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Emacs Hangs on Filesystem Operations on Stale NFS

From: Alexander Shukaev
Subject: Emacs Hangs on Filesystem Operations on Stale NFS
Date: Mon, 11 Jun 2018 12:27:41 +0200
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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. <C-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 <sigaction.h>
#include <sys/stat.h>
#include <unistd.h>
#include <string.h>

static void alarm_handler(int sig)

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));


    newact.sa_flags   = 0;
    newact.sa_handler = alarm_handler;
    sigaction(SIGALRM, &newact, &oldact);


    errno                 = 0;
    const int rc          = stat(path, s);
    const int saved_errno = errno;

    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,

[1] https://lists.gnu.org/archive/html/help-gnu-emacs/2015-11/msg00251.html

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