gh-127716: make memoryview thread-safe

Co-authored-by: ayaangazali <ayaangazali.work@gmail.com>
Co-authored-by: Lu Xiaowei <weixlu420302@gmail.com>

Author: 2 people

Doc/library/threadsafety.rst

@@ -604,3 +604,9 @@ underlying object supports it:
 Resizing or reallocating the underlying object (such as calling
 :meth:`bytearray.resize`) while a memoryview is exported raises
 :exc:`BufferError`. This is enforced regardless of threading.
+
+In the :term:`free-threaded build`, :meth:`~memoryview.release` is
+thread-safe. If another thread has an in-flight operation on the memoryview
+when it is released, the underlying buffer may remain exported until that
+operation completes. Restrictions on the exporter, such as prohibiting
+resizing, remain in effect until then.

Include/cpython/memoryobject.h

@@ -11,7 +11,7 @@
 typedef struct {
     PyObject_HEAD
     int flags;          /* state flags */
-    Py_ssize_t exports; /* number of direct memoryview exports */
+    Py_ssize_t exports; /* registered views and active operations */
     Py_buffer master; /* snapshot buffer obtained from the original exporter */
 } _PyManagedBufferObject;
 

Lib/test/test_free_threading/test_memoryview.py

@@ -0,0 +1,313 @@
+import pickle
+import threading
+import unittest
+
+from test.support import threading_helper
+
+NTHREADS = 8
+
+# Fresh objects expose one-time races under ThreadSanitizer.
+ROUNDS = 20
+ITERS = 20
+
+HASH_DATA_TEMPLATE = bytes(range(256))
+HASH_DATA_REPEAT = 256
+
+READ_OPS = [
+    lambda mv: mv.tobytes(),
+    lambda mv: mv.hex(),
+    lambda mv: mv.tolist(),
+    lambda mv: mv.cast("B"),
+    lambda mv: mv.toreadonly(),
+    lambda mv: memoryview(mv),
+    lambda mv: bytes(mv),
+    lambda mv: pickle.PickleBuffer(mv).release(),
+    lambda mv: mv.obj,
+    lambda mv: mv.format,
+    lambda mv: mv.c_contiguous,
+    lambda mv: mv[0],
+    lambda mv: mv[0:4],
+    lambda mv: mv.count(0),
+    lambda mv: mv.index(0),
+    lambda mv: mv == mv,
+    lambda mv: list(mv),
+    lambda mv: len(mv),
+]
+
+
+def run_racy(func, *args):
+    try:
+        func(*args)
+    except (ValueError, BufferError):
+        pass
+
+
+@threading_helper.requires_working_threading()
+class TestMemoryViewRaces(unittest.TestCase):
+    def assert_exporter_free(self, buf):
+        buf.append(0)
+        del buf[-1]
+
+    def test_concurrent_slicing_keeps_export_count(self):
+        for _ in range(ROUNDS):
+            mv = memoryview(bytes(64))
+            slices = []
+            lock = threading.Lock()
+
+            def make_slices():
+                local = [mv[0:4] for _ in range(ITERS)]
+                local += [memoryview(mv) for _ in range(ITERS)]
+                with lock:
+                    slices.extend(local)
+
+            threading_helper.run_concurrently(make_slices, nthreads=NTHREADS)
+            del slices
+
+            self.assertEqual(bytes(mv[0:4]), b"\x00" * 4)
+
+    def test_concurrent_release(self):
+        buf = bytearray(64)
+
+        for _ in range(ROUNDS):
+            views = [memoryview(buf) for _ in range(NTHREADS)]
+
+            def release(views=views):
+                for mv in views:
+                    run_racy(mv.release)
+
+            threading_helper.run_concurrently(release, nthreads=NTHREADS)
+
+        self.assert_exporter_free(buf)
+
+    def test_release_races_with_reads(self):
+        for _ in range(ROUNDS):
+            buf = bytearray(64)
+            cell = [memoryview(buf)]
+            lock = threading.Lock()
+
+            def releaser():
+                for _ in range(ITERS):
+                    mv = cell[0]
+                    run_racy(mv.release)
+                    with lock:
+                        cell[0] = memoryview(buf)
+
+            def reader():
+                for _ in range(ITERS):
+                    mv = cell[0]
+                    for op in READ_OPS:
+                        run_racy(op, mv)
+
+            threading_helper.run_concurrently(
+                [releaser] * (NTHREADS // 2) + [reader] * (NTHREADS // 2),
