test(kernel): regression test for circular-print mark restoration#23
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PR #20 added the fe_write() cycle-detection fix (borrow GCMARKBIT, clear it in unmarkpairs) but shipped no test for it. This closes that gap. The mark bit lives in the low byte of a pair's car field, so a leaked mark corrupts the car pointer — which makes walking the structure after repr a direct, deterministic check of mark restoration (no GC needed). repr also runs fe_write twice internally (measure + fill), so a stale mark would skew the two passes. Verified the test has teeth: neutering unmarkpairs to a no-op makes the walkability checks segfault (exit 139), which ctest scores as a failure. 20/20 ctest, 23 checks in the kernel file. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Summary
PR #20 added the
fe_write()circular-structure fix (borrowGCMARKBITduring traversal, clear it afterward inunmarkpairs) but shipped no test for it. This adds the missingkernel/circular-printregression test.Why this is a strong test
The mark bit lives in the low byte of a pair's
carfield. So a leaked mark bit doesn't just leave stale state — it corrupts the car pointer. That makes walking the structure afterrepra direct, deterministic check of mark restoration, with no need to force a GC (which would be flaky across arena/pool sizes):(is (repr x) "(1 2 3 ...)")— tail-cycle prints finite, ends in...(is (repr y) "(... 2 3)")— head-cycle (setcarself-reference)(is (cdr (cdr (cdr x))) x)— the cycle still closes onx(pointer identity intact)(is (car (cdr (cdr (cdr x)))) 1)— walking through the back-edge returnsx[0](is (repr x) (repr x))— reprʼing twice yields the same string (marks don't accumulate)repralso runsfe_writetwice internally (measure pass + fill pass), so a stale mark would skew the two passes and corrupt the output — another angle the same test covers.Verified the test has teeth
Neutering
unmarkpairsto a no-op (cycle-detection still works, so it doesn't merely hang) makes the post-print walkability checks segfault (exit 139) — ctest scores that as a failure. With the real fix restored, 20/20 ctest, 23 checks in the kernel file. Kernel source is untouched — test + CHANGELOG only.🤖 Generated with Claude Code