Hang when pass failed to modify the variant shortly before STOP

[emaxx-google]

I experience occasional hangs in particular passes, e.g.:

01:23:45 INFO ===< ClangPass::replace-dependent-typedef >===
01:23:45 INFO (0.0%, 22566 bytes, 541 lines)
01:23:46 INFO (0.0%, 22566 bytes, 541 lines)
// ... the same repeats - even hours later

According to the initial investigation, there seems to be a bug in the replace-dependent-typedef pass, so that it sometimes reports success despite not making any changes in the file. This is normally caught by this check:

cvise/cvise/utils/testing.py

Line 415 in 29fc072

if not self.report_pass_bug(test_env, 'pass failed to modify the variant'):

, however this line is not reached if quit_loop has already been set to True because an earlier invocation returned PassResult.STOP.

The end effect is that the execution gets stuck in this state ~forever: the pass keeps being applied, the file remains the same all the time, and the file comparison check is skipped.

[emaxx-google]

For what it's worth, this particular bug in replace-dependent-typedef will likely be fixed upstream: csmith-project/creduce#281 . However, the potential hang scenario is still very unfortunate because there's no 100% guarantee that no other pass has the same bug.

[emaxx-google]

As for the hang problem itself, one easy fix would probably be to compare the files in TestManager.process_result(). However, we might want to do some code reuse and also to check the other things that process_done_futures() was performing, like the one against max_improvement.