bug#58601: 29.0.50; Infinite loop in byte-compile--first-symbol-with-pos

[Basil L. Contovounesios]

Given this sample input file:

my.el
Description: application/emacs-lisp

Byte-compiling it as follows hangs indefinitely:
  
  emacs -Q -batch -f batch-byte-compile my.el

While it's hanging, might as well poke it:

  pkill -f 'batch-byte-compile my\.el' -SIGUSR2  

To which it yields the following backtrace:

backtrace.txt
Description: Text document

I.e. the 'pure' compile-time form has a cycle, and the DFS of
byte-compile--first-symbol-with-pos gets lost.  Paraphrasing Stefan,
it's acceptable if the compiler mishandles circular source code, but it
should be able to handle circular data.

I don't know why, but I can reproduce the hang only when the form is inside
ert-deftest+should, and not inside a plain defun.

Perhaps byte-compile--first-symbol-with-pos needs to employ something
like byte-run--ssp-seen?  Or does ERT somehow come into play?

Thanks,

-- 
Basil

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.16.0, Xaw3d scroll bars) of 2022-10-18 built on tia
Repository revision: c2b79d9148f21c3717f1eaa2d37b837b0922b94c
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101004
System Description: Debian GNU/Linux bookworm/sid

Configured using:
 'configure 'CFLAGS=-Og -ggdb3' -C --prefix=/home/blc/.local
 --enable-checking=structs --with-file-notification=yes
 --with-x-toolkit=lucid --with-x'

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

Important settings:
  value of $LANG: en_IE.UTF-8
  value of $XMODIFIERS: @im=ibus
  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
  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 emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg rfc6068
epg-config gnus-util text-property-search time-date subr-x 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 rmc iso-transl tooltip eldoc paren electric
uniquify ediff-hook vc-hooks lisp-float-type elisp-mode 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 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 make-network-process emacs)

Memory information:
((conses 16 36538 7652)
 (symbols 48 5112 0)
 (strings 32 13853 1864)
 (string-bytes 1 382346)
 (vectors 16 9260)
 (vector-slots 8 147212 11667)
 (floats 8 23 25)
 (intervals 56 242 0)
 (buffers 1000 10))