\ShowHook of package hook errors if called after the package has already been loaded

[gusbrs]

Brief outline of the bug

An attempt to \ShowHook of a package hook after the package has already been loaded results in error ! File ended while scanning use of \prg_map_break:Nn.

Minimal example showing the bug

\RequirePackage{latexbug}
\documentclass{article}

\usepackage{amsmath}

\ShowHook{package/amsmath/after}

\begin{document}
Hello World.
\end{document}

Log file (required) and possibly PDF file

This is pdfTeX, Version 3.141592653-2.6-1.40.26 (TeX Live 2024) (preloaded format=pdflatex 2024.10.23)  28 OCT 2024 16:22
entering extended mode
 restricted \write18 enabled.
 file:line:error style messages enabled.
 %&-line parsing enabled.
**document.tex
(./document.tex
LaTeX2e <2024-06-01> patch level 2
L3 programming layer <2024-10-09>
(/usr/local/texlive/2024/texmf-dist/tex/latex/latexbug/latexbug.sty
Package: latexbug 2024/06/17 v1.0o Bug-classification
)
(/usr/local/texlive/2024/texmf-dist/tex/latex/base/article.cls
Document Class: article 2024/02/08 v1.4n Standard LaTeX document class
(/usr/local/texlive/2024/texmf-dist/tex/latex/base/size10.clo
File: size10.clo 2024/02/08 v1.4n Standard LaTeX file (size option)
)
\c@part=\count194
\c@section=\count195
\c@subsection=\count196
\c@subsubsection=\count197
\c@paragraph=\count198
\c@subparagraph=\count199
\c@figure=\count266
\c@table=\count267
\abovecaptionskip=\skip49
\belowcaptionskip=\skip50
\bibindent=\dimen141
)
(/usr/local/texlive/2024/texmf-dist/tex/latex/amsmath/amsmath.sty
Package: amsmath 2024/05/23 v2.17q AMS math features
\@mathmargin=\skip51

For additional information on amsmath, use the `?' option.
(/usr/local/texlive/2024/texmf-dist/tex/latex/amsmath/amstext.sty
Package: amstext 2021/08/26 v2.01 AMS text

(/usr/local/texlive/2024/texmf-dist/tex/latex/amsmath/amsgen.sty
File: amsgen.sty 1999/11/30 v2.0 generic functions
\@emptytoks=\toks17
\ex@=\dimen142
))
(/usr/local/texlive/2024/texmf-dist/tex/latex/amsmath/amsbsy.sty
Package: amsbsy 1999/11/29 v1.2d Bold Symbols
\pmbraise@=\dimen143
)
(/usr/local/texlive/2024/texmf-dist/tex/latex/amsmath/amsopn.sty
Package: amsopn 2022/04/08 v2.04 operator names
)
\inf@bad=\count268
LaTeX Info: Redefining \frac on input line 233.
\uproot@=\count269
\leftroot@=\count270
LaTeX Info: Redefining \overline on input line 398.
LaTeX Info: Redefining \colon on input line 409.
\classnum@=\count271
\DOTSCASE@=\count272
LaTeX Info: Redefining \ldots on input line 495.
LaTeX Info: Redefining \dots on input line 498.
LaTeX Info: Redefining \cdots on input line 619.
\Mathstrutbox@=\box52
\strutbox@=\box53
LaTeX Info: Redefining \big on input line 721.
LaTeX Info: Redefining \Big on input line 722.
LaTeX Info: Redefining \bigg on input line 723.
LaTeX Info: Redefining \Bigg on input line 724.
\big@size=\dimen144
LaTeX Font Info:    Redeclaring font encoding OML on input line 742.
LaTeX Font Info:    Redeclaring font encoding OMS on input line 743.
\macc@depth=\count273
LaTeX Info: Redefining \bmod on input line 904.
LaTeX Info: Redefining \pmod on input line 909.
LaTeX Info: Redefining \smash on input line 939.
LaTeX Info: Redefining \relbar on input line 969.
LaTeX Info: Redefining \Relbar on input line 970.
\c@MaxMatrixCols=\count274
\dotsspace@=\muskip17
\c@parentequation=\count275
\dspbrk@lvl=\count276
\tag@help=\toks18
\row@=\count277
\column@=\count278
\maxfields@=\count279
\andhelp@=\toks19
\eqnshift@=\dimen145
\alignsep@=\dimen146
\tagshift@=\dimen147
\tagwidth@=\dimen148
\totwidth@=\dimen149
\lineht@=\dimen150
\@envbody=\toks20
\multlinegap=\skip52
\multlinetaggap=\skip53
\mathdisplay@stack=\toks21
LaTeX Info: Redefining \[ on input line 2953.
LaTeX Info: Redefining \] on input line 2954.
)

