Subject: 28.0.50; Duplicate Edebug instrumentation of lambda form in some cases

From: Philipp Stephani <[email-masked]>
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
 -fsanitize=pointer-subtract''

Configured features:
XPM JPEG TIFF GIF PNG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY INOTIFY
LIBSELINUX GNUTLS FREETYPE HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS GTK3 X11
XDBE XIM MODULES THREADS LIBSYSTEMD JSON PDUMPER

Important settings:
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  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

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Load-path shadows:
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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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