bug#43651: 27.1; compile.el should not parse its own header for errors

[David Landell]

Hi,

I noticed that compilation-mode parses and successfully detect errors
according to set regexp int the compilation buffer header, iow in text
inserted by the mode itself. This is weird since there isn't any error
there to visit.

Reproduce: emacs -Q -l reproduce.el

Expected output:
No error fontification of header line: Compilation started at Sun Sep 27 
11:48:24

Actual output:
Error fontified line: Compilation started at Sun Sep 27 11:48:24

I noticed this in my own package rg.el which is similar to grep.el in
that it reuses compilation mode and has similar regexps for parsing
file name and line number that happens to match the "$CMD started at..."
line. The attached test case somewhat emulates what happens in those
packages. The command output is unrelated though.

Looking at the code in compile.el, my impression is that this would be
taken care of by compilation-filter but for some reason that doesn't happen.

Best regards,
David Landell

reproduce.el
Description: reproduce

In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30, cairo 
version 1.15.10)
 of 2020-09-19 built on lgw01-amd64-054
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Linux Mint 19

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Compilation finished
Making completion list... [2 times]

Configured using:
 'configure --build=x86_64-linux-gnu --prefix=/usr
 '--includedir=${prefix}/include' '--mandir=${prefix}/share/man'
 '--infodir=${prefix}/share/info' --sysconfdir=/etc --localstatedir=/var
 --disable-silent-rules '--libdir=${prefix}/lib/x86_64-linux-gnu'
 '--libexecdir=${prefix}/lib/x86_64-linux-gnu' --disable-maintainer-mode
 --disable-dependency-tracking --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/27.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/27.1/site-lisp:/usr/share/emacs/site-lisp
 --program-suffix=27 --with-modules --with-file-notification=inotify
 --with-mailutils --with-harfbuzz --with-json --with-x=yes
 --with-x-toolkit=gtk3 --with-lcms2 --with-cairo --with-xpm=yes
 --with-gif=yes --with-gnutls=yes --with-jpeg=yes --with-png=yes
 --with-tiff=yes --with-xwidgets 'CFLAGS=-g -O2
 -fdebug-prefix-map=/build/emacs27-YwD3ZV/emacs27-27.1~1.git86d8d76aa3=. 
-fstack-protector-strong
 -Wformat -Werror=format-security -no-pie' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro
 -no-pie''

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS XWIDGETS
LIBSYSTEMD JSON PDUMPER LCMS2 GMP

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

Major mode: Emacs-Lisp

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
compile comint ansi-color ring tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads dbusbind inotify lcms2 dynamic-setting
system-font-setting font-render-setting xwidget-internal cairo
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 49811 10033)
 (symbols 48 6530 1)
 (strings 32 16934 2193)
 (string-bytes 1 560626)
 (vectors 16 10055)
 (vector-slots 8 133282 9060)
 (floats 8 23 180)
 (intervals 56 311 4)
 (buffers 1000 13))