+                nthreads=NTHREADS,
+            )
+
+            cell[0].release()
+            self.assert_exporter_free(buf)
+
+    def test_read_keeps_exporter_alive_after_release(self):
+        size = 1 << 20
+        for _ in range(ROUNDS):
+            exporter = [bytearray(size)]
+            view = memoryview(exporter[0])
+            stale = []
+
+            def reader():
+                try:
+                    for _ in range(4):
+                        data = view.tobytes()
+                        if data.count(0) != len(data):
+                            stale.append(True)
+                            return
+                except ValueError:
+                    pass
+
+            def releaser():
+                view.release()
+                exporter.clear()
+                for _ in range(8):
+                    bytearray(b"\xdb" * size)
+
+            threading_helper.run_concurrently([reader, releaser])
+            self.assertFalse(stale)
+
+    def test_release_races_with_writes(self):
+        for _ in range(ROUNDS):
+            buf = bytearray(8 * NTHREADS)
+            cell = [memoryview(buf)]
+            lock = threading.Lock()
+
+            def releaser():
+                for _ in range(ITERS):
+                    mv = cell[0]
+                    run_racy(mv.release)
+                    with lock:
+                        cell[0] = memoryview(buf)
+
+            def writer(slot):
+                start = slot * 8
+                for _ in range(ITERS):
+                    mv = cell[0]
+                    run_racy(mv.__setitem__, start, 1)
+                    run_racy(mv.__setitem__, slice(start, start + 4), b"abcd")
+
+            workers = [releaser] * (NTHREADS // 2)
+            workers += [lambda s=s: writer(s) for s in range(NTHREADS // 2)]
+            threading_helper.run_concurrently(workers, nthreads=NTHREADS)
+
+            cell[0].release()
+            self.assert_exporter_free(buf)
+
+    def test_release_races_with_buffer_exports(self):
+        for _ in range(ROUNDS):
+            buf = bytearray(64)
+            mv = memoryview(buf)
+
+            def exporter():
+                for _ in range(ITERS):
+                    run_racy(lambda: pickle.PickleBuffer(mv).release())
+
+            def releaser():
+                for _ in range(ITERS):
+                    run_racy(mv.release)
+
+            threading_helper.run_concurrently(
+                [exporter] * (NTHREADS - 1) + [releaser], nthreads=NTHREADS
+            )
+
+            mv.release()
+            self.assert_exporter_free(buf)
+
+    def test_release_with_live_export(self):
+        buf = bytearray(64)
+
+        for _ in range(ROUNDS):
+            mv = memoryview(buf)
+            held = pickle.PickleBuffer(mv)
+
+            def release(mv=mv):
+                try:
+                    mv.release()
+                except BufferError:
+                    pass
+
+            threading_helper.run_concurrently(release, nthreads=NTHREADS)
+
+            self.assertEqual(bytes(mv[0:4]), b"\x00" * 4)
+            held.release()
+            mv.release()
+
+        self.assert_exporter_free(buf)
+
+    def test_compare_two_views_races_with_release(self):
+        for _ in range(ROUNDS):
+            buf = bytearray(64)
+            cell = [memoryview(buf), memoryview(buf)]
+            lock = threading.Lock()
+
+            def releaser(slot):
+                for _ in range(ITERS):
+                    mv = cell[slot]
+                    run_racy(mv.release)
+                    with lock:
+                        cell[slot] = memoryview(buf)
+
+            def comparer():
+                for _ in range(ITERS):
+                    left, right = cell[0], cell[1]
+                    run_racy(lambda: left == right)
+                    run_racy(lambda: left != right)
+
+            threading_helper.run_concurrently(
+                [lambda: releaser(0), lambda: releaser(1)] + [comparer] * 6,
+                nthreads=NTHREADS,
+            )
+
