include-fixer: Don't overwrite buffer changes

[phst]

Summary

Raise a signal if the buffer has been modified before replacing it, to avoid overwriting users' changes

[massberg]

looks good to me

[Eugene.Zelenko]

Committed in rL285060. Please specify "Differential revision: <URL>" as last line of commit message.

include-fixer/tool/clang-include-fixer.el

(let-alist context
         (message "Couldn't find header for '%s'"
                  (let-alist (car .QuerySymbolInfos) .RawIdentifier)))
        (t
         ;; Replace the HeaderInfos list by a single header selected by
         ;; the user.
         (clang-include-fixer--select-header context)
         ;; Call clang-include-fixer again to insert the selected header.
         (clang-include-fixer--start
-         #'clang-include-fixer--replace-buffer
+         (let ((old-tick (buffer-chars-modified-tick)))
+           (lambda (stdout)
+             (when (/= old-tick (buffer-chars-modified-tick))
+               ;; Replacing the buffer now would undo the user’s changes.
+               (user-error (concat "The buffer has been changed "
+                                   "before the header could be inserted")))
+             (clang-include-fixer--replace-buffer stdout)))
          (format "-insert-header=%s"
                  (clang-include-fixer--encode-json context)))))))
   nil)
 
 (defun clang-include-fixer--select-header (context)
   "Prompt the user for a header if necessary.
 CONTEXT must be a clang-include-fixer context object in
 association list format.  If it contains more than one HeaderInfo