The guard worked. I had written it so that no session open in one project could touch another project's files, it had three tests, and all three were green. They had been green for months.
It was blind across 1,029 of the 6,309 lines it was watching. Eight hundred and twenty-four of them executable.
And the three tests kept passing, because all three looked at paths that were already clean.
The defect isn't in the detector: it's in how you test it
You test a detector with cases. The natural thing is to take them from the system itself: feed it the real files and check that it finds nothing, because the system is healthy. A hundred cases, all green, and the feeling of having measured something.
