youknowone · Oct 12, 2026 · Oct 11, 2026 · Oct 12, 2026 · Oct 12, 2026 · Oct 12, 2026
diff --git a/crates/vm/src/builtins/bytearray.rs b/crates/vm/src/builtins/bytearray.rs
@@ -173,11 +173,6 @@ impl PyByteArray {
     fn inner(&self) -> PyDetachingRwLockReadGuard<'_, PyBytesInner> {
         self.inner.read()
     }
-    #[inline]
-    fn inner_mut(&self) -> PyDetachingRwLockWriteGuard<'_, PyBytesInner> {
-        self.inner.write()
-    }
-
     fn __len__(&self) -> usize {
         self.borrow_buf().len()
     }
@@ -781,9 +776,18 @@ impl Initializer for PyByteArray {
     type Args = ByteInnerNewOptions;
 
     fn init(zelf: &Py<Self>, options: Self::Args, vm: &VirtualMachine) -> PyResult<()> {
-        // First unpack bytearray and *then* get a lock to set it.
-        let mut inner = options.get_inner(bytearray_from_object, vm)?;
-        core::mem::swap(&mut *zelf.inner_mut(), &mut inner);
+        // Reject exports and clear existing contents before converting source.
+        zelf.try_resizable(vm)?.elements.clear();
+        // Conversion can call Python, so do not hold the lock across it and
+        // check again for exports acquired by a callback or another thread.
+        let (mut inner, zero_count) =
+            options.get_inner_with_zero_count(bytearray_from_object, vm)?;
+        // A zero integer count performs no further writes, including when its
+        // __index__ callback changed this bytearray or acquired an export.
+        if zero_count {
+            return Ok(());
+        }
+        core::mem::swap(&mut *zelf.try_resizable(vm)?, &mut inner);
         Ok(())
     }
 }
@@ -841,9 +845,11 @@ impl AsBuffer for PyByteArray {
     }
 
     fn as_buffer(zelf: &Py<Self>, _vm: &VirtualMachine) -> PyResult<PyBuffer> {
+        // Keep the length stable until PyBuffer::new retains the export.
+        let inner = zelf.inner();
         Ok(PyBuffer::new(
             zelf.to_owned().into(),
-            BufferDescriptor::simple(zelf.__len__(), false),
+            BufferDescriptor::simple(inner.elements.len(), false),
             &BUFFER_METHODS,
         ))
     }
@@ -855,7 +861,12 @@ impl BufferResizeGuard for PyByteArray {
     fn try_resizable_opt(&self) -> Option<Self::Resizable<'_>> {
         // An export is a borrow someone else still holds, so it is answered
         // before the lock rather than by waiting on it.
-        (self.exports.load(Ordering::SeqCst) == 0).then(|| self.inner.write())
+        if self.exports.load(Ordering::SeqCst) != 0 {
+            return None;
+        }
+        let inner = self.inner.write();
+        // An export may have been acquired while we waited for the lock.
+        (self.exports.load(Ordering::SeqCst) == 0).then_some(inner)
     }
 }
 
@@ -907,7 +918,7 @@ impl AsSequence for PyByteArray {
                 if let Some(value) = value {
                     zelf._setitem_by_index(i, &value, vm)
                 } else {
-                    zelf.borrow_buf_mut().delitem_by_index(vm, i)
+                    zelf.try_resizable(vm)?.elements.delitem_by_index(vm, i)
                 }
             }),
             contains: atomic_func!(|seq, other, vm| {

