Re: Emacs Hangs on Filesystem Operations on Stale NFS

[Alexander Shukaev]

On 06/11/2018 12:27 PM, Alexander Shukaev wrote:
> 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)
> {
>      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

Today I realized that the following code

import os
import signal

class Alarm(Exception):
  pass

def alarm_handler(signum, frame):
  raise Alarm

path = '/mnt/<nfs>'

signal.signal(signal.SIGALRM, alarm_handler)
signal.alarm(3)
try:
  os.stat(path)
  signal.alarm(0)
except Alarm:
  print("Timed out after 3 seconds...")

does not time out in case of 'hard' mounting, which means that the only 
way to time this case out reliably is to spawn a child which attempts to 
perform 'stat' and is then killed by the parent if timed out (similar to 
how 'lsof' does it).  I'm sure such a complex technique would be 
unacceptable for Emacs.

Although I don't like the current behavior as it looks like an editor 
vulnerability to me, which could be (either intentionally or 
unintentionally) used as a potential attack to hang the editor merely by 
pulling a network plug out, I have to admit that we can probably wrap up 
this discussion here since there is not much that could be done by Emacs 
itself to defend.

Regards,
Alexander