GOTEX_TRACE: a runaway names the macro that loops

[tannevaled]

A runaway is the hardest failure this engine reports to act on. The error names the line the document died on, which is almost never the line at fault: loading pgf's parser module (\usepgfmodule{parser}, reached from \usetikzlibrary{svg.path}) reports 6:15 — the document's own \end{document} — for a loop that starts hundreds of expansions earlier, inside pgfkeys. Bisecting the 697-line module by truncation answered nothing, because a half-read file breaks in its own way.

With GOTEX_TRACE set, tripRunaway prints two windows instead:

gotex: runaway at 10:14
  FIRST expansions after the input last moved forward:
    \pgfkeys@parse
    \pgfkeys@possiblerelax
    \pgfkeys@mainstop  x283
  LAST expansions:
    \pgfkeys@mainstop  x400

The first window is where the loop begins, with its caller still in view; the second is what it repeats. Runs are collapsed, so the loop cannot push the caller out of its own report. On the module above the answer was one line, and it named a pgfkeys sentinel — \def\pgfkeys@mainstop{\pgfkeys@mainstop}, "equals only itself" — being expanded where it should have been matched as a delimiter.

Off by default: nothing is recorded and nothing is allocated unless the variable is set. The report goes to traceOut (os.Stderr) so a test can read it back, and both states are tested.

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