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.

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 */

Lib/test/test_free_threading/test_str.py

@@ -0,0 +1,75 @@
+import sys
+import unittest
+
+from itertools import cycle
+from threading import Event, Thread
+from unittest import TestCase
+
+from test.support import threading_helper
+
+@threading_helper.requires_working_threading()
+class TestStr(TestCase):
+    def test_racing_join_extend(self):
+        '''Test joining a string being extended by another thread'''
+        l = []
+        ITERS = 100
+        READERS = 10
+        done_event = Event()
+        def writer_func():
+            for i in range(ITERS):
+                l.extend(map(str, range(i)))
+                l.clear()
+            done_event.set()
+        def reader_func():
+            while not done_event.is_set():
+                ''.join(l)
+        writer = Thread(target=writer_func)
+        readers = []
+        for x in range(READERS):
+            reader = Thread(target=reader_func)
+            readers.append(reader)
+            reader.start()
+
+        writer.start()
+        writer.join()
+        for reader in readers:
+            reader.join()
+
+    def test_racing_join_replace(self):
+        '''
+        Test joining a string of characters being replaced with ephemeral
+        strings by another thread.
+        '''
+        l = [*'abcdefg']
+        MAX_ORDINAL = 1_000
+        READERS = 10
+        done_event = Event()
+
+        def writer_func():
+            for i, c in zip(cycle(range(len(l))),
+                            map(chr, range(128, MAX_ORDINAL))):
+                l[i] = c
+            done_event.set()
+
+        def reader_func():
+            while not done_event.is_set():
+                ''.join(l)
+                ''.join(l)
+                ''.join(l)
+                ''.join(l)
+
+        writer = Thread(target=writer_func)
+        readers = []
+        for x in range(READERS):
+            reader = Thread(target=reader_func)
+            readers.append(reader)
+            reader.start()
+
+        writer.start()
+        writer.join()
+        for reader in readers:
+            reader.join()
+
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/C API/2024-05-21-11-35-11.gh-issue-119247.U6n6mh.rst

@@ -0,0 +1,4 @@
+Added ``Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST`` and
+``Py_END_CRITICAL_SECTION_SEQUENCE_FAST`` macros to make it possible to use
+PySequence_Fast APIs safely when free-threaded, and update str.join to work
+without the GIL using them.

Objects/unicodeobject.c

@@ -44,6 +44,7 @@ OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 #include "pycore_bytesobject.h"   // _PyBytes_Repeat()
 #include "pycore_ceval.h"         // _PyEval_GetBuiltin()
 #include "pycore_codecs.h"        // _PyCodec_Lookup()
+#include "pycore_critical_section.h" // Py_*_CRITICAL_SECTION_SEQUENCE_FAST
 #include "pycore_format.h"        // F_LJUST
 #include "pycore_initconfig.h"    // _PyStatus_OK()
 #include "pycore_interp.h"        // PyInterpreterState.fs_codec
@@ -9559,13 +9560,14 @@ PyUnicode_Join(PyObject *separator, PyObject *seq)
         return NULL;
     }
 
-    /* NOTE: the following code can't call back into Python code,
+    Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST(seq);
-     * so we are sure that fseq won't be mutated.
-     */
 
     items = PySequence_Fast_ITEMS(fseq);
     seqlen = PySequence_Fast_GET_SIZE(fseq);
     res = _PyUnicode_JoinArray(separator, items, seqlen);
+
+    Py_END_CRITICAL_SECTION_SEQUENCE_FAST();
+
     Py_DECREF(fseq);
     return res;
 }