youknowone · Mar 7, 2026 · Mar 7, 2026 · Mar 7, 2026 · coderabbitai · Mar 7, 2026
diff --git a/.cspell.dict/cpython.txt b/.cspell.dict/cpython.txt
@@ -127,8 +127,8 @@ NEWLOCALS
 newsemlockobject
 nfrees
 nkwargs
-nlocalsplus
 nkwelts
+nlocalsplus
 Nondescriptor
 noninteger
 nops
@@ -160,6 +160,7 @@ pylifecycle
 pymain
 pyrepl
 PYTHONTRACEMALLOC
+PYTHONUTF8
 pythonw
 PYTHREAD_NAME
 releasebuffer
@@ -171,9 +172,11 @@ saveall
 scls
 setdict
 setfunc
+setprofileallthreads
 SETREF
 setresult
 setslice
+settraceallthreads
 SLOTDEFINED
 SMALLBUF
 SOABI
@@ -190,8 +193,10 @@ subparams
 subscr
 sval
 swappedbytes
+sysdict
 templatelib
 testconsole
+threadstate
 ticketer
 tmptype
 tok_oldval

diff --git a/.cspell.json b/.cspell.json
@@ -152,11 +152,6 @@
     "IFEXEC",
     // "stat"
     "FIRMLINK",
-    // CPython internal names
-    "PYTHONUTF",
-    "sysdict",
-    "settraceallthreads",
-    "setprofileallthreads"
   ],
   // flagWords - list of words to be always considered incorrect
   "flagWords": [

diff --git a/Lib/test/test_fork1.py b/Lib/test/test_fork1.py
@@ -19,7 +19,6 @@
 
 
 class ForkTest(ForkWait):
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; AssertionError: process 44587 exited with code 1, but exit code 42 is expected
     def test_threaded_import_lock_fork(self):
         """Check fork() in main thread works while a subthread is doing an import"""
         import_started = threading.Event()

diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
@@ -5574,7 +5574,6 @@ def test_fork_warns_when_non_python_thread_exists(self):
         self.assertEqual(err.decode("utf-8"), "")
         self.assertEqual(out.decode("utf-8"), "")
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; AssertionError: b"can't fork at interpreter shutdown" not found in b"Exception ignored in: <function AtFinalization.__del__ at 0xc508b30c0>\nAttributeError: 'NoneType' object has no attribute 'fork'\n"
     def test_fork_at_finalization(self):
         code = """if 1:
             import atexit

diff --git a/crates/common/src/lock.rs b/crates/common/src/lock.rs
@@ -68,43 +68,45 @@ pub type PyMappedRwLockWriteGuard<'a, T> = MappedRwLockWriteGuard<'a, RawRwLock,
 
 // can add fn const_{mutex,rw_lock}() if necessary, but we probably won't need to
 
-/// Reset a `PyMutex` to its initial (unlocked) state after `fork()`.
+/// Reset a lock to its initial (unlocked) state by zeroing its bytes.
 ///
-/// After `fork()`, locks held by dead parent threads would deadlock in the
-/// child. This writes `RawMutex::INIT` via the `Mutex::raw()` accessor,
-/// bypassing the normal unlock path which may interact with parking_lot's
-/// internal waiter queues.
+/// After `fork()`, any lock held by a now-dead thread would remain
+/// permanently locked. We zero the raw bytes (the unlocked state for all
+/// `parking_lot` raw lock types) instead of using the normal unlock path,
+/// which would interact with stale waiter queues.
 ///
 /// # Safety
 ///
 /// Must only be called from the single-threaded child process immediately
 /// after `fork()`, before any other thread is created.
-#[cfg(unix)]
-pub unsafe fn reinit_mutex_after_fork<T: ?Sized>(mutex: &PyMutex<T>) {
-    // Use Mutex::raw() to access the underlying lock without layout assumptions.
-    // parking_lot::RawMutex (AtomicU8) and RawCellMutex (Cell<bool>) both
-    // represent the unlocked state as all-zero bytes.
+/// The type `T` must represent the unlocked state as all-zero bytes
+/// (true for `parking_lot::RawMutex`, `RawRwLock`, `RawReentrantMutex`, etc.).
+pub unsafe fn zero_reinit_after_fork<T>(lock: *const T) {
     unsafe {
-        let raw = mutex.raw() as *const RawMutex as *mut u8;
-        core::ptr::write_bytes(raw, 0, core::mem::size_of::<RawMutex>());
+        core::ptr::write_bytes(lock as *mut u8, 0, core::mem::size_of::<T>());
     }
 }
 
