pythongh-115035: Mark ThreadHandles as non-joinable earlier after forking (python#115042)

This marks dead ThreadHandles as non-joinable earlier in
`PyOS_AfterFork_Child()` before we execute any Python code. The handles
are stored in a global linked list in `_PyRuntimeState` because `fork()`
affects the entire process.

Author: colesbury

Include/internal/pycore_pythread.h

@@ -9,6 +9,7 @@ extern "C" {
 #endif
 
 #include "dynamic_annotations.h" // _Py_ANNOTATE_PURE_HAPPENS_BEFORE_MUTEX
+#include "pycore_llist.h"        // struct llist_node
 
 // Get _POSIX_THREADS and _POSIX_SEMAPHORES macros if available
 #if (defined(HAVE_UNISTD_H) && !defined(_POSIX_THREADS) \
@@ -75,14 +76,22 @@ struct _pythread_runtime_state {
         struct py_stub_tls_entry tls_entries[PTHREAD_KEYS_MAX];
     } stubs;
 #endif
+
+    // Linked list of ThreadHandleObjects
+    struct llist_node handles;
 };
 
+#define _pythread_RUNTIME_INIT(pythread) \
+    { \
+        .handles = LLIST_INIT(pythread.handles), \
+    }
 
 #ifdef HAVE_FORK
 /* Private function to reinitialize a lock at fork in the child process.
    Reset the lock to the unlocked state.
    Return 0 on success, return -1 on error. */
 extern int _PyThread_at_fork_reinit(PyThread_type_lock *lock);
+extern void _PyThread_AfterFork(struct _pythread_runtime_state *state);
 #endif  /* HAVE_FORK */
 
 
@@ -143,12 +152,6 @@ PyAPI_FUNC(int) PyThread_join_thread(PyThread_handle_t);
  */
 PyAPI_FUNC(int) PyThread_detach_thread(PyThread_handle_t);
 
-/*
- * Obtain the new thread ident and handle in a forked child process.
- */
-PyAPI_FUNC(void) PyThread_update_thread_after_fork(PyThread_ident_t* ident,
-                                                   PyThread_handle_t* handle);
-
 #ifdef __cplusplus
 }
 #endif

Include/internal/pycore_runtime_init.h

@@ -16,6 +16,7 @@ extern "C" {
 #include "pycore_parser.h"        // _parser_runtime_state_INIT
 #include "pycore_pyhash.h"        // pyhash_state_INIT
 #include "pycore_pymem_init.h"    // _pymem_allocators_standard_INIT
+#include "pycore_pythread.h"      // _pythread_RUNTIME_INIT
 #include "pycore_runtime_init_generated.h"  // _Py_bytes_characters_INIT
 #include "pycore_signal.h"        // _signals_RUNTIME_INIT
 #include "pycore_tracemalloc.h"   // _tracemalloc_runtime_state_INIT
@@ -90,6 +91,7 @@ extern PyTypeObject _PyExc_MemoryError;
         }, \
         .obmalloc = _obmalloc_global_state_INIT, \
         .pyhash_state = pyhash_state_INIT, \
+        .threads = _pythread_RUNTIME_INIT(runtime.threads), \
         .signals = _signals_RUNTIME_INIT, \
         .interpreters = { \
             /* This prevents interpreters from getting created \

Lib/threading.py

@@ -949,7 +949,6 @@ def _after_fork(self, new_ident=None):
             # This thread is alive.
             self._ident = new_ident
             if self._handle is not None:
-                self._handle.after_fork_alive()
                 assert self._handle.ident == new_ident
             # bpo-42350: If the fork happens when the thread is already stopped
             # (ex: after threading._shutdown() has been called), _tstate_lock
@@ -965,9 +964,7 @@ def _after_fork(self, new_ident=None):
             self._is_stopped = True
             self._tstate_lock = None
             self._join_lock = None
-            if self._handle is not None:
-                self._handle.after_fork_dead()
-                self._handle = None
+            self._handle = None
 
     def __repr__(self):
         assert self._initialized, "Thread.__init__() was not called"

