Limited API is still not fully supported in Python 3.15

[kytta]

Describe the bug

#713 raised an issue that free-threaded wheels cannot be built because of the lack of Limited API. The fix was to disable Limited API on free-threaded 3.14 and below, with hope that it will become stable in 3.15:

aws-crt-python/setup.py

Lines 615 to 619 in 241be07

	# prefer building with stable ABI, so a wheel can work with multiple major versions
	if FREE_THREADED_BUILD and sys.version_info[:2] <= (3, 14):
	# 3.14 free threaded (aka no gil) does not support limited api.
	# disable it for now. 3.15 promises to support limited api + free threading combo
	py_limited_api = False

This is sadly not the case. Using Python 3.15.0rc1+freethreaded (rc1 means the ABI will not be changed), there are still compilation errors (see below).

Regression Issue

• Select this option if this issue appears to be a regression.

Expected Behavior

Installing awscrt under Python 3.15t succeeds

Current Behavior

$ pip install -e .
...
/redacted/path/to/cpython315t/include/python3.15t/object.h:375:12:
      error: call to undeclared function 'Py_IS_TYPE'; ISO C99 and later do not support implicit function declarations
      [-Wimplicit-function-declaration]
        375 |     return Py_IS_TYPE(ob, type) || PyType_IsSubtype(Py_TYPE(ob), type);
            |            ^
      /redacted/path/to/cpython315t/include/python3.15t/object.h:375:53:
      error: call to undeclared function 'Py_TYPE'; ISO C99 and later do not support implicit function declarations
      [-Wimplicit-function-declaration]
        375 |     return Py_IS_TYPE(ob, type) || PyType_IsSubtype(Py_TYPE(ob), type);
            |                                                     ^
      /redacted/path/to/cpython315t/include/python3.15t/object.h:375:53:
      error: incompatible integer to pointer conversion passing 'int' to parameter of type 'PyTypeObject *' (aka 'struct _typeobject *')
      [-Wint-conversion]
        375 |     return Py_IS_TYPE(ob, type) || PyType_IsSubtype(Py_TYPE(ob), type);
            |                                                     ^~~~~~~~~~~
      /redacted/path/to/cpython315t/include/python3.15t/object.h:763:32:
      error: call to undeclared function 'Py_TYPE'; ISO C99 and later do not support implicit function declarations
      [-Wimplicit-function-declaration]
        763 |     return PyType_FastSubclass(Py_TYPE(op), Py_TPFLAGS_TYPE_SUBCLASS);
            |                                ^
      /redacted/path/to/cpython315t/include/python3.15t/object.h:763:12:
      error: incompatible integer to pointer conversion passing 'int' to parameter of type 'PyTypeObject *' (aka 'struct _typeobject *')
      [-Wint-conversion]
        763 |     return PyType_FastSubclass(Py_TYPE(op), Py_TPFLAGS_TYPE_SUBCLASS);
            |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      /redacted/path/to/cpython315t/include/python3.15t/object.h:773:12:
      error: call to undeclared function 'Py_IS_TYPE'; ISO C99 and later do not support implicit function declarations
      [-Wimplicit-function-declaration]
        773 |     return Py_IS_TYPE(op, &PyType_Type);
            |            ^
      6 errors generated.

Reproduction Steps

I was using uv

git clone https://github.com/awslabs/aws-crt-python.git
cd aws-crt-python
git submodule update --init
uv venv -p 3.15t --seed
uv pip install -e .

Possible Solution

I propose we move the upper bound one version further:

diff --git a/setup.py b/setup.py
index bcf0b04..bc60ffe 100644
--- a/setup.py
+++ b/setup.py
@@ -613,9 +613,10 @@ def awscrt_ext():
                     extra_link_args += ['-Wl,--fatal-warnings']
 
     # prefer building with stable ABI, so a wheel can work with multiple major versions
-    if FREE_THREADED_BUILD and sys.version_info[:2] <= (3, 14):
+    if FREE_THREADED_BUILD and sys.version_info[:2] <= (3, 15):
         # 3.14 free threaded (aka no gil) does not support limited api.
-        # disable it for now. 3.15 promises to support limited api + free threading combo
+        # disable it for now. 3.15 does support limited api + free threading combo,
+        # but it's not complete yet
         py_limited_api = False
     elif sys.version_info >= (3, 13):
         # 3.13 deprecates PyWeakref_GetObject(), adds alternative

Additional Information/Context

No response

aws-crt-python version used

0.36.2

Python version used

3.15.0rc1+freethreaded

Operating System and version

macOS Sonoma 14.8.1 (23J30)

[TingDaoK]

https://docs.python.org/3.15/howto/abi3t-migration.html

looks like they do keep the promise, but have an abi3t for 3.15+ with free threading.
We will look into this.

[kytta]

Hah, a lesson for me to not make claims on things I don't know enough about :D Thanks for acknowledging!