-/// Reset a `PyRwLock` to its initial (unlocked) state after `fork()`.
+/// Reset a `PyMutex` after `fork()`. See [`zero_reinit_after_fork`].
+///
+/// # Safety
 ///
-/// Same rationale as [`reinit_mutex_after_fork`] — dead threads' read or
-/// write locks would cause permanent deadlock in the child.
+/// Must only be called from the single-threaded child process immediately
+/// after `fork()`, before any other thread is created.
+#[cfg(unix)]
+pub unsafe fn reinit_mutex_after_fork<T: ?Sized>(mutex: &PyMutex<T>) {
+    unsafe { zero_reinit_after_fork(mutex.raw()) }
+}
+
+/// Reset a `PyRwLock` after `fork()`. See [`zero_reinit_after_fork`].
 ///
 /// # Safety
 ///
 /// Must only be called from the single-threaded child process immediately
 /// after `fork()`, before any other thread is created.
 #[cfg(unix)]
 pub unsafe fn reinit_rwlock_after_fork<T: ?Sized>(rwlock: &PyRwLock<T>) {
-    unsafe {
-        let raw = rwlock.raw() as *const RawRwLock as *mut u8;
-        core::ptr::write_bytes(raw, 0, core::mem::size_of::<RawRwLock>());
-    }
+    unsafe { zero_reinit_after_fork(rwlock.raw()) }
 }
 
 /// Reset a `PyThreadMutex` to its initial (unlocked, unowned) state after `fork()`.

diff --git a/crates/stdlib/src/select.rs b/crates/stdlib/src/select.rs
@@ -280,7 +280,9 @@ mod decl {
 
         loop {
             let mut tv = timeout.map(sec_to_timeval);
-            let res = super::select(nfds, &mut r, &mut w, &mut x, tv.as_mut());
+            let res = vm.allow_threads(|| {
+                super::select(nfds, &mut r, &mut w, &mut x, tv.as_mut())
+            });
 
             match res {
                 Ok(_) => break,

diff --git a/crates/stdlib/src/socket.rs b/crates/stdlib/src/socket.rs
@@ -1105,7 +1105,8 @@ mod _socket {
             loop {
                 if deadline.is_some() || matches!(select, SelectKind::Connect) {
                     let interval = deadline.as_ref().map(|d| d.time_until()).transpose()?;
-                    let res = sock_select(&*self.sock()?, select, interval);
+                    let sock = self.sock()?;
+                    let res = vm.allow_threads(|| sock_select(&sock, select, interval));
                     match res {
                         Ok(true) => return Err(IoOrPyException::Timeout),
                         Err(e) if e.kind() == io::ErrorKind::Interrupted => {
@@ -1118,8 +1119,9 @@ mod _socket {
                 }
 
                 let err = loop {
-                    // loop on interrupt
-                    match f() {
+                    // Detach thread state around the blocking syscall so
+                    // stop-the-world can park this thread (e.g. before fork).
+                    match vm.allow_threads(&mut f) {
                         Ok(x) => return Ok(x),
                         Err(e) if e.kind() == io::ErrorKind::Interrupted => vm.check_signals()?,
                         Err(e) => break e,
@@ -1342,7 +1344,8 @@ mod _socket {
         ) -> Result<(), IoOrPyException> {
             let sock_addr = self.extract_address(address, caller, vm)?;
 
-            let err = match self.sock()?.connect(&sock_addr) {
+            let sock = self.sock()?;
+            let err = match vm.allow_threads(|| sock.connect(&sock_addr)) {
                 Ok(()) => return Ok(()),
                 Err(e) => e,
             };

diff --git a/crates/vm/src/frame.rs b/crates/vm/src/frame.rs
@@ -7016,6 +7016,14 @@ impl ExecutingFrame<'_> {
         Ok(None)
     }
 
