From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED!not-for-mail From: Alexander Shukaev Newsgroups: gmane.emacs.devel Subject: Emacs Hangs on Filesystem Operations on Stale NFS Date: Mon, 11 Jun 2018 12:27:41 +0200 Message-ID: <1727545582523435cab149c2bc857b40@alexander.shukaev.name> NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit X-Trace: blaine.gmane.org 1528712803 29636 195.159.176.226 (11 Jun 2018 10:26:43 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 11 Jun 2018 10:26:43 +0000 (UTC) User-Agent: Roundcube Webmail/1.1.2 To: emacs-devel@gnu.org Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Mon Jun 11 12:26:39 2018 Return-path: Envelope-to: ged-emacs-devel@m.gmane.org Original-Received: from lists.gnu.org ([208.118.235.17]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fSK23-0007Zc-3G for ged-emacs-devel@m.gmane.org; Mon, 11 Jun 2018 12:26:39 +0200 Original-Received: from localhost ([::1]:47569 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fSK48-0005wI-2Z for ged-emacs-devel@m.gmane.org; Mon, 11 Jun 2018 06:28:48 -0400 Original-Received: from eggs.gnu.org ([2001:4830:134:3::10]:51148) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fSK3A-0005uX-7x for emacs-devel@gnu.org; Mon, 11 Jun 2018 06:27:49 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fSK36-0008DI-SZ for emacs-devel@gnu.org; Mon, 11 Jun 2018 06:27:48 -0400 Original-Received: from relay10.mail.gandi.net ([217.70.178.230]:56191) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fSK36-0008BV-Ip for emacs-devel@gnu.org; Mon, 11 Jun 2018 06:27:44 -0400 Original-Received: from webmail.gandi.net (unknown [10.200.201.13]) (Authenticated sender: forum@alexander.shukaev.name) by relay10.mail.gandi.net (Postfix) with ESMTPA id 8D4E3240003 for ; Mon, 11 Jun 2018 10:27:41 +0000 (UTC) X-Sender: emacs@alexander.shukaev.name X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 217.70.178.230 X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Original-Sender: "Emacs-devel" Xref: news.gmane.org gmane.emacs.devel:226189 Archived-At: Hi Everyone, I initiated a discussion back in 2015 [1] about fragility of Emacs in terms of filesystem operations on stale NFS. No solution actually came out of this discussion. I still find this issue very disruptive. Yet another example would be `recentf-cleanup' which is in my case triggered on Emacs start up, when the file comes from stale NFS, the corresponding `file-readable-p' down the stack will hang indefinitely, and there would be no way to unfreeze it apart from issuing 'kill -9' to that Emacs instance. Don't you people find it unacceptable for the daily usage? Well, I do. Such hangs always disrupt daily work and require quite some time to track them down as they are not Lisp-debuggable with e.g. in a straightforward way (these are dead hangs from C code, where even attaching a GDB does not work). Well, enough rant. I think I have a proposal how to fix the issue, even given the blocking nature of Emacs. How about introducing a variable `file-access-timeout' defaulting to `nil', which would reflect a configurable timeout for all access operations (such as `file-readable-p')? This would be achieved via `SIGALARM' in the C code, which would protect every such operation. For example, #include #include #include #include static void alarm_handler(int sig) { return; } int emacs_stat(const char* path, struct stat* s, unsigned int seconds) { struct sigaction newact; struct sigaction oldact; memset(&newact, 0, sizeof(newact)); memset(&oldact, 0, sizeof(oldact)); sigemptyset(&newact.sa_mask); newact.sa_flags = 0; newact.sa_handler = alarm_handler; sigaction(SIGALRM, &newact, &oldact); alarm(seconds); errno = 0; const int rc = stat(path, s); const int saved_errno = errno; alarm(0); sigaction(SIGALRM, &oldact, NULL); errno = saved_errno; return rc; } where `seconds' should be initialized with the value of `file-access-timeout'. The cool advantage of this that I see is that one can then also selectively `let'-bind different values for `file-access-timeout', thus having total control over the use cases in which one wants to protect oneself from indefinite hangs. Kind regards, Alexander [1] https://lists.gnu.org/archive/html/help-gnu-emacs/2015-11/msg00251.html