bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

[Duncan Greatwood]

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While editing the ~/.emacs file in emacs, the machine rebooted (kernel
panic I believe). This left a lock file behind.

The ~/.emacs file is actually a softlink:
.emacs -> /Users/<username>/Dropbox/Documents/Projects/emacs/dotemacs
The fact that it's a softlink may or may not be relevant.

In ~/Dropbox/Documents/Projects/emacs/, there was a file: .#dotemacs

After the reboot, I started emacs, and continued with further edits to
.emacs. Upon saving .emacs, had the following warning:
⛔ Warning (unlock-file): Unlocking file: Invalid
argument, ~/Dropbox/Documents/Projects/emacs/dotemacs, ignored

As per the warning, the save was nonetheless successful.

Invoking file-locked-p directly, I saw the same error, and the following
debug info:
Debugger entered--Lisp error: (file-error "Testing file lock" "Invalid
argument" "~/Dropbox/Documents/Projects/emacs/dotemacs")
  file-locked-p("~/Dropbox/Documents/Projects/emacs/dotemacs")
  eval((file-locked-p "~/Dropbox/Documents/Projects/emacs/dotemacs") t)
  eval-expression((file-locked-p
"~/Dropbox/Documents/Projects/emacs/dotemacs") nil nil 127)
  funcall-interactively(eval-expression (file-locked-p
"~/Dropbox/Documents/Projects/emacs/dotemacs") nil nil 127)
  call-interactively(eval-expression nil nil)
  command-execute(eval-expression)

After removing the lock file manually, the warning on save goes away:
rm ~/Dropbox/Documents/Projects/emacs/.#dotemacs

Access flags on dotemacs file are as follows:
-rw-r--r--@  1 dgreatwood  staff  133806 May 15 16:01 dotemacs
lrwxr-xr-x  1 dgreatwood  staff  59 Dec  6  2015 .emacs -> ...

Thanks.
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bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

[Eli Zaretskii]

> From: Duncan Greatwood <dgreatwood@gmail.com>
> Date: Wed, 15 May 2024 17:53:05 -0700
> 
> While editing the ~/.emacs file in emacs, the machine rebooted (kernel panic I believe). This left a lock file
> behind.
> 
> The ~/.emacs file is actually a softlink:
> .emacs -> /Users/<username>/Dropbox/Documents/Projects/emacs/dotemacs
> The fact that it's a softlink may or may not be relevant.
> 
> In ~/Dropbox/Documents/Projects/emacs/, there was a file: .#dotemacs
> 
> After the reboot, I started emacs, and continued with further edits to .emacs. Upon saving .emacs, had the
> following warning:
> ⛔ Warning (unlock-file): Unlocking file: Invalid argument, ~/Dropbox/Documents/Projects/emacs/dotemacs,
> ignored
> 
> As per the warning, the save was nonetheless successful.
> 
> Invoking file-locked-p directly, I saw the same error, and the following debug info:
> Debugger entered--Lisp error: (file-error "Testing file lock" "Invalid argument"
> "~/Dropbox/Documents/Projects/emacs/dotemacs")
>   file-locked-p("~/Dropbox/Documents/Projects/emacs/dotemacs")
>   eval((file-locked-p "~/Dropbox/Documents/Projects/emacs/dotemacs") t)
>   eval-expression((file-locked-p "~/Dropbox/Documents/Projects/emacs/dotemacs") nil nil 127)
>   funcall-interactively(eval-expression (file-locked-p "~/Dropbox/Documents/Projects/emacs/dotemacs") nil nil
> 127)
>   call-interactively(eval-expression nil nil)
>   command-execute(eval-expression)
> 
> After removing the lock file manually, the warning on save goes away:
> rm ~/Dropbox/Documents/Projects/emacs/.#dotemacs
> 
> Access flags on dotemacs file are as follows:
> -rw-r--r--@  1 dgreatwood  staff  133806 May 15 16:01 dotemacs
> lrwxr-xr-x  1 dgreatwood  staff  59 Dec  6  2015 .emacs -> ...

Thanks.

