tikzexternalize and align environment - errors

Question

I am experiencing a problem when using externalize and the align environment. In some weird cases the pdf of tikz figures are not generated properly and they have to be re-generated every time that the tex file is compiled.

Here is a MWE:

The main file, called test.tex

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{external}
\tikzexternalize % activate with the default mode
\usepackage[cmex10]{amsmath}

\begin{document}

\tikzsetexternalprefix{figs/tikz2pdf/}

\begin{figure}
  \tikzpicturedependsonfile{figs/fig.tikz}
  \tikzsetnextfilename{fig}
  \input{figs/fig.tikz}
\end{figure}

\begin{align}
  A &= 1 \label{A} \nonumber \\ 
  B &= 2 \label{B} 
\end{align}

\end{document}

The content of figs/fig.tikz is:

\begin{tikzpicture}
\draw (0,0) --(1,2);
\end{tikzpicture}

The directory figs/tikz2pdf exists prior to the compilation. I compile the code using "pdflatex -interaction=nonstopmode -shell-escape test.tex". To my understanding this should create figs/tikz2pdf/fig.pdf, but it does not. It successfuly creates test.pdf, but reports the following two errors:

1. ! Package tikz Error: Sorry, the system call 'pdflatex -shell-escape -halt-on-e rror -interaction=batchmode -jobname "figs/tikz2pdf/fig" "\def\tikzexternalreal job{test}\input{test}"' did NOT result in a usable output file 'figs/tikz2pdf/f ig' (expected one of .pdf:.jpg:.jpeg:.png:). Please verify that you have enable d system calls. For pdflatex, this is 'pdflatex -shell-escape'. Sometimes it is also named 'write 18' or something like that. Or maybe the command simply fail ed? Error messages can be found in 'figs/tikz2pdf/fig.log'. If you continue now , I'll try to typeset the picture.

2. ! Package amsmath Error: Multiple \label's: label 'A' will be lost.

Interestingly:

1. if I remove either the "\nonumber" or "\label{A}" from the align environment, figs/tikz2pdf/fig.pdf is created.

2. Once that figs/tikz2pdf/fig.pdf is created once, it is used in following compilations of test.tex, not needing to re-compile figs/fig.tikz

I know that labelling a line in align and then using \nonumber is somehow incongruent. What puzzles me is that the error in the align environment prevents externalize to work properly. And despite all, a proper test.pdf is produced. The overall visible effect is a large compilation time in documents containing many tikz figures.

A final note: I came up with the provided MWE stripping down a long complex document. After that I removed all (accidental) occurrences of simultaneous labelling and usage of \nonumber, after which externalize still does not produce the pdf from tikz figures. I therefore suspect that the previous MWE could be one way to reproduce a more intricate issue.

Any light on this is appreciated. Am I doing something wrong? Am I missing something?

EDIT: It seems that I forgot an occurrence of simultaneous labelling and usage of \nonumber. After its removal externalize does produce the pdf from tikz figures, so this seems to be the key issue.

Answer 1

The process of generating separate pdf figures involves a complete scan over the input document. This scan is optimized in the sense that a lot of expensive macros are disabled unless they are part of the target figure. But it is still a complete scan.

If there is a single error somewhere during that scan, the export will fail. And it seems as if that happened. You will see the root cause explained in figs/tikz2pdf/fig.log.

The fact that you receive a complete test.pdf is caused by -interaction nonstopmode: it reports the error, but continues to process the document. In that case, the external library falls repeats the process as if it would not have attempted the externalization.

To conclude: please repair any errors in your main document, otherwise the external lib will fail as well.