Race condition causes tests to fail

[davidweichiang]

I frequently but intermittently see failures on variable-height_legacy.tex. The log file says

(./tmp-gre/variable-height-6_0_0.gtex
./tmp-gre/variable-height-6_0_0.gtex:1: Text line contains an invalid character.
l.1 ^^@
   ^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@
^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@
^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@...
...
./tmp-gre/variable-height-6_0_0.gtex:1:  ==> Fatal error occurred, no output PDF
 file produced!

Those NUL characters (^^@) suggest that another program was writing to the file at the same time, presumably the run for variable-height.tex which uses the same gabc file.