However, this doesn't provide enough information to investigate the
issue and its causes.  If you can reproduce the problem, please tell
what does

  ls -l ~/Dropbox/Documents/Projects/emacs/.#dotemacs

produce when you see the warnings, because that is the immediate
trigger for the "invalid argument".

bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

[Duncan Greatwood]

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> If you can reproduce the problem, please tell what does
>
>   ls -l ~/Dropbox/Documents/Projects/emacs/.#dotemacs
>
> produce when you see the warnings

As follows:
$ ls -l ~/Dropbox/Documents/Projects/emacs/.#dotemacs
-rw-r--r--@ 1 username  staff  0 May 16 07:13
/Users/username/Dropbox/Documents/Projects/emacs/.#dotemacs


On Thu, May 16, 2024 at 1:43 AM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Duncan Greatwood <dgreatwood@gmail.com>
> > Date: Wed, 15 May 2024 17:53:05 -0700
> >
> > While editing the ~/.emacs file in emacs, the machine rebooted (kernel
> panic I believe). This left a lock file
> > behind.
> >
> > The ~/.emacs file is actually a softlink:
> > .emacs -> /Users/<username>/Dropbox/Documents/Projects/emacs/dotemacs
> > The fact that it's a softlink may or may not be relevant.
> >
> > In ~/Dropbox/Documents/Projects/emacs/, there was a file: .#dotemacs
> >
> > After the reboot, I started emacs, and continued with further edits to
> .emacs. Upon saving .emacs, had the
> > following warning:
> > ⛔ Warning (unlock-file): Unlocking file: Invalid argument,
> ~/Dropbox/Documents/Projects/emacs/dotemacs,
> > ignored
> >
> > As per the warning, the save was nonetheless successful.
> >
> > Invoking file-locked-p directly, I saw the same error, and the following
> debug info:
> > Debugger entered--Lisp error: (file-error "Testing file lock" "Invalid
> argument"
> > "~/Dropbox/Documents/Projects/emacs/dotemacs")
> >   file-locked-p("~/Dropbox/Documents/Projects/emacs/dotemacs")
> >   eval((file-locked-p "~/Dropbox/Documents/Projects/emacs/dotemacs") t)
> >   eval-expression((file-locked-p
> "~/Dropbox/Documents/Projects/emacs/dotemacs") nil nil 127)
> >   funcall-interactively(eval-expression (file-locked-p
> "~/Dropbox/Documents/Projects/emacs/dotemacs") nil nil
> > 127)
> >   call-interactively(eval-expression nil nil)
> >   command-execute(eval-expression)
> >
> > After removing the lock file manually, the warning on save goes away:
> > rm ~/Dropbox/Documents/Projects/emacs/.#dotemacs
> >
> > Access flags on dotemacs file are as follows:
> > -rw-r--r--@  1 dgreatwood  staff  133806 May 15 16:01 dotemacs
> > lrwxr-xr-x  1 dgreatwood  staff  59 Dec  6  2015 .emacs -> ...
>
> Thanks.
>
> However, this doesn't provide enough information to investigate the
> issue and its causes.  If you can reproduce the problem, please tell
> what does
>
>   ls -l ~/Dropbox/Documents/Projects/emacs/.#dotemacs
>
> produce when you see the warnings, because that is the immediate
> trigger for the "invalid argument".
>

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bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

[Eli Zaretskii]

> From: Duncan Greatwood <dgreatwood@gmail.com>
> Date: Thu, 16 May 2024 07:17:59 -0700
> Cc: 70973@debbugs.gnu.org
> 
> > If you can reproduce the problem, please tell what does
> > 
> >   ls -l ~/Dropbox/Documents/Projects/emacs/.#dotemacs
> > 
> > produce when you see the warnings
> 
> As follows:
> $ ls -l ~/Dropbox/Documents/Projects/emacs/.#dotemacs
> -rw-r--r--@ 1 username  staff  0 May 16 07:13
> /Users/username/Dropbox/Documents/Projects/emacs/.#dotemacs

Sorry, that doesn't help.  I thought "ls -l" will show the target of
the symlink, but maybe it doesn't on macOS?  In that case, you should
be able to use the Emacs function file-symlink-p: when called with the
lock file as its argument, it should return the target of the symlink
as a string.

bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

[Duncan Greatwood]

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> I thought "ls -l" will show the target of
> the symlink, but maybe it doesn't on macOS?
It does indeed do that, even on macOS

However, it is the .emacs that is a symlink.

$ ls -l ~/.emacs
lrwxr-xr-x  1 username  staff  59 Dec  6  2015 .emacs ->
/Users/username/Dropbox/Documents/Projects/emacs/dotemacs

The lock file is not a link.

