gh-115999: Add free-threaded specialization for `UNPACK_SEQUENCE` (#126600)

Add free-threaded specialization for `UNPACK_SEQUENCE` opcode.
`UNPACK_SEQUENCE_TUPLE/UNPACK_SEQUENCE_TWO_TUPLE` are already thread safe since tuples are immutable.
`UNPACK_SEQUENCE_LIST` is not thread safe because of nature of lists (there is nothing preventing another thread from adding items to or removing them the list while the instruction is executing). To achieve thread safety we add a critical section to the implementation of `UNPACK_SEQUENCE_LIST`, especially around the parts where we check the size of the list and push items onto the stack.


---------

Co-authored-by: Matt Page <mpage@meta.com>
Co-authored-by: mpage <mpage@cs.stanford.edu>

1 files changed, 20 insertions, 2 deletions

diff --git a/Python/executor_cases.c.h b/Python/executor_cases.c.h
index 5c7138a9421..22de7ca11cd 100644
--- a/Python/executor_cases.c.h
+++ b/Python/executor_cases.c.h
@@ -1711,15 +1711,33 @@
                 UOP_STAT_INC(uopcode, miss);
                 JUMP_TO_JUMP_TARGET();
             }
+            #ifdef Py_GIL_DISABLED
+            PyCriticalSection cs;
+            _PyFrame_SetStackPointer(frame, stack_pointer);
+            PyCriticalSection_Begin(&cs, seq_o);
+            stack_pointer = _PyFrame_GetStackPointer(frame);
+            #endif
             if (PyList_GET_SIZE(seq_o) != oparg) {
-                UOP_STAT_INC(uopcode, miss);
-                JUMP_TO_JUMP_TARGET();
+                #ifdef Py_GIL_DISABLED
+                _PyFrame_SetStackPointer(frame, stack_pointer);
+                PyCriticalSection_End(&cs);
+                stack_pointer = _PyFrame_GetStackPointer(frame);
+                #endif
+                if (true) {
+                    UOP_STAT_INC(uopcode, miss);
+                    JUMP_TO_JUMP_TARGET();
+                }
             }
             STAT_INC(UNPACK_SEQUENCE, hit);
             PyObject **items = _PyList_ITEMS(seq_o);
             for (int i = oparg; --i >= 0; ) {
                 *values++ = PyStackRef_FromPyObjectNew(items[i]);
             }
+            #ifdef Py_GIL_DISABLED
+            _PyFrame_SetStackPointer(frame, stack_pointer);
+            PyCriticalSection_End(&cs);
+            stack_pointer = _PyFrame_GetStackPointer(frame);
+            #endif
             PyStackRef_CLOSE(seq);
             stack_pointer += -1 + oparg;
             assert(WITHIN_STACK_BOUNDS());