GNU bug report logs - #21460
Race condition in tests/tail-2/assert.sh

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From: Pádraig Brady <P <at> draigBrady.com>
To: Ludovic Courtès <ludo <at> gnu.org>,  Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 21460 <at> debbugs.gnu.org, bug-guix <at> gnu.org
Subject: bug#21460: Race condition in tests/tail-2/assert.sh
Date: Fri, 11 Sep 2015 23:49:58 +0100

On 11/09/15 21:55, Ludovic Courtès wrote:
> Paul Eggert <eggert <at> cs.ucla.edu> skribis:
> 
>> Ludovic Courtès wrote:
>>> I think the problem happens when ‘tail’ opens ‘foo’ right in between of
>>> the two notifications: ‘foo’ is still there, and so ‘tail’ doesn’t
>>> report anything.
>>>
>>> Does that make sense?
>>
>> Yes, though if the link count is indeed zero, I'm surprised that
>> 'tail' can open the file -- that sounds like a bug in the kernel.
> 
> Attached is a reproducer; just run it in a loop for a couple of seconds:
> 
> --8<---------------cut here---------------start------------->8---
> $ while ./a.out ; do : ; done
> funny, errno = Success, nlink = 0
> Aborted (core dumped)
> --8<---------------cut here---------------end--------------->8---
> 
> I’m not sure if that’s a kernel bug.  Strictly speaking, inotify works
> as expected: we get a notification for nlink--, which doesn’t mean the
> file has vanished.

Interesting.  It does seem that the IN_ATTRIB is sent before the st_nlink--
takes effect?  That could be a bug.  Or it could be a dcache coherency
issue where the name still references the st_nlink==0 inode.

Note recheck() just open() and close() the file in this case,
but since it doesn't close() the original fd, then there will be
no IN_DELETE_SELF event.

If the above kernel behavior can be explained and is acceptable,
I suppose we could augment recheck() with something like:

diff --git a/src/tail.c b/src/tail.c
index f916d74..e9d5337 100644
--- a/src/tail.c
+++ b/src/tail.c
@@ -1046,6 +1046,18 @@ recheck (struct File_spec *f, bool blocking)
       close_fd (f->fd, pretty_name (f));

     }
+  else if (new_stats.st_nlink == 0) /* XXX: what about multi-linked files.  */
+    {
+      /* It was seen on Linux that a file could be opened
+         even though unlinked as the directory entry (cache)
+         is updated after the IN_ATTRIB is sent for the nlink--.  */
+
+      error (0, f->errnum, _("%s has become inaccessible"),
+             quote (pretty_name (f)));
+
+      close_fd (fd, pretty_name (f));
+      close_fd (f->fd, pretty_name (f));
+      f->fd = -1;
   else
     {
       /* No changes detected, so close new fd.  */

> The conclusion for ‘tail’ would be to wait for the IN_DELETE_SELF event
> before considering the file to be gone.  WDYT?

As mentioned above, tail references the file until it can't open it,
so the IN_DELETE_SELF is only generated upon the close_fd(f->fd) above.

thanks,
Pádraig.

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