On Thu, May 16, 2024 at 8:47 AM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Duncan Greatwood <dgreatwood@gmail.com>
> > Date: Thu, 16 May 2024 07:17:59 -0700
> > Cc: 70973@debbugs.gnu.org
> >
> > > If you can reproduce the problem, please tell what does
> > >
> > >   ls -l ~/Dropbox/Documents/Projects/emacs/.#dotemacs
> > >
> > > produce when you see the warnings
> >
> > As follows:
> > $ ls -l ~/Dropbox/Documents/Projects/emacs/.#dotemacs
> > -rw-r--r--@ 1 username  staff  0 May 16 07:13
> > /Users/username/Dropbox/Documents/Projects/emacs/.#dotemacs
>
> Sorry, that doesn't help.  I thought "ls -l" will show the target of
> the symlink, but maybe it doesn't on macOS?  In that case, you should
> be able to use the Emacs function file-symlink-p: when called with the
> lock file as its argument, it should return the target of the symlink
> as a string.
>

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bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

[Eli Zaretskii]

> From: Duncan Greatwood <dgreatwood@gmail.com>
> Date: Thu, 16 May 2024 08:55:17 -0700
> Cc: 70973@debbugs.gnu.org
> 
> > I thought "ls -l" will show the target of
> > the symlink, but maybe it doesn't on macOS?
> It does indeed do that, even on macOS
> 
> However, it is the .emacs that is a symlink.
> 
> $ ls -l ~/.emacs
> lrwxr-xr-x  1 username  staff  59 Dec  6  2015 .emacs ->
> /Users/username/Dropbox/Documents/Projects/emacs/dotemacs
> 
> The lock file is not a link.

Is that normal on macOS?  Or is that something specific to DropBox?
If you edit a file elsewhere on your system, does Emacs create lock
files that are symlinks?

bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

[Duncan Greatwood]

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AFAIK, there is nothing about the symlink that is macOS or DropBox specific.

Again, ~/.emacs is a symlink to the file in the subfolder of ~/Dropbox.

The lock file is not a symlink.

Emacs does not create lock files that are symlinks AFAIK.

On Thu, May 16, 2024 at 9:09 AM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Duncan Greatwood <dgreatwood@gmail.com>
> > Date: Thu, 16 May 2024 08:55:17 -0700
> > Cc: 70973@debbugs.gnu.org
> >
> > > I thought "ls -l" will show the target of
> > > the symlink, but maybe it doesn't on macOS?
> > It does indeed do that, even on macOS
> >
> > However, it is the .emacs that is a symlink.
> >
> > $ ls -l ~/.emacs
> > lrwxr-xr-x  1 username  staff  59 Dec  6  2015 .emacs ->
> > /Users/username/Dropbox/Documents/Projects/emacs/dotemacs
> >
> > The lock file is not a link.
>
> Is that normal on macOS?  Or is that something specific to DropBox?
> If you edit a file elsewhere on your system, does Emacs create lock
> files that are symlinks?
>

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bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

[Eli Zaretskii]

> From: Duncan Greatwood <dgreatwood@gmail.com>
> Date: Thu, 16 May 2024 09:20:46 -0700
> Cc: 70973@debbugs.gnu.org
> 
> AFAIK, there is nothing about the symlink that is macOS or DropBox specific.
> 
> Again, ~/.emacs is a symlink to the file in the subfolder of ~/Dropbox.
> 
> The lock file is not a symlink.
> 
> Emacs does not create lock files that are symlinks AFAIK.

That is not true.  lock files are normally dangling symlinks,
i.e. their target does not exist.  On a few systems where lock files
are not symlinks (I knew about only one: MS-Windows), lock files are
regular files, but then they are not empty.  And your reports indicate
that it is a regular and empty file:

> As follows:
> $ ls -l ~/Dropbox/Documents/Projects/emacs/.#dotemacs
> -rw-r--r--@ 1 username  staff  0 May 16 07:13 /Users/username/Dropbox/Documents/Projects/emacs/.#dotemacs

This is unusual, because it means the information that a lock file
should record: the user and the process ID that locked the file -- is
not recorded anywhere.  It is usually recorded either in the name of
the symlink's target or (if the lock file is a regular file) in the
file's contents.

So something here is not "normal".  If indeed on macOS lock files are
not symlinks, they should be regular files which are not empty.  If
you could step with a debugger through the code of the C function
create_lock_file and see what happens there when Emacs locks a file
you edit, we could make some progress in investigating this bug.

bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

[Duncan Greatwood]

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>> Emacs does not create lock files that are symlinks AFAIK.
>That is not true.  lock files are normally dangling symlinks,
>i.e. their target does not exist.

Ah, I see.

I just tried editing a different file, client.cc, causing a lockfile to be
created. Sure enough, just as you say, that lockfile is a dangling symlink:
ls -al .#client.cc
lrwxr-xr-x@ 1 username  staff  40 May 16 11:39 .#client.cc ->
username@DMG-MB-Air-15-2024.local.3071

Then, returning to editing ~/.emacs (which, being a symlink, is actually
editing ~/Dropbox/Documents/Projects/emacs/dotemacs).
Again, this creates a dangling symlink as you would expect:
ls -l .#dotemacs
lrwxr-xr-x@ 1 username  staff  40 May 16 11:43 .#dotemacs ->
username@DMG-MB-Air-15-2024.local.3071

At this point, I tried doing a hard reboot (power cycle) to simulate the
kernel crash I mentioned before. But (not surprisingly) after the reboot
the lock file still looks as you would expect.
ls -l .#dotemacs
lrwxr-xr-x@ 1 username  staff  40 May 16 11:43 .#dotemacs ->
username@DMG-MB-Air-15-2024.local.3071

Noting that, in the bad case, the lock file was not only *not* a dangling
symlink as it should be, but was also of zero size, I would speculate that
the kernel crash happened right as emacs was part way through writing the
lockfile to disk.

Taking a quick look at the emacs source, the C function create_lock_file
calls emacs_symlink which in turn calls the OS function symlink.

The OS function symlink is commonly assumed to be atomic I believe (e.g.
see https://rcrowley.org/2010/01/06/things-unix-can-do-atomically.html).
However in this case I would suspect that *the kernel crash terminated the
symlink write before it could fully complete*.

To fix, perhaps emacs needs to check purported lock files and handle the
case where they are not symlinks and/or are of zero size, avoiding the need
to remove the partially-written lock file manually.




On Thu, May 16, 2024 at 11:18 AM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Duncan Greatwood <dgreatwood@gmail.com>
> > Date: Thu, 16 May 2024 09:20:46 -0700
> > Cc: 70973@debbugs.gnu.org
> >
> > AFAIK, there is nothing about the symlink that is macOS or DropBox
> specific.
> >
> > Again, ~/.emacs is a symlink to the file in the subfolder of ~/Dropbox.
> >
> > The lock file is not a symlink.
> >
> > Emacs does not create lock files that are symlinks AFAIK.
>
> That is not true.  lock files are normally dangling symlinks,
> i.e. their target does not exist.  On a few systems where lock files
> are not symlinks (I knew about only one: MS-Windows), lock files are
> regular files, but then they are not empty.  And your reports indicate
> that it is a regular and empty file:
>
> > As follows:
> > $ ls -l ~/Dropbox/Documents/Projects/emacs/.#dotemacs
> > -rw-r--r--@ 1 username  staff  0 May 16 07:13
> /Users/username/Dropbox/Documents/Projects/emacs/.#dotemacs
>
> This is unusual, because it means the information that a lock file
> should record: the user and the process ID that locked the file -- is
> not recorded anywhere.  It is usually recorded either in the name of
> the symlink's target or (if the lock file is a regular file) in the
> file's contents.
>
> So something here is not "normal".  If indeed on macOS lock files are
> not symlinks, they should be regular files which are not empty.  If
> you could step with a debugger through the code of the C function
> create_lock_file and see what happens there when Emacs locks a file
> you edit, we could make some progress in investigating this bug.
>

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bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

[Eli Zaretskii]

> From: Duncan Greatwood <dgreatwood@gmail.com>
> Date: Thu, 16 May 2024 12:27:05 -0700
> Cc: 70973@debbugs.gnu.org
> 
> I just tried editing a different file, client.cc, causing a lockfile to be created. Sure enough, just as you say, that
> lockfile is a dangling symlink:
> ls -al .#client.cc
> lrwxr-xr-x@ 1 username  staff  40 May 16 11:39 .#client.cc -> username@DMG-MB-Air-15-2024.local.3071
> 
> Then, returning to editing ~/.emacs (which, being a symlink, is actually editing ~
> /Dropbox/Documents/Projects/emacs/dotemacs).
> Again, this creates a dangling symlink as you would expect:
> ls -l .#dotemacs
> lrwxr-xr-x@ 1 username  staff  40 May 16 11:43 .#dotemacs -> username@DMG-MB-Air-15-2024.local.3071
> 
> At this point, I tried doing a hard reboot (power cycle) to simulate the kernel crash I mentioned before. But
> (not surprisingly) after the reboot the lock file still looks as you would expect.
> ls -l .#dotemacs
> lrwxr-xr-x@ 1 username  staff  40 May 16 11:43 .#dotemacs -> username@DMG-MB-Air-15-2024.local.3071
> 
> Noting that, in the bad case, the lock file was not only not a dangling symlink as it should be, but was also of
> zero size, I would speculate that the kernel crash happened right as emacs was part way through writing the
> lockfile to disk.
> 
> Taking a quick look at the emacs source, the C function create_lock_file calls emacs_symlink which in turn
> calls the OS function symlink. 
> 
> The OS function symlink is commonly assumed to be atomic I believe (e.g. see
> https://rcrowley.org/2010/01/06/things-unix-can-do-atomically.html). However in this case I would suspect that
> the kernel crash terminated the symlink write before it could fully complete.
> 
> To fix, perhaps emacs needs to check purported lock files and handle the case where they are not symlinks
> and/or are of zero size, avoiding the need to remove the partially-written lock file manually.

I'm not sure we should silently sweep these rare and special cases
under the carpet.  The warning is just a warning, and manually
deleting the lock file fixes even that.

So I'm not sure we should do anything here, as long as the conclusion
is that this happened due to a system crash in an opportune moment.

bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

[Duncan Greatwood]

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I would agree that the issue was likely due to a system crash at an
inopportune moment.

Of course, if emacs did detect a bad lock file, it could prompt the user
for confirmation before removing / replacing it. No need to be silent.

The status quo, requiring a user to find and remove the lock file manually,
seems burdensome to my eye, albeit the issue is presumably rare. But
certainly it is a matter of taste.



On Thu, May 16, 2024 at 12:52 PM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Duncan Greatwood <dgreatwood@gmail.com>
> > Date: Thu, 16 May 2024 12:27:05 -0700
> > Cc: 70973@debbugs.gnu.org
> >
> > I just tried editing a different file, client.cc, causing a lockfile to
> be created. Sure enough, just as you say, that
> > lockfile is a dangling symlink:
> > ls -al .#client.cc
> > lrwxr-xr-x@ 1 username  staff  40 May 16 11:39 .#client.cc ->
> username@DMG-MB-Air-15-2024.local.3071
> >
> > Then, returning to editing ~/.emacs (which, being a symlink, is actually
> editing ~
> > /Dropbox/Documents/Projects/emacs/dotemacs).
> > Again, this creates a dangling symlink as you would expect:
> > ls -l .#dotemacs
> > lrwxr-xr-x@ 1 username  staff  40 May 16 11:43 .#dotemacs ->
> username@DMG-MB-Air-15-2024.local.3071
> >
> > At this point, I tried doing a hard reboot (power cycle) to simulate the
> kernel crash I mentioned before. But
> > (not surprisingly) after the reboot the lock file still looks as you
> would expect.
> > ls -l .#dotemacs
> > lrwxr-xr-x@ 1 username  staff  40 May 16 11:43 .#dotemacs ->
> username@DMG-MB-Air-15-2024.local.3071
> >
> > Noting that, in the bad case, the lock file was not only not a dangling
> symlink as it should be, but was also of
> > zero size, I would speculate that the kernel crash happened right as
> emacs was part way through writing the
> > lockfile to disk.
> >
> > Taking a quick look at the emacs source, the C function create_lock_file
> calls emacs_symlink which in turn
> > calls the OS function symlink.
> >
> > The OS function symlink is commonly assumed to be atomic I believe (e.g.
> see
> > https://rcrowley.org/2010/01/06/things-unix-can-do-atomically.html).
> However in this case I would suspect that
> > the kernel crash terminated the symlink write before it could fully
> complete.
> >
> > To fix, perhaps emacs needs to check purported lock files and handle the
> case where they are not symlinks
> > and/or are of zero size, avoiding the need to remove the
> partially-written lock file manually.
>
> I'm not sure we should silently sweep these rare and special cases
> under the carpet.  The warning is just a warning, and manually
> deleting the lock file fixes even that.
>
> So I'm not sure we should do anything here, as long as the conclusion
> is that this happened due to a system crash in an opportune moment.
>

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bug#70973: 29.1; "Unlocking file: Invalid argument" Warning saving via a softlink with stale file lock

[Eli Zaretskii]

> From: Duncan Greatwood <dgreatwood@gmail.com>
> Date: Thu, 16 May 2024 14:36:37 -0700
> Cc: 70973@debbugs.gnu.org
> 
> I would agree that the issue was likely due to a system crash at an inopportune moment.
> 
> Of course, if emacs did detect a bad lock file, it could prompt the user for confirmation before removing /
> replacing it. No need to be silent.
> 
> The status quo, requiring a user to find and remove the lock file manually, seems burdensome to my eye,
> albeit the issue is presumably rare. But certainly it is a matter of taste.

No further comments, so I'm now closing this bug.