bug#50431: revert-buffer doesn't preserve read-onlyness

[Richard Stallman]

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  > Making revert-buffer preserve user-initiated readedness changes isn't
  > trivial.  We'd need to record whether the buffer is read-only because it
  > was read from a read-only file (and vice versa), and then use that data
  > point in revert-buffer (to see whether we should respect the current
  > readedness state or not).

How about just this: keep buffer-read-only unchanged?

If it differs from the file's permissions, almost certainly
the user changed it and will beglad of revert-buffer preserves it.

The other case is that the file permissions have been changed
since visiting the file.  But users won't be shocked if
revert-buffer fails to change buffer-read-only in that case,
and they will know what to do.


-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)