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gh-157377: Don't flush the thread-local allocation count in gc.get_count #157381

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gh-157377: Don't flush the thread-local allocation count in gc.get_count #157381

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gh-157377: Don't flush the thread-local allocation count in gc.get_co…
ngoldbaum Sep 12, 2026
81ca259
gh-157377: clamp gc.get_count() return value at zero
ngoldbaum Sep 13, 2026
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19 changes: 19 additions & 0 deletions Lib/test/test_gc.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1658,6 +1658,25 @@ def test_indirect_calls_with_gc_disabled(self):
finally:
gc.enable()

def test_get_count_nonnegative(self):
xs = [[] for _ in range(1500)]
gc.collect()
del xs[:1000]
self.assertGreaterEqual(gc.get_count()[0], 0)

@gc_threshold(1000, 0, 0)
def test_get_count_does_not_prevent_collection(self):
junk = []
gc.collect()
detector = GC_Detector()
for _ in range(10000):
junk.append([])
gc.get_count()
if detector.gc_happened:
break
else:
self.fail("gc didn't happen after 10000 iterations")

# Ensure that setting *threshold0* to zero disables collection.
@gc_threshold(0)
def test_threshold_zero(self):
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4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-09-12-22-29-51.gh-issue-157377.O6WYlM.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
Fix :func:`gc.get_count` on the free-threaded build resetting the thread-local
allocation counter that schedules automatic garbage collection. A thread that
called it while allocating could prevent cyclic garbage from ever being
collected.
11 changes: 7 additions & 4 deletions Modules/gcmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -221,9 +221,12 @@ gc_get_count_impl(PyObject *module)
_PyThreadStateImpl *tstate = (_PyThreadStateImpl *)_PyThreadState_GET();
struct _gc_thread_state *gc = &tstate->gc;

// Flush the local allocation count to the global count
_Py_atomic_add_int(&gcstate->young.count, (int)gc->alloc_count);
gc->alloc_count = 0;
// Don't flush: record_allocation() checks the threshold only when it fills.
int young = _Py_atomic_load_int_relaxed(&gcstate->young.count);
young += (int)gc->alloc_count;

@kumaraditya303 kumaraditya303 Sep 13, 2026

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I think young needs to be clamped with if (young < 0) young = 0; otherwise negative values can be reported like:

xs = [[] for _ in range(600)]; gc.collect(); del xs[:400]
gc.get_count()  ->  (-173, 0, 0)     # with the PR applied

@ngoldbaum ngoldbaum Sep 13, 2026

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Makes sense and is consistent with the clamping of the global count done in gh-142048. When that happened the behavior of this function from gh-112529 was not revisited.

if (young < 0) {
young = 0;
}
#endif

#ifndef Py_GIL_DISABLED
Expand All @@ -233,7 +236,7 @@ gc_get_count_impl(PyObject *module)
gcstate->generations[2].count);
#else
return Py_BuildValue("(iii)",
_Py_atomic_load_int_relaxed(&gcstate->young.count),
young,
gcstate->old[0].count,
gcstate->old[1].count);
#endif
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