gh-116738: Make _heapq module thread-safe (#135036)

Use critical sections to make heapq methods that update the heap thread-safe when the GIL is disabled.

---------

Co-authored-by: mpage <mpage@meta.com>

Lib/test/test_free_threading/test_heapq.py

@@ -0,0 +1,240 @@
+import unittest
+
+import heapq
+
+from enum import Enum
+from threading import Thread, Barrier
+from random import shuffle, randint
+
+from test.support import threading_helper
+from test import test_heapq
+
+
+NTHREADS = 10
+OBJECT_COUNT = 5_000
+
+
+class Heap(Enum):
+    MIN = 1
+    MAX = 2
+
+
+@threading_helper.requires_working_threading()
+class TestHeapq(unittest.TestCase):
+    def setUp(self):
+        self.test_heapq = test_heapq.TestHeapPython()
+
+    def test_racing_heapify(self):
+        heap = list(range(OBJECT_COUNT))
+        shuffle(heap)
+
+        self.run_concurrently(
+            worker_func=heapq.heapify, args=(heap,), nthreads=NTHREADS
+        )
+        self.test_heapq.check_invariant(heap)
+
+    def test_racing_heappush(self):
+        heap = []
+
+        def heappush_func(heap):
+            for item in reversed(range(OBJECT_COUNT)):
+                heapq.heappush(heap, item)
+
+        self.run_concurrently(
+            worker_func=heappush_func, args=(heap,), nthreads=NTHREADS
+        )
+        self.test_heapq.check_invariant(heap)
+
+    def test_racing_heappop(self):
+        heap = self.create_heap(OBJECT_COUNT, Heap.MIN)
+
+        # Each thread pops (OBJECT_COUNT / NTHREADS) items
+        self.assertEqual(OBJECT_COUNT % NTHREADS, 0)
+        per_thread_pop_count = OBJECT_COUNT // NTHREADS
+
+        def heappop_func(heap, pop_count):
+            local_list = []
+            for _ in range(pop_count):
+                item = heapq.heappop(heap)
+                local_list.append(item)
+
+            # Each local list should be sorted
+            self.assertTrue(self.is_sorted_ascending(local_list))
+
+        self.run_concurrently(
+            worker_func=heappop_func,
+            args=(heap, per_thread_pop_count),
+            nthreads=NTHREADS,
+        )
+        self.assertEqual(len(heap), 0)
+
+    def test_racing_heappushpop(self):
+        heap = self.create_heap(OBJECT_COUNT, Heap.MIN)
+        pushpop_items = self.create_random_list(-5_000, 10_000, OBJECT_COUNT)
+
+        def heappushpop_func(heap, pushpop_items):
+            for item in pushpop_items:
+                popped_item = heapq.heappushpop(heap, item)
+                self.assertTrue(popped_item <= item)
+
+        self.run_concurrently(
+            worker_func=heappushpop_func,
+            args=(heap, pushpop_items),
+            nthreads=NTHREADS,
+        )
+        self.assertEqual(len(heap), OBJECT_COUNT)
+        self.test_heapq.check_invariant(heap)
+
+    def test_racing_heapreplace(self):
+        heap = self.create_heap(OBJECT_COUNT, Heap.MIN)
+        replace_items = self.create_random_list(-5_000, 10_000, OBJECT_COUNT)
+
+        def heapreplace_func(heap, replace_items):
+            for item in replace_items:
+                heapq.heapreplace(heap, item)
+
+        self.run_concurrently(
+            worker_func=heapreplace_func,
+            args=(heap, replace_items),
+            nthreads=NTHREADS,
+        )
+        self.assertEqual(len(heap), OBJECT_COUNT)
+        self.test_heapq.check_invariant(heap)
+
+    def test_racing_heapify_max(self):
+        max_heap = list(range(OBJECT_COUNT))
+        shuffle(max_heap)
+
+        self.run_concurrently(
+            worker_func=heapq.heapify_max, args=(max_heap,), nthreads=NTHREADS
+        )
+        self.test_heapq.check_max_invariant(max_heap)
+
+    def test_racing_heappush_max(self):
+        max_heap = []
+
+        def heappush_max_func(max_heap):
+            for item in range(OBJECT_COUNT):
+                heapq.heappush_max(max_heap, item)
+
+        self.run_concurrently(
+            worker_func=heappush_max_func, args=(max_heap,), nthreads=NTHREADS
+        )
+        self.test_heapq.check_max_invariant(max_heap)
+
+    def test_racing_heappop_max(self):
