gh-126688: Reinit import lock after fork (#126692)

The PyMutex implementation supports unlocking after fork because we
clear the list of waiters in parking_lot.c. This doesn't work as well
for _PyRecursiveMutex because on some systems, such as SerenityOS, the
thread id is not preserved across fork().

Author: colesbury

Include/internal/pycore_import.h

@@ -21,6 +21,7 @@ extern int _PyImport_SetModuleString(const char *name, PyObject* module);
 
 extern void _PyImport_AcquireLock(PyInterpreterState *interp);
 extern void _PyImport_ReleaseLock(PyInterpreterState *interp);
+extern void _PyImport_ReInitLock(PyInterpreterState *interp);
 
 // This is used exclusively for the sys and builtins modules:
 extern int _PyImport_FixupBuiltin(

Misc/NEWS.d/next/Core_and_Builtins/2024-11-11-17-02-48.gh-issue-126688.QiOXUi.rst

@@ -0,0 +1,2 @@
+Fix a crash when calling :func:`os.fork` on some operating systems,
+including SerenityOS.

Modules/posixmodule.c

@@ -678,6 +678,7 @@ PyOS_AfterFork_Child(void)
     _PyEval_StartTheWorldAll(&_PyRuntime);
     _PyThreadState_DeleteList(list);
 
+    _PyImport_ReInitLock(tstate->interp);
     _PyImport_ReleaseLock(tstate->interp);
 
     _PySignal_AfterFork();

Python/import.c

@@ -122,6 +122,13 @@ _PyImport_ReleaseLock(PyInterpreterState *interp)
     _PyRecursiveMutex_Unlock(&IMPORT_LOCK(interp));
 }
 
+void
+_PyImport_ReInitLock(PyInterpreterState *interp)
+{
+    // gh-126688: Thread id may change after fork() on some operating systems.
+    IMPORT_LOCK(interp).thread = PyThread_get_thread_ident_ex();
+}
+
 
 /***************/
 /* sys.modules */