gh-119247: Add macros to use PySequence_Fast safely in free-threaded build (#119315)

Add `Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST` and
`Py_END_CRITICAL_SECTION_SEQUENCE_FAST` macros and update `str.join` to use
them. Also add a regression test that would crash reliably without this
patch.

1 files changed, 22 insertions, 0 deletions

diff --git a/Include/internal/pycore_critical_section.h b/Include/internal/pycore_critical_section.h
index 573d09a..7bebcdb 100644
--- a/Include/internal/pycore_critical_section.h
+++ b/Include/internal/pycore_critical_section.h
@@ -108,6 +108,26 @@ extern "C" {
         _PyCriticalSection2_End(&_cs2);                                 \
     }
 
+// Specialized version of critical section locking to safely use
+// PySequence_Fast APIs without the GIL. For performance, the argument *to*
+// PySequence_Fast() is provided to the macro, not the *result* of
+// PySequence_Fast(), which would require an extra test to determine if the
+// lock must be acquired.
+# define Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST(original)              \
+    {                                                                   \
+        PyObject *_orig_seq = _PyObject_CAST(original);                 \
+        const bool _should_lock_cs = PyList_CheckExact(_orig_seq);      \
+        _PyCriticalSection _cs;                                         \
+        if (_should_lock_cs) {                                          \
+            _PyCriticalSection_Begin(&_cs, &_orig_seq->ob_mutex);       \
+        }
+
+# define Py_END_CRITICAL_SECTION_SEQUENCE_FAST()                        \
+        if (_should_lock_cs) {                                          \
+            _PyCriticalSection_End(&_cs);                               \
+        }                                                               \
+    }
+
 // Asserts that the mutex is locked.  The mutex must be held by the
 // top-most critical section otherwise there's the possibility
 // that the mutex would be swalled out in some code paths.
@@ -137,6 +157,8 @@ extern "C" {
 # define Py_END_CRITICAL_SECTION()
 # define Py_BEGIN_CRITICAL_SECTION2(a, b)
 # define Py_END_CRITICAL_SECTION2()
+# define Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST(original)
+# define Py_END_CRITICAL_SECTION_SEQUENCE_FAST()
 # define _Py_CRITICAL_SECTION_ASSERT_MUTEX_LOCKED(mutex)
 # define _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(op)
 #endif  /* !Py_GIL_DISABLED */