+        max_heap = self.create_heap(OBJECT_COUNT, Heap.MAX)
+
+        # Each thread pops (OBJECT_COUNT / NTHREADS) items
+        self.assertEqual(OBJECT_COUNT % NTHREADS, 0)
+        per_thread_pop_count = OBJECT_COUNT // NTHREADS
+
+        def heappop_max_func(max_heap, pop_count):
+            local_list = []
+            for _ in range(pop_count):
+                item = heapq.heappop_max(max_heap)
+                local_list.append(item)
+
+            # Each local list should be sorted
+            self.assertTrue(self.is_sorted_descending(local_list))
+
+        self.run_concurrently(
+            worker_func=heappop_max_func,
+            args=(max_heap, per_thread_pop_count),
+            nthreads=NTHREADS,
+        )
+        self.assertEqual(len(max_heap), 0)
+
+    def test_racing_heappushpop_max(self):
+        max_heap = self.create_heap(OBJECT_COUNT, Heap.MAX)
+        pushpop_items = self.create_random_list(-5_000, 10_000, OBJECT_COUNT)
+
+        def heappushpop_max_func(max_heap, pushpop_items):
+            for item in pushpop_items:
+                popped_item = heapq.heappushpop_max(max_heap, item)
+                self.assertTrue(popped_item >= item)
+
+        self.run_concurrently(
+            worker_func=heappushpop_max_func,
+            args=(max_heap, pushpop_items),
+            nthreads=NTHREADS,
+        )
+        self.assertEqual(len(max_heap), OBJECT_COUNT)
+        self.test_heapq.check_max_invariant(max_heap)
+
+    def test_racing_heapreplace_max(self):
+        max_heap = self.create_heap(OBJECT_COUNT, Heap.MAX)
+        replace_items = self.create_random_list(-5_000, 10_000, OBJECT_COUNT)
+
+        def heapreplace_max_func(max_heap, replace_items):
+            for item in replace_items:
+                heapq.heapreplace_max(max_heap, item)
+
+        self.run_concurrently(
+            worker_func=heapreplace_max_func,
+            args=(max_heap, replace_items),
+            nthreads=NTHREADS,
+        )
+        self.assertEqual(len(max_heap), OBJECT_COUNT)
+        self.test_heapq.check_max_invariant(max_heap)
+
+    @staticmethod
+    def is_sorted_ascending(lst):
+        """
+        Check if the list is sorted in ascending order (non-decreasing).
+        """
+        return all(lst[i - 1] <= lst[i] for i in range(1, len(lst)))
+
+    @staticmethod
+    def is_sorted_descending(lst):
+        """
+        Check if the list is sorted in descending order (non-increasing).
+        """
+        return all(lst[i - 1] >= lst[i] for i in range(1, len(lst)))
+
+    @staticmethod
+    def create_heap(size, heap_kind):
+        """
+        Create a min/max heap where elements are in the range (0, size - 1) and
+        shuffled before heapify.
+        """
+        heap = list(range(OBJECT_COUNT))
+        shuffle(heap)
+        if heap_kind == Heap.MIN:
+            heapq.heapify(heap)
+        else:
+            heapq.heapify_max(heap)
+
+        return heap
+
+    @staticmethod
+    def create_random_list(a, b, size):
+        """
+        Create a list of random numbers between a and b (inclusive).
+        """
+        return [randint(-a, b) for _ in range(size)]
+
+    def run_concurrently(self, worker_func, args, nthreads):
+        """
+        Run the worker function concurrently in multiple threads.
+        """
+        barrier = Barrier(nthreads)
+
+        def wrapper_func(*args):
+            # Wait for all threads to reach this point before proceeding.
+            barrier.wait()
+            worker_func(*args)
+
+        with threading_helper.catch_threading_exception() as cm:
+            workers = (
+                Thread(target=wrapper_func, args=args) for _ in range(nthreads)
+            )
+            with threading_helper.start_threads(workers):
+                pass
+
+            # Worker threads should not raise any exceptions
+            self.assertIsNone(cm.exc_value)
+
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2025-06-02-13-57-40.gh-issue-116738.ycJsL8.rst

@@ -0,0 +1 @@
+Make methods in :mod:`heapq` thread-safe on the :term:`free threaded <free threading>` build.

