fix: check if .gnuplot is up-to-date in a more reliable way

[muzimuzhi]

Motivation for this change

Fixes #1397

Checklist

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• Code changes are licensed under GPLv2 + LPPLv1.3c
• Documentation changes are licensed under FDLv1.2

[muzimuzhi]

The manual was built without errors, but the job failed at a latter step "Deploy tlcontrib", with

E: The repository 'http://deb.debian.org/debian-security testing-security Release' does not have a Release file.

pgf/.github/workflows/doc.yml

Lines 58 to 65 in 757f4c2

	- name: Deploy tlcontrib
	env:
	GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
	run: |
	apt-get update -qq
	apt-get install -qq libxml-parser-perl libxml-xpath-perl libtext-unidecode-perl tree
	cp build/doc/pgfmanual.pdf ../
	bash ci/update_tlcontrib.sh

Probably an issue with the image registry.gitlab.com/islandoftex/images/texlive:latest?

---

Update: Somehow the step now passed.

[muzimuzhi]

This change was buggy.

By using \pgf@plot@code in replace of #2 in writing to .gnuplot, it has to be defined. But currently \pgf@plot@code is only defined if the .gnuplot exists.

A minimal reproducing example:

% copied from https://github.com/pgf-tikz/pgf/issues/1397
\documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
    \draw plot[domain=-2:2] function{x**2};
\end{tikzpicture}
\end{document}

If the \jobname.pgf-plot.gnuplot doesn't exist before the compilation, an error is raised:

! Undefined control sequence.
<write> \pgf@plot@code 

Usually the error disappears in a following compilation, but the pgfplots manual building was broken because of the -halt-on-error option. See pgf-tikz/pgfplots#510 (comment).

The fix is simple, but I need some more time to setup a new l3build test suite which checks with shell escape, so .

diff --git a/tex/generic/pgf/modules/pgfmoduleplot.code.tex b/tex/generic/pgf/modules/pgfmoduleplot.code.tex
index 4dbaa14f..1b070a89 100644
--- a/tex/generic/pgf/modules/pgfmoduleplot.code.tex
+++ b/tex/generic/pgf/modules/pgfmoduleplot.code.tex
@@ -481,6 +481,7 @@
   \pgfutilpreparefilename{#1.table}%
   \let\pgf@plottablefile=\pgfretval
   \let\pgf@plottablefile@quoted=\pgfretvalquoted
+  \edef\pgf@plot@code{\pgfutil@trimspaces@e{#2}}%
   \begingroup
   \endlinechar=-1 % suppress white space at end
   % Check, whether it is up-to-date
@@ -490,7 +491,6 @@
     \pgfutil@read\pgfutil@inputcheck to\pgf@temp% ignored
     \pgfutil@read\pgfutil@inputcheck to\pgf@plot@line%
     \closein\pgfutil@inputcheck
-    \edef\pgf@plot@code{\pgfutil@trimspaces@e{#2}}%
     \ifx\pgf@plot@code\pgf@plot@line%
       \openin\pgfutil@inputcheck=\pgfretval\relax
       \ifeof\pgfutil@inputcheck%

[muzimuzhi]

New PR #1412 is proposed. It comes with a set of tests.