bug#18550: eww-history-browse may end up calling eww-restore-history in an arbitrary buffer

[Ivan Shmakov]

Package:  emacs
Severity: normal

        Shortly: go to *eww*, H, C-x 1, C-x b *foo* RET, C-x b RET, RET,
        – the EWW history entry will be rendered in *foo*, /not/ *eww*.

        As of 0ee10aff098b, eww-history-browse uses quit-window to leave
        the history buffer, switching to the “next” buffer – as per
        (buffer-list), – and calls eww-restore-history right there.

        While the user may generally be expected to use M-x eww-history
        (which makes the current *eww* buffer the “next” one), select an
        entry, and M-x eww-history-browse it, – it’s also possible for
        the user to switch buffers arbitrarily between the calls.  This
        way, quit-window may select a completely unrelated buffer, and
        eww-restore-history will then call (erase-buffer) for it.

        (Naturally, that gets much worse if the buffer that the user
        choose to take a look at actually has some important data.)

        As for #16211, my suggestion would be to employ a separate
        buffer-local variable to track the current “browsing” buffer.
        In the patch MIMEd, I assume that it’s named eww-current-buffer
        (just like in the #16211 patch), although eww-browsing-buffer
        seems like a more appropriate name to me now.

        As suggested, the code behaves as before when the variable is
        nil.  It may make sense to raise an error in that case instead,
        so to avoid possible corruption of an arbitrary buffer.

PS.  Beware of the loose line numbers in the diff.

-- 
FSF associate member #7257  http://boycottsystemd.org/  … 3013 B6A0 230E 334A

--- a/lisp/net/eww.el
+++ b/lisp/net/eww.el
@@ -1290,9 +1290,11 @@ defun eww-list-histories ()
   (interactive)
   (when (null eww-history)
     (error "No eww-histories are defined"))
-  (let ((eww-history-trans eww-history))
+  (let ((eww-history-trans eww-history)
+       (buffer (current-buffer)))
     (set-buffer (get-buffer-create "*eww history*"))
     (eww-history-mode)
+    (setq-local eww-current-buffer buffer)
     (let ((inhibit-read-only t)
          (domain-length 0)
          (title-length 0)
@@ -1302,6 +1304,9 @@ defun eww-history-browse ()
     (unless history
       (error "No history on the current line"))
-    (quit-window)
+    (let ((buffer eww-current-buffer))
+      (quit-window)
+      (when buffer
+       (switch-to-buffer buffer)))
     (eww-restore-history history)))
 
 (defvar eww-history-mode-map