diff --git a/crates/vm/src/bytes_inner.rs b/crates/vm/src/bytes_inner.rs
@@ -225,12 +225,24 @@ impl ByteInnerNewOptions {
     /// read: `bytes()` and `bytearray()` differ in whether they ask it how long
     /// it is.
     pub fn get_inner(self, from_object: FromObject, vm: &VirtualMachine) -> PyResult<PyBytesInner> {
-        match (self.source, self.encoding, self.errors) {
+        self.get_inner_with_zero_count(from_object, vm)
+            .map(|(inner, _)| inner)
+    }
+
+    pub(crate) fn get_inner_with_zero_count(
+        self,
+        from_object: FromObject,
+        vm: &VirtualMachine,
+    ) -> PyResult<(PyBytesInner, bool)> {
+        let mut zero_count = false;
+        let inner = match (self.source, self.encoding, self.errors) {
             (OptionalArg::Present(obj), OptionalArg::Missing, OptionalArg::Missing) => {
                 // Try __index__ first to handle int-like objects that might raise custom exceptions
                 if let Some(index_result) = obj.try_index_opt(vm) {
                     match index_result {
-                        Ok(index) => Self::get_value_from_size(&index, vm),
+                        Ok(index) => Self::get_value_from_size(&index, vm).inspect(|inner| {
+                            zero_count = inner.elements.is_empty();
+                        }),
                         Err(e) => {
                             // Only propagate non-TypeError exceptions
                             // TypeError means the object doesn't support __index__, so fall back
@@ -266,7 +278,8 @@ impl ByteInnerNewOptions {
             (OptionalArg::Present(_), OptionalArg::Missing, OptionalArg::Present(_)) => {
                 Err(vm.new_type_error(STRING_WITHOUT_ENCODING.to_owned()))
             }
-        }
+        }?;
+        Ok((inner, zero_count))
     }
 }
 

diff --git a/crates/vm/src/stdlib/_io.rs b/crates/vm/src/stdlib/_io.rs
@@ -762,10 +762,13 @@ mod _io {
                             )));
                         }
                         let n = n as usize;
-                        let mut bytes = b.borrow_buf_mut();
-                        bytes.truncate(n);
-                        // FIXME: try to use Arc::unwrap on the bytearray to get at the inner buffer
-                        bytes.clone().to_pyobject(vm)
+                        let mut inner = b.try_resizable(vm)?;
+                        let bytes = &mut inner.elements;
+                        bytes
+                            .try_reserve_exact(n.saturating_sub(bytes.len()))
+                            .map_err(|_| vm.no_memory_error())?;
+                        bytes.resize(n, 0);
+                        core::mem::take(bytes).to_pyobject(vm)
                     }
                 })
             } else {

