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Merged
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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Revise InternalDocs/garbage_collector.md.
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@nascheme
nascheme committed Aug 7, 2025
commit 4fe8de0b117de1adc7dc6103df930c1bf28dae5c
26 changes: 16 additions & 10 deletions InternalDocs/garbage_collector.md
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Expand Up @@ -329,15 +329,16 @@ Once the GC knows the list of unreachable objects, a very delicate process start
with the objective of completely destroying these objects. Roughly, the process
follows these steps in order:

1. Handle and clear weak references (if any). Weak references to unreachable objects
are set to `None`. If the weak reference has an associated callback, the callback
is enqueued to be called once the clearing of weak references is finished. We only
invoke callbacks for weak references that are themselves reachable. If both the weak
reference and the pointed-to object are unreachable we do not execute the callback.
This is partly for historical reasons: the callback could resurrect an unreachable
object and support for weak references predates support for object resurrection.
Ignoring the weak reference's callback is fine because both the object and the weakref
are going away, so it's legitimate to say the weak reference is going away first.
1. Handle weak references with callbacks (if any). If the weak reference has
an associated callback, the callback is enqueued to be called after the weak
reference is cleared. We only invoke callbacks for weak references that
are themselves reachable. If both the weak reference and the pointed-to
object are unreachable we do not execute the callback. This is partly for
historical reasons: the callback could resurrect an unreachable object
and support for weak references predates support for object resurrection.
Ignoring the weak reference's callback is fine because both the object and
the weakref are going away, so it's legitimate to say the weak reference is
going away first.
2. If an object has legacy finalizers (`tp_del` slot) move it to the
`gc.garbage` list.
3. Call the finalizers (`tp_finalize` slot) and mark the objects as already
Expand All @@ -346,7 +347,12 @@ follows these steps in order:
4. Deal with resurrected objects. If some objects have been resurrected, the GC
finds the new subset of objects that are still unreachable by running the cycle
detection algorithm again and continues with them.
5. Call the `tp_clear` slot of every object so all internal links are broken and
5. Clear any weak references that still refer to unreachable objects. The
`wr_object` attribute for these weakrefs are set to `None`. Note that some
of these weak references maybe have been newly created during the running of
finalizers in step 3. Also, clear any weak references that are part of the
unreachable set.
6. Call the `tp_clear` slot of every object so all internal links are broken and
the reference counts fall to 0, triggering the destruction of all unreachable
objects.

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