Reset the exit code when the test harness finishes a passing run

[nohwnd]

test.ps1 could end with a non-zero $LASTEXITCODE while every test passed. CI ends the step with exit $LASTEXITCODE, so a fully green run goes red with no failing test in it. Which test ran last decides whether it happens, which is what would make it intermittent.

The cause is that Invoke-InNewProcess calls the child as a native command, so the child's exit code lands in the caller's $LASTEXITCODE, and several tests run a child that exits non-zero on purpose.

What I got wrong first, since the PR history shows it

My first attempt made the helper restore the caller's value. That is wrong: propagating the child's exit code is intended and tested. Exitcode is set to the number of failed tests and the process exits when tests fail asserts $LASTEXITCODE | Verify-Equal 1 in the caller after the child ran, and two more tests assert 0. CI failed on every leg and was right to.

So the helper is untouched, and the fix is one place: test.ps1 sets the exit code to 0 once it has decided the run passed.

It cannot hide a failure

test.ps1 does not decide pass/fail from $LASTEXITCODE:

if (0 -lt $result.Count) { ... if ($CI) { exit 1 } }   # P tests failed
if ("Failed" -eq $r.Result) { throw "Run failed!" }     # Pester run failed

Both come before the reset and both override it. The bug is one-directional: green runs going red, never red runs going green.

No test

This is the harness's own exit path, so there is nothing to assert on from inside the suite it runs. The behaviour it protects, the child's code reaching the caller, is already covered by the three tests named above, and those now pass unchanged.

What this does not claim

main went red once today on PS7 - Ubuntu 22.04 with exit code 1, every suite green and no error text, and a re-run passed. This is the right shape but I could not prove it was the cause: the known leaking test leaves 2, not 1, and the tests added in #3020 do not make their child exit non-zero (checked, the child ends on a string so it exits 0 before and after that change). If a silent non-zero exit shows up again after this lands, that rules this class out.

🤖