bug#69863: 30.0.50; elisp-flymake-byte-compile hangs forever if buffer contains circular list

bug#69863: 30.0.50; elisp-flymake-byte-compile hangs forever if buffer contains circular list

[sbaugh]

1. emacs -Q
2. In *scratch*, M-x flymake-mode
3. In *scratch*, insert this text:
#1=(1 . #1#)
4. M-x proced
5. Observe that there's a Emacs subprocess at 100% CPU which will never finish

The subprocess is running byte-compile-file, which when it sees a
circular list, runs forever at 100% CPU.

In turn this is because macroexpand-all has that behavior when passed a
circular list, e.g. (macroexpand-all '#1=(1 . #1#))

I'm not sure how this could be fixed, maybe elisp-flymake-byte-compile
could have a timeout on its subprocess?  Or maybe byte-compile-file
could invoke read in such a way that it errors if it reads an infinite
structure?  Or maybe macroexpand-all should detect infinite structures
and abort?


In GNU Emacs 30.0.50 (build 3, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.16.0, Xaw3d scroll bars) of 2024-03-17 built on earth
Repository revision: 39239982403f01a37d42d1cd8db0b2ed0b48b50c
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12101007
System Description: Debian GNU/Linux 12 (bookworm)

Configured using:
 'configure -C --with-x-toolkit=lucid'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3
THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XAW3D XDBE XIM
XINPUT2 XPM LUCID ZLIB

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

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-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
  minibuffer-regexp-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr pp flymake project comp-run emacsbug message
mailcap yank-media puny dired dired-loaddefs rfc822 mml mml-sec
password-cache epa derived epg rfc6068 epg-config gnus-util 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-print debug backtrace cus-start cus-load find-func time-date subr-x
cl-extra shortdoc comp-common rx help-fns radix-tree help-mode warnings
icons compile text-property-search comint ansi-osc ansi-color ring
vc-git diff-mode easy-mmode vc-dispatcher cl-loaddefs cl-lib
bug-reference byte-opt gv bytecomp byte-compile thingatpt rmc iso-transl
tooltip cconv eldoc paren electric uniquify ediff-hook vc-hooks
lisp-float-type elisp-mode mwheel term/x-win x-win term/common-win x-dnd
touch-screen tool-bar dnd fontset image regexp-opt fringe tabulated-list
replace newcomment text-mode lisp-mode prog-mode register page tab-bar
menu-bar rfn-eshadow isearch easymenu timer select scroll-bar mouse
jit-lock font-lock syntax font-core term/tty-colors frame minibuffer
nadvice seq simple cl-generic indonesian philippine 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 emoji-zwj charscript charprop
case-table epa-hook jka-cmpr-hook help abbrev obarray oclosure
cl-preloaded button loaddefs theme-loaddefs faces cus-face macroexp
files window text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget keymap hashtable-print-readable backquote
threads dbusbind inotify lcms2 dynamic-setting system-font-setting
font-render-setting cairo x-toolkit xinput2 x multi-tty move-toolbar
make-network-process native-compile emacs)

Memory information:
((conses 16 136447 45220) (symbols 48 9286 0) (strings 32 35565 2968)
 (string-bytes 1 984707) (vectors 16 20041)
 (vector-slots 8 229862 11860) (floats 8 104 356)
 (intervals 56 1154 69) (buffers 984 22))