Modules/_heapqmodule.c

@@ -11,7 +11,7 @@ annotated by François Pinard, and converted to C by Raymond Hettinger.
 #endif
 
 #include "Python.h"
-#include "pycore_list.h"          // _PyList_ITEMS()
+#include "pycore_list.h"          // _PyList_ITEMS(), _PyList_AppendTakeRef()
 
 #include "clinic/_heapqmodule.c.h"
 
@@ -117,6 +117,7 @@ siftup(PyListObject *heap, Py_ssize_t pos)
 }
 
 /*[clinic input]
+@critical_section heap
 _heapq.heappush
 
     heap: object(subclass_of='&PyList_Type')
@@ -128,13 +129,22 @@ Push item onto heap, maintaining the heap invariant.
 
 static PyObject *
 _heapq_heappush_impl(PyObject *module, PyObject *heap, PyObject *item)
-/*[clinic end generated code: output=912c094f47663935 input=7c69611f3698aceb]*/
+/*[clinic end generated code: output=912c094f47663935 input=f7a4f03ef8d52e67]*/
 {
-    if (PyList_Append(heap, item))
+    if (item == NULL) {
+        PyErr_BadInternalCall();
         return NULL;
+    }
 
-    if (siftdown((PyListObject *)heap, 0, PyList_GET_SIZE(heap)-1))
+    // In a free-threaded build, the heap is locked at this point.
+    // Therefore, calling _PyList_AppendTakeRef() is safe and no overhead.
+    if (_PyList_AppendTakeRef((PyListObject *)heap, Py_NewRef(item))) {
         return NULL;
+    }
+
+    if (siftdown((PyListObject *)heap, 0, PyList_GET_SIZE(heap)-1)) {
+        return NULL;
+    }
     Py_RETURN_NONE;
 }
 
@@ -171,6 +181,7 @@ heappop_internal(PyObject *heap, int siftup_func(PyListObject *, Py_ssize_t))
 }
 
 /*[clinic input]
+@critical_section heap
 _heapq.heappop
 
     heap: object(subclass_of='&PyList_Type')
@@ -181,7 +192,7 @@ Pop the smallest item off the heap, maintaining the heap invariant.
 
 static PyObject *
 _heapq_heappop_impl(PyObject *module, PyObject *heap)
-/*[clinic end generated code: output=96dfe82d37d9af76 input=91487987a583c856]*/
+/*[clinic end generated code: output=96dfe82d37d9af76 input=ed396461b153dd51]*/
 {
     return heappop_internal(heap, siftup);
 }
@@ -207,6 +218,7 @@ heapreplace_internal(PyObject *heap, PyObject *item, int siftup_func(PyListObjec
 
 
 /*[clinic input]
+@critical_section heap
 _heapq.heapreplace
 
     heap: object(subclass_of='&PyList_Type')
@@ -226,12 +238,13 @@ this routine unless written as part of a conditional replacement:
 
 static PyObject *
 _heapq_heapreplace_impl(PyObject *module, PyObject *heap, PyObject *item)
-/*[clinic end generated code: output=82ea55be8fbe24b4 input=719202ac02ba10c8]*/
+/*[clinic end generated code: output=82ea55be8fbe24b4 input=9be1678b817ef1a9]*/
 {
     return heapreplace_internal(heap, item, siftup);
 }
 
 /*[clinic input]
+@critical_section heap
 _heapq.heappushpop
 
     heap: object(subclass_of='&PyList_Type')
@@ -246,7 +259,7 @@ a separate call to heappop().
 