+    /// Read a cached descriptor pointer and validate it against the expected
+    /// type version, using a lock-free double-check pattern:
+    ///   1. read pointer  →  incref (try_to_owned)
+    ///   2. re-read version + pointer and confirm they still match
+    ///
+    /// This matches the read-side pattern used in LOAD_ATTR_METHOD_WITH_VALUES
+    /// and friends: no read-side lock, relying on the write side to invalidate
+    /// the version tag before swapping the pointer.
     #[inline]
     fn try_read_cached_descriptor(
         &self,
@@ -7026,7 +7034,12 @@ impl ExecutingFrame<'_> {
         if descr_ptr == 0 {
             return None;
         }
+        // SAFETY: `descr_ptr` was a valid `*mut PyObject` when the writer
+        // stored it, and the writer keeps a strong reference alive in
+        // `InlineCacheEntry`.  `try_to_owned_from_ptr` performs a
+        // conditional incref that fails if the object is already freed.
         let cloned = unsafe { PyObject::try_to_owned_from_ptr(descr_ptr as *mut PyObject) }?;
+        // Double-check: version tag still matches AND pointer unchanged.
         if self.code.instructions.read_cache_u32(cache_base + 1) == expected_type_version
             && self.code.instructions.read_cache_ptr(cache_base + 5) == descr_ptr
         {
@@ -7044,8 +7057,9 @@ impl ExecutingFrame<'_> {
         type_version: u32,
         descr_ptr: usize,
     ) {
-        // Publish descriptor cache atomically as a tuple:
+        // Publish descriptor cache with version-invalidation protocol:
         // invalidate version first, then write payload, then publish version.
+        // Reader double-checks version+ptr after incref, so no writer lock needed.
         unsafe {
             self.code.instructions.write_cache_u32(cache_base + 1, 0);
             self.code
@@ -7065,7 +7079,6 @@ impl ExecutingFrame<'_> {
         metaclass_version: u32,
         descr_ptr: usize,
     ) {
-        // Same publish protocol as write_cached_descriptor(), plus metaclass guard.
         unsafe {
             self.code.instructions.write_cache_u32(cache_base + 1, 0);
             self.code

diff --git a/crates/vm/src/object/core.rs b/crates/vm/src/object/core.rs
@@ -17,7 +17,7 @@ use super::{
 };
 use crate::object::traverse_object::PyObjVTable;
 use crate::{
-    builtins::{PyDict, PyDictRef, PyType, PyTypeRef},
+    builtins::{PyDictRef, PyType, PyTypeRef},
     common::{
         atomic::{Ordering, PyAtomic, Radium},
         linked_list::{Link, Pointers},
@@ -916,6 +916,12 @@ impl InstanceDict {
     pub fn replace(&self, d: PyDictRef) -> PyDictRef {
         core::mem::replace(&mut self.d.write(), d)
     }
+
+    /// Consume the InstanceDict and return the inner PyDictRef.
+    #[inline]
+    pub fn into_inner(self) -> PyDictRef {
+        self.d.into_inner()
+    }
 }
 
 impl<T: PyPayload> PyInner<T> {
@@ -1668,11 +1674,19 @@ impl PyObject {
         }
 
         // 2. Clear dict and member slots (subtype_clear)
-        if let Some(ext) = obj.0.ext_ref() {
-            if let Some(dict) = ext.dict.as_ref() {
-                let dict_ref = dict.get();
-                // Clear dict entries to break cycles, then collect the dict itself
-                PyDict::clear(&dict_ref);
+        // Use mutable access to actually detach the dict, matching CPython's
+        // Py_CLEAR(*_PyObject_GetDictPtr(self)) which NULLs the dict pointer
+        // without clearing dict contents. This is critical because the dict
+        // may still be referenced by other live objects (e.g. function.__globals__).
+        if obj.0.has_ext() {
+            let self_addr = (ptr as *const u8).addr();
+            let ext_ptr = core::ptr::with_exposed_provenance_mut::<ObjExt>(
+                self_addr.wrapping_sub(EXT_OFFSET),
+            );
+            let ext = unsafe { &mut *ext_ptr };
+            if let Some(old_dict) = ext.dict.take() {
+                // Get the dict ref before dropping InstanceDict
+                let dict_ref = old_dict.into_inner();
                 result.push(dict_ref.into());
             }
             for slot in ext.slots.iter() {

diff --git a/crates/vm/src/stdlib/imp.rs b/crates/vm/src/stdlib/imp.rs
@@ -16,7 +16,7 @@ mod lock {
 
     #[pyfunction]
     fn acquire_lock(_vm: &VirtualMachine) {
-        IMP_LOCK.lock()
+        acquire_lock_for_fork()
     }
-    #[pyfunction]
-    fn acquire_lock(_vm: &VirtualMachine) {
-        IMP_LOCK.lock()
-        acquire_lock_for_fork()
-    }
+    #[pyfunction]
+    fn acquire_lock(vm: &VirtualMachine) {
+        vm.allow_threads(acquire_lock_for_fork)
+    }
-    #[pyfunction]
-    fn acquire_lock(_vm: &VirtualMachine) {
-        IMP_LOCK.lock()
-        acquire_lock_for_fork()
-    }
+    #[pyfunction]
+    fn acquire_lock(vm: &VirtualMachine) {
+        vm.allow_threads(acquire_lock_for_fork)
+    }
 
