Preserve permissions after backup?

Preserve permissions after backup?

[Kai Großjohann]

A Tramp user is reporting that the permissions of a root-owned file
change after he edits it with Tramp (the su method).  I'm guessing
that's because Tramp does backup by renaming and creates a new file
with default permissions.

Is there some machinery in Emacs which is designed to prevent things
like this from happening?  I guess there must be a bug in Tramp which
prevents this machinery from working.

I investigated the backup-by-copying variables but these don't talk
about the permissions, only the owner and group.

Or is backup-by-copying the right approach and I (and the user) just
need to make sure that it is used for the file(s) in question?
-- 
Ambibibentists unite!

Re: Preserve permissions after backup?

[Eli Zaretskii]

On Sun, 26 Jan 2003, Kai =?iso-8859-1?q?Gro=DFjohann?= wrote:

> A Tramp user is reporting that the permissions of a root-owned file
> change after he edits it with Tramp (the su method).  I'm guessing
> that's because Tramp does backup by renaming and creates a new file
> with default permissions.
> 
> Is there some machinery in Emacs which is designed to prevent things
> like this from happening?

I think that machinery is in this line of files.el:basic-save-buffer:

   (set-file-modes buffer-file-name setmodes)

[and similar lines elsewhere in basic-save-buffer's subroutines].

Perhaps Tramp sabotaged some of the code that tells Emacs there's a need 
to set file's permissions, or maybe set-file-modes itself fails under 
Tramp?

Re: Preserve permissions after backup?

[Richard Stallman]

    A Tramp user is reporting that the permissions of a root-owned file
    change after he edits it with Tramp (the su method).  I'm guessing
    that's because Tramp does backup by renaming and creates a new file
    with default permissions.

    Is there some machinery in Emacs which is designed to prevent things
    like this from happening?

Yes.  The functions to save the file are supposed to record the permissions
of the old file and apply them to the new file.  This must
be malfunctioning with Tramp.