in Python here, but the same would work in C). Basically, in Twisted's system process runner, there's a "hack" [1] which basically assumes readability (as in a poll() loop) of the writable end of a pipe indicates that the pipe is closed and/or there's an error. Although the "hack" is disabled by default, in practice it is always forced to be enabled [2]. One could maybe consider this a bug in Twisted since I can't find anything in POSIX which states this behavior (though I could be missing it). Nevertheless, on Linux this *is* the behavior: >>> import os >>> import select >>> r, w = os.pipe() >>> poller = select.poll() >>> os.close(r) >>> poller.register(w, select.POLLIN) >>> print(poller.poll(1000)) [(4, 8)] where 8 indicates that POLLERR is set on fd 4, indicating in this case that if we tried to write to the pipe we would get a broken pipe error. However, on Cygwin the same code returns an empty list. I don't know if this *should* be fixed, but it would be nice. It's slightly tricky though. In Cygwin's poll there's a line [3] that does something similar for sockets--if the socket is not connected it sets POLLERR in the results. One could do something similar for pipes, but there isn't an existing internal API to do this conveniently. What one might want is something that calls NtQueryInformationFile like in pipe_data_available [4], and checks the NamedPipeState flag. But that something doesn't exist yet. Any ideas? Erik [1] https://github.com/twisted/twisted/blob/bb371b3b602b1547c6e603d3b234bb18ded6d67c/src/twisted/internet/process.py#L139 [2] https://github.com/twisted/twisted/blob/bb371b3b602b1547c6e603d3b234bb18ded6d67c/src/twisted/internet/process.py#L764 [3] https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/poll.cc;h=ea45b70adacaca96edbc46d7b8ffe1cd8a94270e;hb=HEAD#l122 [4] https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/select.cc;h=f5a993850408477a65d9ecda444785d0db1f35a8;hb=HEAD#l567 -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple