bug#46397: 27.1; Cannot delete buffer pointing to a file in a path that includes a file

[Eli Zaretskii <eliz@gnu.org>]

> From: Matt Armstrong <matt@rfc20.org>
> Cc: Paul Eggert <eggert@cs.ucla.edu>,  gmatta@gmail.com,
>   46397@debbugs.gnu.org,  craven@gmx.net
> Date: Wed, 10 Feb 2021 14:39:36 -0800
> 
> > We should not ignore these errors, we should ask the user what to do
> > about them.  The user can tell us the error can be ignored, but we
> > should not decide that without asking.
> 
> I think Paul's commit is a good one. I'll try to explain why.
> 
> The commit does not silently ignore ENOTDIR. Instead, it is explicitly
> handles that particular error code it in a way that honors the lock file
> API contract.

I said "silently" because the user is left unaware of what Emacs did
in this case.  We don't even show a warning or any other informative
message.

> In this case, Paul's commit changes the current_lock_owner() function
> such that it returns zero upon ENOTDIR. The caller must interpret the
> zero return as meaning "at the time current_lock_owner() was called,
> nobody owned the lock file...or the lock file was obsolete."
> 
> ENOTDIR has a specific meaning that we can rely on. Both ENOENT and
> ENOTDIR imply that the file was definitely not on disk at the time of
> the call. Because of this, current_lock_owner() can safely conclude that
> nobody owned the lock.

"Definitely"? "safely"?  How do you arrive at that conclusion?

The Posix spec of 'unlink' says:

  [ENOTDIR]
      A component of the path prefix names an existing file that is
      neither a directory nor a symbolic link to a directory, or the
      path argument contains at least one non- <slash> character and
      ends with one or more trailing <slash> characters and the last
      pathname component names an existing file that is neither a
      directory nor a symbolic link to a directory.

It doesn't even say which component of the file name is not a
directory, nor does it distinguish between the two different use cases
that could cause ENOTDIR.  How can current_lock_owner decide, on these
shaky grounds alone, that nobody owned the lock, let alone do that
'safely"?

My point is that the values of errno provide too little information
for a safe decision here, one that couldn't possibly be wrong.  It
could be the scenario that triggered this bug report, but it could be
something entirely different.  We just don't know enough, and any
assumptions in this situation can definitely err.

Which is why I still think that we need to bring the user into the
loop.  Users will know what could or did happen, and even if they
don't, they are in charge of resolving the situation.  These problems
are rare enough to not make prompting the user for the appropriate
action an annoyance, so there's no good reason not to do so.  Doing so
will, as a nice bonus, also solve similar problems for any other value
of errno, once and for all.