diff --git a/extra_tests/snippets/builtin_bytearray_exports.py b/extra_tests/snippets/builtin_bytearray_exports.py
@@ -0,0 +1,136 @@
+"""Resizing paths must respect outstanding bytearray exports."""
+
+import io
+
+from testutils import assert_raises
+
+for args in ((), (b"abc",), (b"x",), (b"",), (object(),)):
+    value = bytearray(b"abc")
+    with memoryview(value) as view:
+        with assert_raises(BufferError):
+            value.__init__(*args)
+        assert value == b"abc"
+        assert view.tobytes() == b"abc"
+    value.__init__(b"released")
+    assert value == b"released"
+
+value = bytearray(b"abc")
+with assert_raises(TypeError):
+    value.__init__(object())
+assert value == b""
+value = bytearray(b"abc")
+value.__init__(value)
+assert value == b""
+
+
+class ExportDuringInit:
+    def __init__(self, count, payload=b""):
+        self.count = count
+        self.payload = payload
+        self.calls = 0
+
+    def __index__(self):
+        self.calls += 1
+        value.extend(self.payload)
+        self.view = memoryview(value)
+        return self.count
+
+
+value = bytearray(b"abc")
+source = ExportDuringInit(2)
+with assert_raises(BufferError):
+    value.__init__(source)
+assert value == b""
+assert source.view.tobytes() == b""
+assert source.calls == 1
+source.view.release()
+value.extend(b"released")
+assert value == b"released"
+
+value = bytearray(b"abc")
+source = ExportDuringInit(0)
+value.__init__(source)
+assert value == b""
+assert source.view.tobytes() == b""
+source.view.release()
+
+value = bytearray(b"abc")
+source = ExportDuringInit(0, b"xy")
+value.__init__(source)
+assert value == b"xy"
+assert source.view.tobytes() == b"xy"
+assert source.calls == 1
+source.view.release()
+
+
+class FailingIndex:
+    def __index__(self):
+        assert value == b""
+        raise ValueError("conversion failed")
+
+
+value = bytearray(b"abc")
+with assert_raises(ValueError):
+    value.__init__(FailingIndex())
+assert value == b""
+
+for key in (0, slice(0, 1)):
+    value = bytearray(b"abc")
+    with memoryview(value) as view:
+        with assert_raises(BufferError):
+            del value[key]
+        assert view.tobytes() == b"abc"
+    del value[key]
+    assert value == b"bc"
+
+
+class Reader(io.RawIOBase):
+    def __init__(self, count, retain, payload=b"abcd"):
+        self.count = count
+        self.retain = retain
+        self.payload = payload
+
+    def readinto(self, buffer):
+        buffer[:] = self.payload
+        self.buffer = buffer
+        if self.retain:
+            self.view = memoryview(buffer)
+        return self.count
+
+
+for count in (0, 2, 4):
+    reader = Reader(count, False)
+    assert reader.read(4) == b"abcd"[:count]
+    assert reader.buffer == b""
+
+    reader = Reader(count, True)
+    with assert_raises(BufferError):
+        reader.read(4)
+    assert reader.view.tobytes() == b"abcd"
+    reader.view.release()
+    reader.buffer.clear()
+
+for count in (-1, 5):
+    reader = Reader(count, True)
+    with assert_raises(ValueError):
+        reader.read(4)
+    assert reader.view.tobytes() == b"abcd"
+    reader.view.release()
+
+reader = Reader(None, True)
+assert reader.read(4) is None
+assert reader.view.tobytes() == b"abcd"
+reader.view.release()
+
+# The callback can shrink the temporary buffer, so slicing by the original
+# requested size without checking the current buffer is not safe.
+reader = Reader(2, False, b"xy")
+assert reader.read(4) == b"xy"
+assert reader.buffer == b""
+
+reader = Reader(4, False, b"xy")
+result = reader.read(4)
+assert len(result) == 4
+assert result[:2] == b"xy"
+# The contents of the newly grown region are unspecified by CPython.
+assert reader.buffer == b""
diff --git a/extra_tests/snippets/builtin_bytearray_threading.py b/extra_tests/snippets/builtin_bytearray_threading.py
@@ -0,0 +1,55 @@
+"""Acquiring a bytearray export must be atomic with respect to resizing."""
+
+import threading
+
+
+def exercise_exports():
+    payload = b"x" * 65536
+    value = bytearray(payload)
+    barrier = threading.Barrier(3, timeout=5)
+    errors = []
+
+    def worker(operation):
+        try:
+            for _ in range(256):
+                barrier.wait()
+                operation()
+                barrier.wait()
+        except BaseException as exc:
+            errors.append(exc)
+            barrier.abort()
+
+    def resize():
+        try:
+            value.clear()
+            value.extend(payload)
+        except BufferError:
+            pass
+
+    def decode():
+        result = value.decode("ascii")
+        assert result in ("", "x" * len(payload))
+
+    def export():
+        with memoryview(value) as view:
+            size = len(view)
+            assert size in (0, len(payload))
+            # The acquired export keeps its original length and contents even
+            # if the resizing thread runs between these two reads.
+            assert view.tobytes() == payload[:size]
+            assert len(view) == size
+
+    threads = [
+        threading.Thread(target=worker, args=(operation,), daemon=True)
+        for operation in (resize, decode, export)
+    ]
+    for thread in threads:
+        thread.start()
+    for thread in threads:
+        thread.join(timeout=10)
+    assert not any(thread.is_alive() for thread in threads), "worker did not finish"
+    if errors:
+        raise errors[0]
+
+
+exercise_exports()