Sporadic error in `NumberOfRightCongruences` in GitHub Actions in `tst/standard/libsemigroups/sims1.tst`

[wilfwilson]

See e.g. the log of this failed macOS / GAP v4.12 / 64-bit job:

testing: /tmp/gaproot/pkg/Semigroups//tst/standard/libsemigroups/sims1.tst
######## > Diff in:
/tmp/gaproot/pkg/Semigroups//tst/standard/libsemigroups/sims1.tst:226
# Input is:
NumberOfRightCongruences(S);
# Expected output:
6
# But found:
5
########
     606 ms (234 ms GC) and 12.1MB allocated for libsemigroups/sims1.tst

Note that (as far as I can tell) this was in the run-through of the test with acting methods disabled; the tests had all passed fine with acting methods enabled. Also, the pull request in question (#1120) solely changes the concurrency settings of the GitHub Actions workflow files and can't possibly have been the cause of this failure.

To me it suggests some sort of uncommon memory corruption issue...

[james-d-mitchell]

Thanks for the report @wilfwilson I think this is caused by a data race in the Sims low-index congruence code in libsemigroups, that we have resolved in v3. So hopefully this will disappear when we finally update the Semigroups package to work with libsemigroups v3.