vc-mode-line ancient, root-specific code for buffer-read-only

[Glenn Morris <rgm@gnu.org>]

Hi,

vc-mode-line contains this:

    ;; If the user is root, and the file is not owner-writable,
    ;; then pretend that we can't write it
    ;; even though we can (because root can write anything).
    ;; This way, even root cannot modify a file that isn't locked.
    (and (equal file buffer-file-name)
         (not buffer-read-only)
         (zerop (user-real-uid))
         (zerop (logand (file-modes buffer-file-name) 128))
         (setq buffer-read-only t)))

Some version of this has been present since at least 1996.

It is buggy in that it has a race condition: (file-modes buffer-file-name)
will return nil if the file has been deleted, and logand will error.

Eg this happens if you run make lisp/vc/vc-bzr-tests as root, which is
presumably what prompted Debian to disable one of the tests

https://sources.debian.org/patches/emacs/1:26.1+1-1/0010-Mark-vc-bzr-test-fauilt-bzr-autoloads-as-unstable-fo.patch/


I would like to suggest just removing that vc-mode-line code.
It's for locking VCS, is buggy, and doesn't even make sense.
root cannot "write anything", unless we are concerned with odd cases
like a file being writable by other but not owner. Ie locked files would
be marked read-only for root anyway.