bug#35208: 27.0.50; Gnus nndraft error when saving message buffer

[Basil L. Contovounesios]

Trying to save a draft, e.g. as follows:

0. HOME=$(mktemp -d) emacs -Q
1. M-x toggle-debug-on-error RET
2. (setq gnus-select-method '(nnnil)) C-j
3. M-x gnus RET
4. m
5. foo C-x C-s

results in the following error:

--8<---------------cut here---------------start------------->8---
Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p nil)
  nndraft-update-unread-articles()
  run-hooks(after-save-hook)
  basic-save-buffer(t)
  save-buffer(1)
  funcall-interactively(save-buffer 1)
  call-interactively(save-buffer nil nil)
  command-execute(save-buffer)
--8<---------------cut here---------------end--------------->8---

Reevaluating nndraft-update-unread-articles and repeating step (5) gives
the following backtrace:

--8<---------------cut here---------------start------------->8---
Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p nil)
  =(0 nil)
  (if (= 0 (gnus-group-group-unread)) (progn (delete-region (point-at-bol) 
(progn (forward-line 1) (point)))))
  (if (and gnus-permanently-visible-groups (string-match 
gnus-permanently-visible-groups group)) nil (gnus-group-goto-group group) (if 
(= 0 (gnus-group-group-unread)) (progn (delete-region (point-at-bol) (progn 
(forward-line 1) (point))))))
  (while --dolist-tail-- (setq group (car --dolist-tail--)) (if (and 
gnus-permanently-visible-groups (string-match gnus-permanently-visible-groups 
group)) nil (gnus-group-goto-group group) (if (= 0 (gnus-group-group-unread)) 
(progn (delete-region (point-at-bol) (progn (forward-line 1) (point)))))) (setq 
--dolist-tail-- (cdr --dolist-tail--)))
  (let ((--dolist-tail-- groups) group) (while --dolist-tail-- (setq group (car 
--dolist-tail--)) (if (and gnus-permanently-visible-groups (string-match 
gnus-permanently-visible-groups group)) nil (gnus-group-goto-group group) (if 
(= 0 (gnus-group-group-unread)) (progn (delete-region (point-at-bol) (progn 
(forward-line 1) (point)))))) (setq --dolist-tail-- (cdr --dolist-tail--))))
  (save-excursion (let ((--dolist-tail-- groups) group) (while --dolist-tail-- 
(setq group (car --dolist-tail--)) (if (and gnus-permanently-visible-groups 
(string-match gnus-permanently-visible-groups group)) nil 
(gnus-group-goto-group group) (if (= 0 (gnus-group-group-unread)) (progn 
(delete-region (point-at-bol) (progn ... ...))))) (setq --dolist-tail-- (cdr 
--dolist-tail--)))))
  (let* ((groups (mapcar #'(lambda (elem) (gnus-group-prefixed-name (car elem) 
(list ... ""))) (nnmail-get-active))) (gnus-group-marked (copy-sequence 
groups)) (gnus-get-new-news-hook nil) (inhibit-read-only t)) 
(gnus-group-get-new-news-this-group nil t) (save-excursion (let 
((--dolist-tail-- groups) group) (while --dolist-tail-- (setq group (car 
--dolist-tail--)) (if (and gnus-permanently-visible-groups (string-match 
gnus-permanently-visible-groups group)) nil (gnus-group-goto-group group) (if 
(= 0 (gnus-group-group-unread)) (progn (delete-region ... ...)))) (setq 
--dolist-tail-- (cdr --dolist-tail--))))))
  (save-current-buffer (set-buffer gnus-group-buffer) (let* ((groups (mapcar 
#'(lambda (elem) (gnus-group-prefixed-name ... ...)) (nnmail-get-active))) 
(gnus-group-marked (copy-sequence groups)) (gnus-get-new-news-hook nil) 
(inhibit-read-only t)) (gnus-group-get-new-news-this-group nil t) 
(save-excursion (let ((--dolist-tail-- groups) group) (while --dolist-tail-- 
(setq group (car --dolist-tail--)) (if (and gnus-permanently-visible-groups 
(string-match gnus-permanently-visible-groups group)) nil 
(gnus-group-goto-group group) (if (= 0 ...) (progn ...))) (setq --dolist-tail-- 
(cdr --dolist-tail--)))))))
  nndraft-update-unread-articles()
  run-hooks(after-save-hook)
  basic-save-buffer(t)
  save-buffer(1)
  funcall-interactively(save-buffer 1)
  call-interactively(save-buffer nil nil)
  command-execute(save-buffer)
--8<---------------cut here---------------end--------------->8---

I'm temporarily working around this by
changing (zerop (gnus-group-group-unread))
to (eq 0 (gnus-group-group-unread)),
but a larger problem seems to be at play.

Any tips?  Thanks,

-- 
Basil

Gnus v5.13
In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
 of 2019-04-09 built on thunk
Repository revision: 44b306d3510e54432b76724583ea9405f1c90686
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description: Debian GNU/Linux buster/sid

Configured using:
 'configure 'CC=ccache gcc' 'CFLAGS=-O2 -march=native' --config-cache
 --prefix=/home/blc/.local --with-mailutils --with-x-toolkit=lucid
 --with-modules --with-file-notification=yes --with-x'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS
GLIB NOTIFY INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT
LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM MODULES THREADS
LIBSYSTEMD JSON PDUMPER LCMS2 GMP