Fix for bug #39344 (Gnus selecting deleted buffer) incomplete.

[Karl Fogel]

Hi.  I just encountered bug#39344 today (get a "Selecting deleted buffer" error 
when quitting Gnus).  While I was debugging it, coming to the exact same 
initial workaround that Steve Berman came to in the bug ticket, Sam Steingold 
committed this intended fix:

  commit bb3b0990d8505342ae14841144778df9e650ebdc
  Commit:     Sam Steingold <address@hidden>
  CommitDate: Wed Jan 29 11:17:50 2020 -0500
  
      fix bug#39344
      
      * gnus.el (gnus-add-buffer): Use `cl-pushnew' instead of `push' to
        avoid duplicate entries.

I recompiled to get Sam's fix, but the bug is still present.  

After some debugging, I still don't know why the bug remains, though I've 
learned a lot about Gnus buffer management, so that's nice.  My guess is that 
this problem is related to earlier commit 5ee3c37634f, in which many functions 
in Gnus were changed to use `gnus-get-buffer-create' and thus now store the 
created buffer in in `gnus-buffers'.  I did wonder about the fact that 
`gnus-buffers' is both a variable and a function (with only the latter pruning 
out the killed buffers) -- that seems like it could a potential confusion 
source and/or bug source in general, but I don't know if it has anything to do 
with the current bug.

Anyway, before I go deeper, Sam, I thought I'd post this in case your fix is 
working for you and you didn't realize that it isn't working for someone else.  
If you can't reproduce the problem anymore after commit bb3b0990d, then I will 
continue to debug.  However, if you can reproduce it, then I'll assume you are 
working on it (and you surely would do so much more efficiently then I would).

Best regards,
-Karl