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

[Mike Kupfer]

(Adding Bill Wohler, who has a better grasp than I about why MH-E does
some things.)

Matt Armstrong wrote:

> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > I think we should audit all the callers of unlock_buffer and
> > unlock_file, and see if signaling an error there is really the best
> > alternative.
[...]
>       * lisp/mh-e/mh-show.el (mh-clean-msg-header, mh-unvisit-file):
>         hard to say, very old code...
>       * lisp/mh-e/mh-comp.el (mh-read-draft): ditto.

I'm not sure I completely understanding the logic behind those calls to
unlock-buffer, but I'll take a stab at it.

mh-unvisit-file: this function's purpose in life is to disassociate the
current buffer from the underlying file.  (MH-E reuses a per-folder
buffer when displaying a message.)

mh-read-draft: this case is similar to mh-unvisit-file.  In the relevant
code path, the user's configuration allows for only a single "draft"
buffer, and MH-E is cleaning up a potentially old draft buffer for
reuse.

mh-clean-msg-header: this one confuses me.  mh-clean-msg-header edits
the current buffer to reflect the user's preferences for which mail
headers to display.  But there are several places where
mh-clean-msg-header is used, and I haven't figured out if the buffer
ever has an associated file when mh-clean-msg-header is called.  So I'm
not even sure the call to unlock-buffer actually has any value here.

But to Eli's question, I think a signal is fine for MH-E if the lockfile
can't be removed for some reason.  An uncaught signal could leave the
current buffer in an odd state, but the user can simply kill the buffer
and retry whatever operation she had attempted.  Or if the buffer has
something that is worth saving, the user can attempt to save the buffer
somewhere, perhaps a different filesystem (e.g., if the original
filesystem went read-only due to the OS detecting a problem).

I don't understand the proposal for unlock-buffer (or something under
it) to prompt the user.  IIUC, the proposal is for a prompt like

> /tmp/x/y lock is invalid; assume unlocked? (yes or no)

I assume that if the user responds with "yes", unlock-buffer returns and
the caller is none the wiser.  If the user responds with "no", what
happens?

mike