LOCK-BUFFER when two buffer visits the same buffer

LOCK-BUFFER when two buffer visits the same buffer

[Qiantan Hong]

I do lock-buffer in the first buffer, then in the second buffer,
and I killed the second buffer, the lock is gone — even it seems
that buffer one should hold the lock!

Re: LOCK-BUFFER when two buffer visits the same buffer

[Tassilo Horn]

Qiantan Hong <qhong@mit.edu> writes:

> I do lock-buffer in the first buffer, then in the second buffer, and I
> killed the second buffer, the lock is gone — even it seems that buffer
> one should hold the lock!

More details, please.  Do you just call (lock-buffer) or (lock-buffer
"~/file")?  One emacs instance or two?

As I understand the docs, file locks are not associated with a buffer
but the emacs instance and (lock-buffer) is just a shorthand for

  (when (buffer-modified-p)
    (lock-file (buffer-file-name)))

Bye,
Tassilo

Re: LOCK-BUFFER when two buffer visits the same buffer

[Eli Zaretskii]

> From: Qiantan Hong <qhong@mit.edu>
> Date: Sat, 11 Dec 2021 10:00:00 +0000
> Accept-Language: en-US
> 
> I do lock-buffer in the first buffer, then in the second buffer,
> and I killed the second buffer, the lock is gone — even it seems
> that buffer one should hold the lock!

lock-buffer doesn't support what you are evidently trying to do.  Its
intended use is to prevent simultaneous modifications of a file by two
users, or by a single user from two separate instances of Emacs.  You
want reference counting (and probably also locking the file in the
filesystem), and that isn't supported.

Re: LOCK-BUFFER when two buffer visits the same buffer

[Qiantan Hong]

> lock-buffer doesn't support what you are evidently trying to do.  Its
> intended use is to prevent simultaneous modifications of a file by two
> users, or by a single user from two separate instances of Emacs.  You
> want reference counting (and probably also locking the file in the
> filesystem), and that isn't supported.
Figured that out, thanks. I could add reference counting myself,
but it probably doesn’t worth the effort at the moment.
The scenario comes up in the case of recursive transaction,
and guess I’ll just drop support for recursive transaction for now.

>> I do lock-buffer in the first buffer, then in the second buffer, and I
>> killed the second buffer, the lock is gone — even it seems that buffer
>> one should hold the lock!
> 
> More details, please.  Do you just call (lock-buffer) or (lock-buffer
> "~/file")?  One emacs instance or two?
It’s one Emacs instance.
I called (lock-buffer) after I set the buffer-file-truename of a temporary buffer.
In my case, (lock-buffer FILE) seems to make more sense, but I don’t
know how to unlock it. Therefore I just fake it with a temporary buffer
(so I can unlock it by doing (unlock-buffer) in that buffer later, or 
just kill it directly)