-> The generic hook 'package/amsmath/after':
> The hook is not declared.
> Code chunks:
)
Runaway argument?
{\int_gdecr:N \g__kernel_prg_map_int \cs_gset_eq:Nc \__prop_pair:wn {\ETC.
! File ended while scanning use of \prg_map_break:Nn.
<inserted text> 
                \par 
<*> document.tex
                
I suspect you have forgotten a `}', causing me
to read past where you wanted me to stop.
I'll try to recover; but if the error is serious,
you'd better type `E' or `X' now and fix your file.

! Emergency stop.
<*> document.tex
                
*** (job aborted, no legal \end found)

 
Here is how much of TeX's memory you used:
 2294 strings out of 473577
 46787 string characters out of 5728195
 416206 words of memory out of 5000000
 25265 multiletter control sequences out of 15000+600000
 558069 words of font info for 36 fonts, out of 8000000 for 9000
 1141 hyphenation exceptions out of 8191
 56i,0n,65p,204b,1346s stack positions out of 10000i,1000n,20000p,200000b,200000s
!  ==> Fatal error occurred, no output PDF file produced!

[muzimuzhi]

Oh \ShowHook maps over an internal l3prop variable \g__hook_<hook-name>_code_prop (multiple times, see lines 6189 and 6240) without checking its existence first.

Need to check if there are similar remaining cases.

Update: The behavior that mapping over an undeclared prop raises errors only happens since l3kernel 2024-02-13 added linked property variable.

latex2e/base/lthooks.dtx

Lines 6185 to 6189 in b1e172c

	\@@_log_line:x { Code~chunks: }
	\prop_if_empty:cTF { g_@@_#1_code_prop }
	{ \@@_log_line_indent:x { --- } }
	{
	\prop_map_inline:cn { g_@@_#1_code_prop }

latex2e/base/lthooks.dtx

Lines 6238 to 6240 in b1e172c

	\@@_log_line:x { Rules: }
	\bool_set_true:N \l_@@_tmpa_bool
	\@@_list_rules:nn {#1}

[muzimuzhi]

Aside: currently \hook_if_empty:nTF is true even if the corresponding \g__hook_<hook-name>_code_prop does not exist. Is this by design?

latex2e/base/lthooks.dtx

Lines 7056 to 7069 in b1e172c

	\prg_new_conditional:Npnn \hook_if_empty:n #1 { p , T , F , TF }
	{
	\if:w
	T
	\prop_if_exist:cT { g_@@_#1_code_prop }
	{ \prop_if_empty:cF { g_@@_#1_code_prop } { F } }
	\@@_cs_if_empty:cF { @@_toplevel~#1 } { F }
	\@@_cs_if_empty:cF { @@_next~#1 } { F }
	T
	\prg_return_true:
	\else:
	\prg_return_false:
	\fi:
	}

[FrankMittelbach]

Aside: currently \hook_if_empty:nTF is true even if the corresponding \g__hook_<hook-name>_code_prop does not exist. Is this by design?

of the top of my head, yes. A conditional is either true or false, and saying that a not (yet) existing hook "empty" makes more sense than claiming it is non-empty. If I remember correctly it is documented as this.

[FrankMittelbach]

Update: The behavior that mapping over an undeclared prop raises errors only happens since l3kernel 2024-02-13 added linked property variable.

hmm, yes, I can see that this may have stopped working but did so in the past (by mistake more or less).