diff --git a/Include/internal/pycore_critical_section.h b/Include/internal/pycore_critical_section.h
index 573d09a09683ef..7bebcdb67c7169 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 */
diff --git a/Lib/test/test_free_threading/test_str.py b/Lib/test/test_free_threading/test_str.py
new file mode 100644
index 00000000000000..f5e93b7de0aa9b
--- /dev/null
+++ b/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()
diff --git a/Misc/NEWS.d/next/C API/2024-05-21-11-35-11.gh-issue-119247.U6n6mh.rst b/Misc/NEWS.d/next/C API/2024-05-21-11-35-11.gh-issue-119247.U6n6mh.rst
new file mode 100644
index 00000000000000..3b2cdc8cf2dc5c
--- /dev/null
+++ b/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.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 057b417074ebea..480b6713905c70 100644
--- a/Objects/unicodeobject.c
+++ b/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,
- * so we are sure that fseq won't be mutated.
- */
+ Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST(seq);
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;
}
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