Corner case in hydra-eval-jobset existing build scheduled check?

I pushed a derivation for mfixgui and mfixsolver that had an invalid url in a fetchurl field. The hydra build failed as expected.

( fetchurl {
    url = "https://mfix.netl.doe.gov/s3/71e94306/f00d8cf543e7a1f42c41a545baef58c4/source/mfix/mfix-${version}.tar.gz";
    sha256 = "1bnwvgmrmz9y32fsnnqb4x2ab4yh9iyg1d7f3wc2w6q9fsfnzpj4";
} )

trying https://mfix.netl.doe.gov/s3/71e94306/f00d8cf543e7a1f42c41a545baef58c4/source/mfix/mfix-19.3.0.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:--  0:00:02 --:--:--     0
100 34322    0 34322    0     0   9147      0 --:--:--  0:00:03 --:--:-- 84536
100   220  100   220    0     0     56      0  0:00:03  0:00:03 --:--:--    56
100 12542    0 12542    0     0   3125      0 --:--:--  0:00:04 --:--:--  3125
hash mismatch in fixed-output derivation '/nix/store/8qmh1xvg53d3la951xz710617ll9bwp7-mfix-19.3.0.tar.gz':
  wanted: sha256:1bnwvgmrmz9y32fsnnqb4x2ab4yh9iyg1d7f3wc2w6q9fsfnzpj4
  got:    sha256:1h43xxf315hwn7s3fz7xphkilgwh3m1ga7h5ysr25m60nwfh0mb1

• derivation with incorrect url
• hydra evaluation with incorrect url

I then amended the commit with the corrected url value and force pushed. The hydra build is continuing to fail and showing a build log with the original failed download step with the incorrect url.

( fetchurl {
    url = "https://www.sharcnet.ca/~tyson/nix/mfix-${version}.tar.gz";
    sha256 = "1bnwvgmrmz9y32fsnnqb4x2ab4yh9iyg1d7f3wc2w6q9fsfnzpj4";
} )

trying https://mfix.netl.doe.gov/s3/71e94306/f00d8cf543e7a1f42c41a545baef58c4/source/mfix/mfix-19.3.0.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:--  0:00:02 --:--:--     0
100 34322    0 34322    0     0   9147      0 --:--:--  0:00:03 --:--:-- 84536
100   220  100   220    0     0     56      0  0:00:03  0:00:03 --:--:--    56
100 12542    0 12542    0     0   3125      0 --:--:--  0:00:04 --:--:--  3125
hash mismatch in fixed-output derivation '/nix/store/8qmh1xvg53d3la951xz710617ll9bwp7-mfix-19.3.0.tar.gz':
  wanted: sha256:1bnwvgmrmz9y32fsnnqb4x2ab4yh9iyg1d7f3wc2w6q9fsfnzpj4
  got:    sha256:1h43xxf315hwn7s3fz7xphkilgwh3m1ga7h5ysr25m60nwfh0mb1

• derivation with correct url
• hydra evaluation with corrected url

After much hair pulling, I eventually stepped through the hydra-eval-jobset script in perl debug mode and believe I have discovered a corner case. The checkBuild function checks for previous builds with the following search, and doesn't add a new build if it finds a match

my ($prevBuild) = $prevEval->builds->search(
    # The "project" and "jobset" constraints are                                                        
    # semantically unnecessary (because they're implied by                                              
    # the eval), but they give a factor 1000 speedup on                                                 
    # the Nixpkgs jobset with PostgreSQL.                                                               
    { project => $jobset->project->name, jobset => $jobset->name, job => $jobName,
      name => $firstOutputName, path => $firstOutputPath },
    { rows => 1, columns => ['id'], join => ['buildoutputs'] });

I think this causes problems in the case I've laid out above as the overall output path remains the same after the updated url push because the only change made was to the derivation for a fixed output input.

Start with the hydra-eval-jobset checkBuild function and the prevEval->builds->search call shown in the report. Compare evaluations 550 and 551 with the two mfixgui.nix revisions, and verify whether the corrected fixed-output URL reuses the previous output path and skips the build; done means this case schedules the corrected build.