 static PyObject *
 _heapq_heappushpop_impl(PyObject *module, PyObject *heap, PyObject *item)
-/*[clinic end generated code: output=67231dc98ed5774f input=5dc701f1eb4a4aa7]*/
+/*[clinic end generated code: output=67231dc98ed5774f input=db05c81b1dd92c44]*/
 {
     PyObject *returnitem;
     int cmp;
@@ -371,6 +384,7 @@ heapify_internal(PyObject *heap, int siftup_func(PyListObject *, Py_ssize_t))
 }
 
 /*[clinic input]
+@critical_section heap
 _heapq.heapify
 
     heap: object(subclass_of='&PyList_Type')
@@ -381,7 +395,7 @@ Transform list into a heap, in-place, in O(len(heap)) time.
 
 static PyObject *
 _heapq_heapify_impl(PyObject *module, PyObject *heap)
-/*[clinic end generated code: output=e63a636fcf83d6d0 input=53bb7a2166febb73]*/
+/*[clinic end generated code: output=e63a636fcf83d6d0 input=aaaaa028b9b6af08]*/
 {
     return heapify_internal(heap, siftup);
 }
@@ -481,6 +495,7 @@ siftup_max(PyListObject *heap, Py_ssize_t pos)
 }
 
 /*[clinic input]
+@critical_section heap
 _heapq.heappush_max
 
     heap: object(subclass_of='&PyList_Type')
@@ -492,9 +507,16 @@ Push item onto max heap, maintaining the heap invariant.
 
 static PyObject *
 _heapq_heappush_max_impl(PyObject *module, PyObject *heap, PyObject *item)
-/*[clinic end generated code: output=c869d5f9deb08277 input=4743d7db137b6e2b]*/
+/*[clinic end generated code: output=c869d5f9deb08277 input=c437e3d1ff8dcb70]*/
 {
-    if (PyList_Append(heap, item)) {
+    if (item == NULL) {
+        PyErr_BadInternalCall();
+        return NULL;
+    }
+
+    // In a free-threaded build, the heap is locked at this point.
+    // Therefore, calling _PyList_AppendTakeRef() is safe and no overhead.
+    if (_PyList_AppendTakeRef((PyListObject *)heap, Py_NewRef(item))) {
         return NULL;
     }
 
@@ -506,6 +528,7 @@ _heapq_heappush_max_impl(PyObject *module, PyObject *heap, PyObject *item)
 }
 
 /*[clinic input]
+@critical_section heap
 _heapq.heappop_max
 
     heap: object(subclass_of='&PyList_Type')
@@ -516,12 +539,13 @@ Maxheap variant of heappop.
 
 static PyObject *
 _heapq_heappop_max_impl(PyObject *module, PyObject *heap)
-/*[clinic end generated code: output=2f051195ab404b77 input=e62b14016a5a26de]*/
+/*[clinic end generated code: output=2f051195ab404b77 input=5d70c997798aec64]*/
 {
     return heappop_internal(heap, siftup_max);
 }
 
 /*[clinic input]
+@critical_section heap
 _heapq.heapreplace_max
 
     heap: object(subclass_of='&PyList_Type')
@@ -533,12 +557,13 @@ Maxheap variant of heapreplace.
 
 static PyObject *
 _heapq_heapreplace_max_impl(PyObject *module, PyObject *heap, PyObject *item)
-/*[clinic end generated code: output=8770778b5a9cbe9b input=21a3d28d757c881c]*/
+/*[clinic end generated code: output=8770778b5a9cbe9b input=fe70175356e4a649]*/
 {
     return heapreplace_internal(heap, item, siftup_max);
 }
 
 /*[clinic input]
+@critical_section heap
 _heapq.heapify_max
 
     heap: object(subclass_of='&PyList_Type')
@@ -549,12 +574,13 @@ Maxheap variant of heapify.
 
 static PyObject *
 _heapq_heapify_max_impl(PyObject *module, PyObject *heap)
-/*[clinic end generated code: output=8401af3856529807 input=edda4255728c431e]*/
+/*[clinic end generated code: output=8401af3856529807 input=4eee63231e7d1573]*/
 {
     return heapify_internal(heap, siftup_max);
 }
 
