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gh-135552: Make the GC clear weakrefs later by nascheme · Pull Request #136189 · python/cpython · GitHub
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gh-135552: Make the GC clear weakrefs later #136189

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Merged
nascheme merged 20 commits into from
Aug 7, 2025
Merged

gh-135552: Make the GC clear weakrefs later #136189

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42abb05
Make the GC clear weakrefs later.
nascheme Jun 30, 2025
17a4f9e
Remove inaccurate comment.
nascheme Jul 1, 2025
12f0b5c
Run clear_weakrefs() with world stopped.
nascheme Jul 1, 2025
2f3daba
Defer weakref clears only for refs to classes.
nascheme Jul 2, 2025
123bc25
Ensure weakrefs with callbacks are cleared early.
nascheme Jul 3, 2025
496c0b1
Add NEWS.
nascheme Jul 3, 2025
8a553d1
Add comment about wrlist iteration.
nascheme Jul 3, 2025
01c882f
Merge 'origin/main' into gh-135552-wr-clear-later
nascheme Jul 3, 2025
0060602
Merge 'origin/main' into gh-135552-wr-clear-later
nascheme Jul 8, 2025
900022b
Revise NEWS.
nascheme Jul 8, 2025
84bd123
Defer clear for weakrefs without callbacks.
nascheme Jul 8, 2025
bb29ea1
Add unit test for GH-132413.
nascheme Jul 8, 2025
073409b
Add additional tests for weakref clearing.
nascheme Jul 9, 2025
135223e
Revert unneeded changes to unit tests.
nascheme Jul 9, 2025
fe9e58a
Use assertIsNone() and assertIsNotNone().
nascheme Jul 23, 2025
70ef387
Merge 'origin/main' into gh-135552-wr-clear-later
nascheme Jul 23, 2025
a7551cf
Merge 'origin/main' into gh-135552-wr-clear-later
nascheme Aug 6, 2025
4fe8de0
Revise InternalDocs/garbage_collector.md.
nascheme Aug 7, 2025
24fd972
Revise comments about weakref handling.
nascheme Aug 7, 2025
eca79ce
Merge branch 'origin/main' into gh-135552-wr-clear-later
nascheme Aug 7, 2025
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Run clear_weakrefs() with world stopped.
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@nascheme
nascheme committed Jul 1, 2025
commit 12f0b5c6a495c830dede2e1638f0eab0c9750de9
34 changes: 18 additions & 16 deletions Python/gc_free_threading.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2255,11 +2255,28 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state,
call_weakref_callbacks(state);
finalize_garbage(state);

// Handle any objects that may have resurrected after the finalization.
_PyEval_StopTheWorld(interp);
// Handle any objects that may have resurrected after the finalization.
err = handle_resurrected_objects(state);
// Clear free lists in all threads
_PyGC_ClearAllFreeLists(interp);
if (err == 0) {
// Clear weakrefs to objects in the unreachable set. No Python-level
// code must be allowed to access those unreachable objects. During
// delete_garbage(), finalizers outside the unreachable set might
// run and if those weakrefs were not cleared, that could reveal
// unreachable objects.
//
// We used to clear weakrefs earlier, before calling finalizers.
// That causes at least two problems. First, the finalizers could
// create new weakrefs, that refer to unreachable objects. Those
// would not be cleared and could cause the problem described above
// (see GH-91636 as an example). Second, we need the weakrefs in the
// tp_subclasses to *not* be cleared so that caches based on the type
// version are correctly invalidated (see GH-135552 as a bug caused by
// this).
clear_weakrefs(state);
}
_PyEval_StartTheWorld(interp);

if (err < 0) {
Expand All @@ -2271,21 +2288,6 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state,
return;
}

// Clear weakrefs to objects in the unreachable set. No Python-level
// code must be allowed to access those unreachable objects. During
// delete_garbage(), finalizers outside the unreachable set might run
// and if those weakrefs were not cleared, that could reveal unreachable
// objects.
//
// We used to clear weakrefs earlier, before calling finalizers. That
// causes at least two problems. First, the finalizers could create
// new weakrefs, that refer to unreachable objects. Those would not be
// cleared and could cause the problem described above (see GH-91636 as
// an example). Second, we need the weakrefs in the tp_subclasses to
// *not* be cleared so that caches based on the type version are correctly
// invalidated (see GH-135552 as a bug caused by this).
clear_weakrefs(state);

// Call tp_clear on objects in the unreachable set. This will cause
// the reference cycles to be broken. It may also cause some objects
// to be freed.
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