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bug#42701: 28.0.50; Duplicate Edebug instrumentation of lambda form in s

From: Philipp Stephani
Subject: bug#42701: 28.0.50; Duplicate Edebug instrumentation of lambda form in some cases
Date: Mon, 03 Aug 2020 18:47:28 +0200

Create a file /tmp/edebug.el:

$ cat /tmp/edebug.el 
(defun f () (if-let (x (funcall (lambda (y) 1) 2)) 3 4))

Visit the file:

$ emacs -Q -l subr-x -l edebug /tmp/edebug.el

Now instrument the function `f' using C-u C-M-x.  The *Messages* buffer
will now contain

Edebug: edebug-anon0 [2 times]
Edebug: f

The [2 times] indicates the problem: Edebug has instrumented two
definitions with the same (generated) symbol.  This is a problem when
using Edebug for e.g. coverage instrumentation, as the coverage
information is attached to the symbol itself (as a symbol property), and
duplicate symbols when instrumenting code lead to subtle errors such as
mismatching vector lengths for the position of the breakpoints and the
hit counts.

I'd speculate that this issue is similar to Bug#41988 in that Edebug
defines instrumented symbols even when backtracking later.  In this
case, Edebug backtracks to the legacy (SYM VAL) form, but has already
partially matched the ((SYM VAL)) form, including instrumenting the
lambda form therein.  I guess Edebug should perform some kind of
two-phase instrumentation and instrument subforms only when a form has
been chosen after backtracking.  Since this is somewhat difficult to
implement without rewriting larger parts of Edebug, it might be more
feasible to regenerate anonymous symbols after a failed match.

In GNU Emacs 28.0.50 (build 72, x86_64-pc-linux-gnu, GTK+ Version 3.24.18, 
cairo version 1.16.0)
 of 2020-08-03
Repository revision: 16b7f413a9ff819c374e07ee927c1fd2b4138109
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Debian GNU/Linux rodete

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --enable-gcc-warnings=warn-only
 --enable-gtk-deprecation-warnings --without-pop --with-mailutils
 --enable-checking=all --enable-check-lisp-object-type --with-modules
 'CFLAGS=-O1 -ggdb3 -fno-omit-frame-pointer -fsanitize=address
 -fsanitize=undefined -fsanitize=pointer-compare

Configured features:

Important settings:
  value of $LANG: en_US.utf8
  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
  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.

(shadow sort mail-extr emacsbug message rmc dired dired-loaddefs rfc822
mml easymenu mml-sec epa epg epg-config gnus-util rmail rmail-loaddefs
text-property-search 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 phst skeleton derived edmacro
kmacro pcase ffap thingatpt url url-proxy url-privacy url-expand
url-methods url-history url-cookie url-domsuf url-util url-parse
auth-source cl-seq eieio eieio-core cl-macs eieio-loaddefs
password-cache json map url-vars mailcap subr-x rx gnutls puny seq
byte-opt gv bytecomp byte-compile cconv dbus xml compile comint
ansi-color ring cl-loaddefs cl-lib 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 dynamic-setting system-font-setting
font-render-setting cairo move-toolbar gtk x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 68791 8062)
 (symbols 48 8629 1)
 (strings 32 23741 1958)
 (string-bytes 1 764239)
 (vectors 16 13112)
 (vector-slots 8 166367 5382)
 (floats 8 25 30)
 (intervals 56 221 0)
 (buffers 992 11))

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