 /*[clinic input]
+@critical_section heap
 _heapq.heappushpop_max
 
     heap: object(subclass_of='&PyList_Type')
@@ -569,7 +595,7 @@ a separate call to heappop_max().
 
 static PyObject *
 _heapq_heappushpop_max_impl(PyObject *module, PyObject *heap, PyObject *item)
-/*[clinic end generated code: output=ff0019f0941aca0d input=525a843013cbd6c0]*/
+/*[clinic end generated code: output=ff0019f0941aca0d input=24d0defa6fd6df4a]*/
 {
     PyObject *returnitem;
     int cmp;

Modules/clinic/_heapqmodule.c.h

@@ -2,6 +2,7 @@
 preserve
 [clinic start generated code]*/
 
+#include "pycore_critical_section.h"// Py_BEGIN_CRITICAL_SECTION()
 #include "pycore_modsupport.h"    // _PyArg_CheckPositional()
 
 PyDoc_STRVAR(_heapq_heappush__doc__,
@@ -32,7 +33,9 @@ _heapq_heappush(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
     }
     heap = args[0];
     item = args[1];
+    Py_BEGIN_CRITICAL_SECTION(heap);
     return_value = _heapq_heappush_impl(module, heap, item);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
@@ -61,7 +64,9 @@ _heapq_heappop(PyObject *module, PyObject *arg)
         goto exit;
     }
     heap = arg;
+    Py_BEGIN_CRITICAL_SECTION(heap);
     return_value = _heapq_heappop_impl(module, heap);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
@@ -103,7 +108,9 @@ _heapq_heapreplace(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
     }
     heap = args[0];
     item = args[1];
+    Py_BEGIN_CRITICAL_SECTION(heap);
     return_value = _heapq_heapreplace_impl(module, heap, item);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
@@ -140,7 +147,9 @@ _heapq_heappushpop(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
     }
     heap = args[0];
     item = args[1];
+    Py_BEGIN_CRITICAL_SECTION(heap);
     return_value = _heapq_heappushpop_impl(module, heap, item);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
@@ -169,7 +178,9 @@ _heapq_heapify(PyObject *module, PyObject *arg)
         goto exit;
     }
     heap = arg;
+    Py_BEGIN_CRITICAL_SECTION(heap);
     return_value = _heapq_heapify_impl(module, heap);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
@@ -203,7 +214,9 @@ _heapq_heappush_max(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
     }
     heap = args[0];
     item = args[1];
+    Py_BEGIN_CRITICAL_SECTION(heap);
     return_value = _heapq_heappush_max_impl(module, heap, item);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
@@ -232,7 +245,9 @@ _heapq_heappop_max(PyObject *module, PyObject *arg)
         goto exit;
     }
     heap = arg;
+    Py_BEGIN_CRITICAL_SECTION(heap);
     return_value = _heapq_heappop_max_impl(module, heap);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
@@ -266,7 +281,9 @@ _heapq_heapreplace_max(PyObject *module, PyObject *const *args, Py_ssize_t nargs
     }
     heap = args[0];
     item = args[1];
+    Py_BEGIN_CRITICAL_SECTION(heap);
     return_value = _heapq_heapreplace_max_impl(module, heap, item);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
@@ -295,7 +312,9 @@ _heapq_heapify_max(PyObject *module, PyObject *arg)
         goto exit;
     }
     heap = arg;
+    Py_BEGIN_CRITICAL_SECTION(heap);
     return_value = _heapq_heapify_max_impl(module, heap);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
@@ -332,9 +351,11 @@ _heapq_heappushpop_max(PyObject *module, PyObject *const *args, Py_ssize_t nargs
     }
     heap = args[0];
     item = args[1];
+    Py_BEGIN_CRITICAL_SECTION(heap);
     return_value = _heapq_heappushpop_max_impl(module, heap, item);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=f55d8595ce150c76 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=e83d50002c29a96d input=a9049054013a1b77]*/