bug#45650: 28.0.50; nnmaildir discards line count

[Justus-dev]

With the nnmaildir backend, NOV data are created with a line count of
-1, causing the %L summary-line format specifier to show a question
mark.

The culprit is this section in nnmaildir--update-nov in nnmaildir.el:

        (save-restriction
          (if (search-forward "\n\n" nil 'noerror)
              (progn
                (setq nov-mid (count-lines (point) (point-max)))
                (narrow-to-region (point-min) (1- (point))))
            (setq nov-mid 0))
          (goto-char (point-min))
          (delete-char 1)
          (setq nov (nnheader-parse-head t)
                field (or (mail-header-lines nov) 0)))
        (unless (or (zerop field) (nnmaildir--param pgname 'distrust-Lines:))
          (setq nov-mid field))

This code assumes that if no Lines: header is found, mail-header-lines
returns 0.  However, mail-header-lines returns -1 in this case, causing
the correctly-counted number of lines in nov-mid to be overwritten with
-1.

Replacing (zerop field) by (<= field 0) fixes it.

While I think this is indeed a correct immediate fix, this function
should be refactored:

- Why count the number of lines in the message only to discard the
  result if a Lines: header is found afterwards? (Well, fixing this
  won't have much impact: Of the 161594 messages in my nnmaildir
  folders, only 2982 contain a Lines: header.)

- [unrelated] Above the quoted code section, the function checks if the
  NOV data have to be (re)created by parsing the message; if not, it
  raises an exception to return early.  This construct should be
  replaced by a conditional, perhaps splitting out the second half into
  its own function for readability.

I don't think any of the below is significant, except perhaps for the
fact that this problem was not present in Emacs 27.


In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo 
version 1.17.4)
 of 2021-01-03 built on piater
Repository revision: dc5dab469242c40803891518efe9c9f6bd650cea
Repository branch: makepkg
Windowing system distributor 'System Description: Arch Linux

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --mandir=/usr/share/man --with-gameuser=:games
 --with-sound=alsa --with-modules --without-gconf --without-gsettings
 --with-nativecomp --with-pgtk --with-x-toolkit=gtk3 --without-xaw3d
 --without-m17n-flt --with-cairo --with-xwidgets
 --without-compress-install 'CFLAGS=-march=x86-64 -mtune=generic -O2
 -pipe -fno-plt -g -fuse-ld=gold -g -fuse-ld=gold'
 CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GLIB NOTIFY INOTIFY ACL
GNUTLS LIBXML2 FREETYPE HARFBUZZ LIBOTF ZLIB TOOLKIT_SCROLL_BARS GTK3
PGTK XIM MODULES NATIVE_COMP THREADS XWIDGETS LIBSYSTEMD JSON PDUMPER
LCMS2

Important settings:
  value of $LC_COLLATE: de_AT.UTF-8
  value of $LC_MONETARY: de_AT.UTF-8
  value of $LC_TIME: de_AT.UTF-8
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Group

Minor modes in effect:
  semantic-minor-modes-format: ((:eval (if (or semantic-highlight-edits-mode 
semantic-show-unmatched-syntax-mode)  S)))
  gnus-agent-group-mode: t
  gnus-topic-mode: t
  shell-dirtrack-mode: t
  gnus-undo-mode: t
  show-paren-mode: t
  override-global-mode: t
  pdfgrep-mode: t
  pdf-occur-global-minor-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
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/usr/share/emacs/site-lisp/haskell-mode/haskell-decl-scan hides 
/usr/share/emacs/site-lisp/haskell-mode/build-26.1/haskell-decl-scan
/usr/share/emacs/site-lisp/haskell-mode/haskell-debug hides 
/usr/share/emacs/site-lisp/haskell-mode/build-26.1/haskell-debug
/usr/share/emacs/site-lisp/haskell-mode/haskell-customize hides 
/usr/share/emacs/site-lisp/haskell-mode/build-26.1/haskell-customize
/usr/share/emacs/site-lisp/haskell-mode/haskell-completions hides 
/usr/share/emacs/site-lisp/haskell-mode/build-26.1/haskell-completions
/usr/share/emacs/site-lisp/haskell-mode/haskell-complete-module hides 
/usr/share/emacs/site-lisp/haskell-mode/build-26.1/haskell-complete-module
/usr/share/emacs/site-lisp/haskell-mode/haskell-compile hides 
/usr/share/emacs/site-lisp/haskell-mode/build-26.1/haskell-compile
/usr/share/emacs/site-lisp/haskell-mode/haskell-compat hides 
/usr/share/emacs/site-lisp/haskell-mode/build-26.1/haskell-compat
/usr/share/emacs/site-lisp/haskell-mode/haskell-commands hides 
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/usr/share/emacs/site-lisp/haskell-mode/haskell-collapse hides 
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/usr/share/emacs/site-lisp/haskell-mode/haskell-checkers hides 
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/usr/share/emacs/site-lisp/haskell-mode/haskell-cabal hides 
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/usr/share/emacs/site-lisp/haskell-mode/haskell-c2hs hides 
/usr/share/emacs/site-lisp/haskell-mode/build-26.1/haskell-c2hs
/usr/share/emacs/site-lisp/haskell-mode/haskell-align-imports hides 
/usr/share/emacs/site-lisp/haskell-mode/build-26.1/haskell-align-imports
/usr/share/emacs/site-lisp/haskell-mode/ghci-script-mode hides 
/usr/share/emacs/site-lisp/haskell-mode/build-26.1/ghci-script-mode
/usr/share/emacs/site-lisp/haskell-mode/ghc-core hides 
/usr/share/emacs/site-lisp/haskell-mode/build-26.1/ghc-core

Features:
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gnus-bcklg gnus-draft gnus-cite mail-extr gnus-async qp sort gnus-ml
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Memory information:
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 (symbols 48 55146 3)
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