bug#34418: 27.0.50; Flymake adds markup to buffers not specified in `flymake-make-diagnostic'

[Philipp]

1. Create two scratch buffers *a* and *b* (e.g. C-x b *a* RET).

2. Insert some text into the buffers (e.g. "text a" and "text b",
   respectively)

3. Add a trivial Flymake backend to buffer *b*.  E.g., select *b* and
   run M-: with the following code:
   
   (add-hook 'flymake-diagnostic-functions
             (lambda (report-fn &rest _args)
                    (funcall report-fn (list (with-current-buffer "*a*"
                    (flymake-make-diagnostic (current-buffer)
                                             (point-min) (point-max)
                     :error "message"))))))
                     
   Note that this backend adds a diagnostic for buffer *a*, not *b*.
   
4. Enable Flymake mode in buffer *b*.

Buffer *b* will now show a diagnostic, even though it was reported for
buffer *a*.  This should either add a diagnostic for buffer *a* or
signal an error.

See
https://lists.gnu.org/archive/html/help-gnu-emacs/2018-12/msg00170.html
for more context.


In GNU Emacs 27.0.50 (build 37, x86_64-apple-darwin18.2.0, NS appkit-1671.20 
Version 10.14.3 (Build 18D109))
 of 2019-02-10 built on p
Repository revision: 24695e8977c3d048e60d1fe3714b789567c2213c
Repository branch: master
Windowing system distributor 'Apple', version 10.3.1671
System Description:  Mac OS X 10.14.3

Recent messages:
C-+ is undefined
Region has 0 lines, 0 words, and 0 characters.
M-+ is undefined
Use +,-,0 for further adjustment [5 times]
Loading flymake...done
next-line: End of buffer
user-error: End of history; no default available
((lambda (report-fn &rest _args) (funcall report-fn (list ...))) 
flymake-proc-legacy-flymake)
Flymake mode enabled in current buffer [2 times]
mwheel-scroll: Beginning of buffer [4 times]
mwheel-scroll: Beginning of buffer
Configured using:
 'configure --without-xml2 --with-modules --without-pop --with-mailutils
 --enable-gcc-warnings=warn-only --enable-checking=yes
 --enable-check-lisp-object-type 'CFLAGS=-ggdb3 -O0''

Configured features:
IMAGEMAGICK NOTIFY KQUEUE ACL GNUTLS ZLIB TOOLKIT_SCROLL_BARS NS MODULES
THREADS JSON PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: de_DE.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Messages

Minor modes in effect:
  tooltip-mode: t
  global-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
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt bytecomp
byte-compile cconv dired dired-loaddefs format-spec rfc822 mml mml-sec
password-cache epa derived epg epg-config gnus-util rmail rmail-loaddefs
time-date mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev
gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util
mail-prsvr mail-utils cl-macs gv cl-seq flymake-proc flymake easymenu
compile comint ansi-color ring warnings thingatpt cl-loaddefs cl-lib
face-remap elec-pair tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel term/ns-win ns-win ucs-normalize mule-util
term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame 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 minibuffer 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 kqueue cocoa ns lcms2 multi-tty make-network-process
emacs)

Memory information:
((conses 16 56619 10244)
 (symbols 48 6929 1)
 (strings 32 17753 1870)
 (string-bytes 1 600979)
 (vectors 16 11313)
 (vector-slots 8 139126 16778)
 (floats 8 29 130)
 (intervals 56 213 28)
 (buffers 992 14))