     #[pyfunction]
@@ -34,6 +34,16 @@ mod lock {
         IMP_LOCK.is_locked()
     }
 
+    pub(super) fn acquire_lock_for_fork() {
+        IMP_LOCK.lock();
+    }
+
+    pub(super) fn release_lock_after_fork_parent() {
+        if IMP_LOCK.is_locked() && IMP_LOCK.is_owned_by_current_thread() {
+            unsafe { IMP_LOCK.unlock() };
+        }
+    }
+
     /// Reset import lock after fork() — only if held by a dead thread.
     ///
     /// `IMP_LOCK` is a reentrant mutex. If the *current* (surviving) thread
@@ -47,22 +57,44 @@ mod lock {
     pub(crate) unsafe fn reinit_after_fork() {
         if IMP_LOCK.is_locked() && !IMP_LOCK.is_owned_by_current_thread() {
             // Held by a dead thread — reset to unlocked.
-            // Same pattern as RLock::_at_fork_reinit in thread.rs.
-            unsafe {
-                let old: &crossbeam_utils::atomic::AtomicCell<RawRMutex> =
-                    core::mem::transmute(&IMP_LOCK);
-                old.swap(RawRMutex::INIT);
-            }
+            unsafe { rustpython_common::lock::zero_reinit_after_fork(&IMP_LOCK) };
+        }
+    }
+
+    /// Match CPython's `_PyImport_ReInitLock()` + `_PyImport_ReleaseLock()`
+    /// behavior in the post-fork child:
+    /// 1) if ownership metadata is stale (dead owner / changed tid), reset;
+    /// 2) if current thread owns the lock, release it.
+    #[cfg(unix)]
+    pub(super) unsafe fn after_fork_child_reinit_and_release() {
+        unsafe { reinit_after_fork() };
+        if IMP_LOCK.is_locked() && IMP_LOCK.is_owned_by_current_thread() {
+            unsafe { IMP_LOCK.unlock() };
         }
     }
 }
 
 /// Re-export for fork safety code in posix.rs
+#[cfg(feature = "threading")]
+pub(crate) fn acquire_imp_lock_for_fork() {
+    lock::acquire_lock_for_fork();
+}
+
+#[cfg(feature = "threading")]
+pub(crate) fn release_imp_lock_after_fork_parent() {
+    lock::release_lock_after_fork_parent();
+}
+
 #[cfg(all(unix, feature = "threading"))]
 pub(crate) unsafe fn reinit_imp_lock_after_fork() {
     unsafe { lock::reinit_after_fork() }
 }
 
+#[cfg(all(unix, feature = "threading"))]
+pub(crate) unsafe fn after_fork_child_imp_lock_release() {
+    unsafe { lock::after_fork_child_reinit_and_release() }
+}
+
 #[cfg(not(feature = "threading"))]
 #[pymodule(sub)]
 mod lock {

diff --git a/crates/vm/src/stdlib/io.rs b/crates/vm/src/stdlib/io.rs
@@ -5015,13 +5015,13 @@ mod _io {
 
         if let Some(tio) = obj.downcast_ref::<TextIOWrapper>() {
             unsafe { reinit_thread_mutex_after_fork(&tio.data) };
-            if let Some(guard) = tio.data.lock() {
-                if let Some(ref data) = *guard {
-                    if let Some(ref decoder) = data.decoder {
-                        reinit_io_locks(decoder);
-                    }
-                    reinit_io_locks(&data.buffer);
+            if let Some(guard) = tio.data.lock()
+                && let Some(ref data) = *guard
+            {
+                if let Some(ref decoder) = data.decoder {
+                    reinit_io_locks(decoder);
                 }
+                reinit_io_locks(&data.buffer);
             }
             return;
         }
@@ -5044,7 +5044,6 @@ mod _io {
         if let Some(brw) = obj.downcast_ref::<BufferedRWPair>() {
             unsafe { reinit_thread_mutex_after_fork(&brw.read.data) };
             unsafe { reinit_thread_mutex_after_fork(&brw.write.data) };
-            return;
         }
     }