bug#39344: 28.0.50; Gnus: Selecting deleted buffer

[Robert Pluim]

>>>>> On Wed, 29 Jan 2020 14:33:20 +0100, Stephen Berman 
>>>>> <stephen.berman@gmx.net> said:

    Stephen> On Wed, 29 Jan 2020 14:25:10 +0100 Robert Pluim <rpluim@gmail.com> 
wrote:
    >>>>>>> On Wed, 29 Jan 2020 14:07:16 +0100, Stephen Berman
    >> <stephen.berman@gmx.net> said:
    Stephen> I see this too.  The patch below appears to fix it for me, but I 
don't
    Stephen> know if it's just a workaround.
    >> 
    >> That canʼt be right. 'gnus-buffers' already checks 'buffer-live-p'.

    Stephen> Hm, odd, because when I stepped through gnus-clear-system, the 
error
    Stephen> occurred here:

    Stephen>    ;; Kill Gnus buffers.
    Stephen>    (do-auto-save t)
    Stephen>    (dolist (buffer (gnus-buffers))
    Stephen>      (with-current-buffer buffer   <===

    Stephen> and after adding the buffer-live-p check before that line, the 
error no
    Stephen> longer occurs.

So one of Gnus' buffers is being killed in the middle of that
dolist. The only way I can see that happening is if the same buffer is
in there twice, in which case the following would work as well.

diff --git a/lisp/gnus/gnus.el b/lisp/gnus/gnus.el
index 6df26b4af8..2fc14c5fff 100644
--- a/lisp/gnus/gnus.el
+++ b/lisp/gnus/gnus.el
@@ -671,7 +671,7 @@ gnus-kill-buffer
 
 (defun gnus-buffers ()
   "Return a list of live Gnus buffers."
-  (setq gnus-buffers (seq-filter #'buffer-live-p gnus-buffers)))
+  (setq gnus-buffers (delete-dups (seq-filter #'buffer-live-p gnus-buffers))))
 
 (defalias 'gnus-prune-buffers #'gnus-buffers)