Modules/_threadmodule.c

@@ -44,6 +44,7 @@ get_thread_state(PyObject *module)
 
 typedef struct {
     PyObject_HEAD
+    struct llist_node node;  // linked list node (see _pythread_runtime_state)
     PyThread_ident_t ident;
     PyThread_handle_t handle;
     char joinable;
@@ -59,13 +60,26 @@ new_thread_handle(thread_module_state* state)
     self->ident = 0;
     self->handle = 0;
     self->joinable = 0;
+
+    HEAD_LOCK(&_PyRuntime);
+    llist_insert_tail(&_PyRuntime.threads.handles, &self->node);
+    HEAD_UNLOCK(&_PyRuntime);
+
     return self;
 }
 
 static void
 ThreadHandle_dealloc(ThreadHandleObject *self)
 {
     PyObject *tp = (PyObject *) Py_TYPE(self);
+
+    // Remove ourself from the global list of handles
+    HEAD_LOCK(&_PyRuntime);
+    if (self->node.next != NULL) {
+        llist_remove(&self->node);
+    }
+    HEAD_UNLOCK(&_PyRuntime);
+
     if (self->joinable) {
         int ret = PyThread_detach_thread(self->handle);
         if (ret) {
@@ -77,6 +91,28 @@ ThreadHandle_dealloc(ThreadHandleObject *self)
     Py_DECREF(tp);
 }
 
+void
+_PyThread_AfterFork(struct _pythread_runtime_state *state)
+{
+    // gh-115035: We mark ThreadHandles as not joinable early in the child's
+    // after-fork handler. We do this before calling any Python code to ensure
+    // that it happens before any ThreadHandles are deallocated, such as by a
+    // GC cycle.
+    PyThread_ident_t current = PyThread_get_thread_ident_ex();
+
+    struct llist_node *node;
+    llist_for_each_safe(node, &state->handles) {
+        ThreadHandleObject *hobj = llist_data(node, ThreadHandleObject, node);
+        if (hobj->ident == current) {
+            continue;
+        }
+
+        // Disallow calls to detach() and join() as they could crash.
+        hobj->joinable = 0;
+        llist_remove(node);
+    }
+}
+
 static PyObject *
 ThreadHandle_repr(ThreadHandleObject *self)
 {
@@ -91,21 +127,6 @@ ThreadHandle_get_ident(ThreadHandleObject *self, void *ignored)
 }
 
 
-static PyObject *
-ThreadHandle_after_fork_alive(ThreadHandleObject *self, void* ignored)
-{
-    PyThread_update_thread_after_fork(&self->ident, &self->handle);
-    Py_RETURN_NONE;
-}
-
-static PyObject *
-ThreadHandle_after_fork_dead(ThreadHandleObject *self, void* ignored)
-{
-    // Disallow calls to detach() and join() as they could crash.
-    self->joinable = 0;
-    Py_RETURN_NONE;
-}
-
 static PyObject *
 ThreadHandle_detach(ThreadHandleObject *self, void* ignored)
 {
@@ -157,8 +178,6 @@ static PyGetSetDef ThreadHandle_getsetlist[] = {
 
 static PyMethodDef ThreadHandle_methods[] =
 {
-    {"after_fork_alive", (PyCFunction)ThreadHandle_after_fork_alive, METH_NOARGS},
-    {"after_fork_dead", (PyCFunction)ThreadHandle_after_fork_dead, METH_NOARGS},
     {"detach", (PyCFunction)ThreadHandle_detach, METH_NOARGS},
     {"join", (PyCFunction)ThreadHandle_join, METH_NOARGS},
     {0, 0}

Python/pystate.c

@@ -517,6 +517,8 @@ _PyRuntimeState_ReInitThreads(_PyRuntimeState *runtime)
         return _PyStatus_NO_MEMORY();
     }
 
+    _PyThread_AfterFork(&runtime->threads);
+
     return _PyStatus_OK();
 }
 #endif

Python/thread_nt.h

@@ -242,10 +242,6 @@ PyThread_detach_thread(PyThread_handle_t handle) {
     return (CloseHandle(hThread) == 0);
 }
 
-void
-PyThread_update_thread_after_fork(PyThread_ident_t* ident, PyThread_handle_t* handle) {
-}
-
 /*
  * Return the thread Id instead of a handle. The Id is said to uniquely identify the
  * thread in the system

Python/thread_pthread.h

@@ -339,16 +339,6 @@ PyThread_detach_thread(PyThread_handle_t th) {
     return pthread_detach((pthread_t) th);
 }
 
-void
-PyThread_update_thread_after_fork(PyThread_ident_t* ident, PyThread_handle_t* handle) {
-    // The thread id might have been updated in the forked child
-    pthread_t th = pthread_self();
-    *ident = (PyThread_ident_t) th;
-    *handle = (PyThread_handle_t) th;
-    assert(th == (pthread_t) *ident);
-    assert(th == (pthread_t) *handle);
-}
-
 /* XXX This implementation is considered (to quote Tim Peters) "inherently
    hosed" because:
      - It does not guarantee the promise that a non-zero integer is returned.