GNU bug report logs - #74653
tail --follow=name doesn't exit when using inotify and the file is moved

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Göran Uddeborg <goeran <at> uddeborg.se>
To: bug-coreutils <at> gnu.org
Subject: tail --follow=name doesn't exit when using inotify and the file
 is moved
Date: Mon, 2 Dec 2024 17:19:21 +0100

When using "tail --follow=name", but without "--retry", on a file
supporting inotify, the command doesn't finish if the file is moved to
a new name.

Repeat this way on a local filesystem:

    echo apa > apa
    tail --follow=name apa &
    mv apa bepa

"tail" will print an error message saying "No such file or directory",
but it will continue running. I would expect a following message "no
files remaining" and "tail" to exit.

If the file system doesn't support inotify, if "apa" is on NFS for
example, "tail" exits as expected.

If the file is removed rather than moved ("rm apa") "tail" also exits
as expected.

This is tested on Fedora 41 using

coreutils-9.5-11.fc41.x86_64
kernel-6.11.6-300.fc41.x86_64

Message #8 received at 74653 <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Göran Uddeborg <goeran <at> uddeborg.se>, 74653 <at> debbugs.gnu.org
Subject: Re: bug#74653: tail --follow=name doesn't exit when using inotify and
 the file is moved
Date: Wed, 4 Dec 2024 11:55:29 +0000

On 02/12/2024 16:19, Göran Uddeborg wrote:
> When using "tail --follow=name", but without "--retry", on a file
> supporting inotify, the command doesn't finish if the file is moved to
> a new name.
> 
> Repeat this way on a local filesystem:
> 
>      echo apa > apa
>      tail --follow=name apa &
>      mv apa bepa
> 
> "tail" will print an error message saying "No such file or directory",
> but it will continue running. I would expect a following message "no
> files remaining" and "tail" to exit.
> 
> If the file system doesn't support inotify, if "apa" is on NFS for
> example, "tail" exits as expected.
> 
> If the file is removed rather than moved ("rm apa") "tail" also exits
> as expected.
> 
> This is tested on Fedora 41 using
> 
> coreutils-9.5-11.fc41.x86_64
> kernel-6.11.6-300.fc41.x86_64

Yes that is a bug.
The info docs state this:

"In that [renamed file] case, use ‘--follow=name’ to track the named file,
 perhaps by reopening it periodically to see if it has been removed
 and recreated by some other program.  Note that the inotify-based
 implementation handles this case without the need for any periodic reopening."

But that description only alludes to the implementation,
and not any functional difference I would say.
I.e. inotify should not imply --retry.

Interestingly if you `rm depa` before recreating "apa" in the example above,
then tail does exit.  Also tail will exit immediately if "apa" is not present
at startup.  Also tail will exit if you move "apa" to a different directory.
All of those existing behaviors gives us latitude to adjust this behavior I think,
so that tail does in fact exit for the case above with inotify.

BTW one can test the non-inotify behavior on any file system
with the undocumented ---disable-inotify option.

thanks,
Pádraig

Message #13 received at 74653-done <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Göran Uddeborg <goeran <at> uddeborg.se>,
 74653-done <at> debbugs.gnu.org
Subject: Re: bug#74653: tail --follow=name doesn't exit when using inotify and
 the file is moved
Date: Thu, 5 Dec 2024 12:55:29 +0000

[Message part 1 (text/plain, inline)]

On 04/12/2024 11:55, Pádraig Brady wrote:
> On 02/12/2024 16:19, Göran Uddeborg wrote:
>> When using "tail --follow=name", but without "--retry", on a file
>> supporting inotify, the command doesn't finish if the file is moved to
>> a new name.
>>
>> Repeat this way on a local filesystem:
>>
>>       echo apa > apa
>>       tail --follow=name apa &
>>       mv apa bepa
>>
>> "tail" will print an error message saying "No such file or directory",
>> but it will continue running. I would expect a following message "no
>> files remaining" and "tail" to exit.
>>
>> If the file system doesn't support inotify, if "apa" is on NFS for
>> example, "tail" exits as expected.
>>
>> If the file is removed rather than moved ("rm apa") "tail" also exits
>> as expected.
>>
>> This is tested on Fedora 41 using
>>
>> coreutils-9.5-11.fc41.x86_64
>> kernel-6.11.6-300.fc41.x86_64
> 
> Yes that is a bug.
> The info docs state this:
> 
> "In that [renamed file] case, use ‘--follow=name’ to track the named file,
>    perhaps by reopening it periodically to see if it has been removed
>    and recreated by some other program.  Note that the inotify-based
>    implementation handles this case without the need for any periodic reopening."
> 
> But that description only alludes to the implementation,
> and not any functional difference I would say.
> I.e. inotify should not imply --retry.
> 
> Interestingly if you `rm depa` before recreating "apa" in the example above,
> then tail does exit.  Also tail will exit immediately if "apa" is not present
> at startup.  Also tail will exit if you move "apa" to a different directory.
> All of those existing behaviors gives us latitude to adjust this behavior I think,
> so that tail does in fact exit for the case above with inotify.
> 
> BTW one can test the non-inotify behavior on any file system
> with the undocumented ---disable-inotify option.

I'll push the attached later to fix this.

Marking this as done.

thanks,
Pádraig

[tail-unfollow-renames.patch (text/x-patch, attachment)]

Message #16 received at 74653 <at> debbugs.gnu.org (full text, mbox):

From: Göran Uddeborg <goeran <at> uddeborg.se>
To: 74653 <at> debbugs.gnu.org
Subject: bug#74653: closed (Re: bug#74653: tail --follow=name doesn't exit
 when using inotify and the file is moved)
Date: Sat, 7 Dec 2024 19:55:03 +0100

Thank you for a quick follow up and prompt fix of this issue!

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