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From: Sean McAfee <eefacm@gmail.com>
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Subject: Processing a file that may already be open
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I'd like to be able to write a routine that opens a file and reads it,
and potentially modifies its contents and saves it back to disk.  The
slightly tricky bit is that if there are any open buffers visiting that
file, they should be updated immediately.  (I don't want to modify the
file behind any buffers' backs, that is.)

I've written the following macro to encapsulate what seems to be the
necessary logic:

(defmacro with-visited-file (path &rest body)
  (let ((retain (gensym))
        (buffer (gensym)))
    `(let* ((,retain (find-buffer-visiting ,path))
            (,buffer (find-file ,path)))
       (unwind-protect
           (save-excursion
             (save-restriction
               (widen)
               (beginning-of-buffer)
               ,@body))
         (or ,retain (kill-buffer ,buffer))))))

It seems to work okay, but are there any lurking gotchas?  Or, is there
any better approach?