+            for mv in cell:
+                mv.release()
+            self.assert_exporter_free(buf)
+
+    def test_release_parent_keeps_child_valid(self):
+        for _ in range(ROUNDS):
+            buf = bytearray(range(64))
+            parent = memoryview(buf)
+            child = parent[0:32]
+
+            def use_child():
+                for _ in range(ITERS):
+                    self.assertEqual(child[0], 0)
+                    child.tobytes()
+
+            def release_parent():
+                parent.release()
+
+            threading_helper.run_concurrently(
+                [release_parent] + [use_child] * (NTHREADS - 1),
+                nthreads=NTHREADS,
+            )
+
+            self.assertEqual(child.tobytes(), bytes(range(32)))
+            child.release()
+            self.assert_exporter_free(buf)
+
+    def test_concurrent_iteration(self):
+        for _ in range(ROUNDS):
+            buf = bytearray(range(64))
+            cell = [memoryview(buf)]
+            lock = threading.Lock()
+
+            def releaser():
+                for _ in range(ITERS):
+                    mv = cell[0]
+                    run_racy(mv.release)
+                    with lock:
+                        cell[0] = memoryview(buf)
+
+            def iterator():
+                for _ in range(ITERS):
+                    run_racy(list, cell[0])
+
+            threading_helper.run_concurrently(
+                [releaser] * (NTHREADS // 2) + [iterator] * (NTHREADS // 2),
+                nthreads=NTHREADS,
+            )
+
+            cell[0].release()
+            self.assert_exporter_free(buf)
+
+    def test_iterator_exhaustion_drops_last_reference(self):
+        def loop():
+            for _ in range(ROUNDS * ITERS):
+                self.assertEqual(list(iter(memoryview(b"ab"))), [97, 98])
+
+        threading_helper.run_concurrently(loop, nthreads=NTHREADS)
+
+    def test_concurrent_hash(self):
+        for _ in range(ROUNDS):
+            data = HASH_DATA_TEMPLATE * HASH_DATA_REPEAT
+            mv = memoryview(data)
+            results = []
+            lock = threading.Lock()
+
+            def hasher():
+                local = {hash(mv) for _ in range(ITERS)}
+                with lock:
+                    results.append(local)
+
+            threading_helper.run_concurrently(hasher, nthreads=NTHREADS)
+            self.assertEqual({h for s in results for h in s}, {hash(data)})
+
+    def test_concurrent_hash_and_release(self):
+        for _ in range(ROUNDS):
+            mv = memoryview(HASH_DATA_TEMPLATE * HASH_DATA_REPEAT)
+
+            def work(mv=mv):
+                run_racy(hash, mv)
+                run_racy(mv.release)
+
+            threading_helper.run_concurrently(work, nthreads=NTHREADS)
+
+
+if __name__ == "__main__":
+    unittest.main()

Lib/test/test_memoryview.py

@@ -91,6 +91,13 @@ def test_iter(self):
             m = self._view(b)
             self.assertEqual(list(m), [m[i] for i in range(len(m))])
 
+    def test_released_empty_iterator(self):
+        for tp in self._types:
+            m = self._view(tp(b""))
+            iterator = iter(m)
+            m.release()
+            self.assertRaises(StopIteration, next, iterator)
+
     def test_count(self):
         for tp in self._types:
             b = tp(self._source)

Misc/NEWS.d/next/Core_and_Builtins/2026-08-21-14-37-50.gh-issue-127716.KrPth8.rst

@@ -0,0 +1,9 @@
+On the free-threaded build, :class:`memoryview` operations are now thread safe:
+an operation that reads the buffer holds the managed buffer for its duration,
+so a concurrent :meth:`~memoryview.release` can no longer free the buffer under
+it; slicing a memoryview from several threads no longer loses buffer
+registrations and raises ``ValueError: operation forbidden on released
+memoryview object``; and :func:`hash` of a memoryview is no longer a data race.
+As a result :meth:`~memoryview.release` may now return while an operation
+started by another thread is still running, see
+:ref:`thread-safety-memoryview`.