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

[Ivan Shmakov <ivan@siamics.net>]

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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

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--- 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