Add regression tests for task stack scanning by the GC - #1422
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A GAP object held only in a C stack frame of a Julia task must survive collections that run while the task is suspended (parked in a Julia call, or busy on another thread). This went wrong when the task stack rescan optimization in GAP's julia_gc.c skipped old tasks, the cause of long-elusive crashes; see #1032, #1224 and gap-system/gap#6525. The tests hold a canary object solely via a wrapper bag in a GAP kernel argument temporary and detect premature collection through per-canary finalizers, covering automatic and explicit incremental collections and, in a subprocess with two Julia threads and two GC mark threads, a never-switching task on a second thread. Only automatic collections exercise the historically buggy path, so the driver churns allocations rather than calling GC.gc. Fixes #996. AI disclosure: tests developed with the assistance of Claude Code. Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #1422 +/- ##
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+ Coverage 83.33% 83.35% +0.01%
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Files 62 62
Lines 5048 5048
Branches 27 27
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+ Hits 4207 4208 +1
+ Misses 841 840 -1 🚀 New features to boost your workflow:
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Since the patch loop moved into the "Build GAP" step (b115fdc), the relative patchdir resolved inside the GAP checkout in /tmp/GAPROOT, so the patches -- including the julia_gc-rescan.patch working around the task stack scanning bug -- were silently never applied. The new GC task tests detect exactly that bug, turning the silent gap into CI failures. AI disclosure: diagnosed and fixed with the assistance of Claude Code. Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
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| mv GAPROOT /tmp/GAPROOT | ||
| cd /tmp/GAPROOT | ||
| patchdir=.github/workflows/GAP_patches/${{ matrix.gap-version }} | ||
| patchdir=${GITHUB_WORKSPACE}/.github/workflows/GAP_patches/${{ matrix.gap-version }} |
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This seems a bit unrelated to the rest of this PR (but looks fine anyway)
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A GAP object held only in a C stack frame of a Julia task must survive collections that run while the task is suspended (parked in a Julia call, or busy on another thread). This went wrong when the task stack rescan optimization in GAP's julia_gc.c skipped old tasks, the cause of long-elusive crashes; see #1032, #1224 and gap-system/gap#6525.
The tests hold a canary object solely via a wrapper bag in a GAP kernel argument temporary and detect premature collection through per-canary finalizers, covering automatic and explicit incremental collections and, in a -t2 subprocess, a never-switching task on a second thread. Only automatic collections exercise the historically buggy path, so the driver churns allocations rather than calling GC.gc.
Resolves #996
AI disclosure: tests developed with the assistance of Claude Code.