Add support for free-threading builds of CPython

.github/workflows/test.yml

@@ -20,6 +20,7 @@ jobs:
           - '3.11'
           - '3.12'
           - '3.13'
+          - '3.13t'
         arch:
           - 'arm64'
           - 'x86'
@@ -58,35 +59,35 @@ jobs:
       # Activate Python development mode so we get warnings.
       PYTHONDEVMODE: '1'
     steps:
+      - uses: actions/checkout@v4
+
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.py }}
           architecture: ${{ matrix.arch }}
 
       - name: Install Rust
-        if: matrix.arch == 'x64'
+        if: matrix.arch == 'x64' && !startsWith(matrix.py, '3.13')
         uses: dtolnay/rust-toolchain@v1
         with:
           toolchain: stable
 
-      - uses: actions/checkout@v4
-
       - name: Install Dependencies
         shell: bash
         run: |
           python -m pip install --require-hashes -r ci/requirements.txt
 
       # TODO enable once PyO3 supports 3.13.
       - name: Build (Rust)
-        if: matrix.arch == 'x64' && matrix.py != '3.13'
+        if: matrix.arch == 'x64' && !startsWith(matrix.py, '3.13')
         env:
           PIP_CONSTRAINT: 'ci/constraints.txt'
         run: |
           python -m pip install --config-settings='--build-option=--rust-backend' -e .
 
       - name: Build (No Rust)
-        if: matrix.arch != 'x64' || matrix.py == '3.13'
+        if: matrix.arch != 'x64' || startsWith(matrix.py, '3.13')
         run: |
           python -m pip install -e .
 
@@ -95,13 +96,15 @@ jobs:
           pytest --numprocesses=auto --hypothesis-profile=${HYPOTHESIS_PROFILE} -v tests/
 
       - name: Test CFFI Backend
+        # CFFI doesn't yet support the free-threaded build of CPython
+        if: matrix.py != '3.13t'
         env:
           PYTHON_ZSTANDARD_IMPORT_POLICY: 'cffi'
         run: |
           pytest --numprocesses=auto --hypothesis-profile=${HYPOTHESIS_PROFILE} -v tests/
 
       - name: Test Rust Backend
-        if: matrix.arch == 'x64'
+        if: matrix.arch == 'x64' && !startsWith(matrix.py, '3.13')
         # Rust backend is currently experimental. So ignore failures in it.
         continue-on-error: true
         env:

c-ext/backend_c.c

@@ -210,6 +210,10 @@ void zstd_module_init(PyObject *m) {
     Py_DECREF(feature);
 #endif
 
+#ifdef Py_GIL_DISABLED
+    PyUnstable_Module_SetGIL(m, Py_MOD_GIL_NOT_USED);
+#endif
+
     if (PyObject_SetAttrString(m, "backend_features", features) == -1) {
         return;
     }
@@ -313,7 +317,7 @@ size_t roundpow2(size_t i) {
 int safe_pybytes_resize(PyObject **obj, Py_ssize_t size) {
     PyObject *tmp;
 
-    if ((*obj)->ob_refcnt == 1) {
+    if (Py_REFCNT(*obj) == 1) {
         return _PyBytes_Resize(obj, size);
     }
 

docs/installing.rst

@@ -44,6 +44,10 @@ One way to do this is to depend on the ``zstandard[cffi]`` dependency.
 e.g. ``pip install 'zstandard[cffi]'`` or add ``zstandard[cffi]`` to your
 pip requirements file.
 
+CFFI does not yet support the free-threaded build of CPython so the CFFI
+backend is disabled at build time for free-threaded Python regardless of
+whether or not the ``cffi`` is specified.
+
 Legacy Format Support
 =====================
 
@@ -84,7 +88,8 @@ All Install Arguments
    Do not compile the CFFI-based backend.
 
 ``--rust-backend``
-   Compile the Rust backend (not yet feature complete).
+   Compile the Rust backend (not yet feature complete and not supported
+   on the free-threaded build or Python 3.13).
 
 If you invoke ``setup.py``, simply pass the aforementioned arguments. e.g.
 ``python3.9 setup.py --no-cffi-backend``. If using ``pip``, use the

setup.py

@@ -10,6 +10,7 @@
 import os
 import platform
 import sys
+import sysconfig
 
 from setuptools import setup
 
@@ -36,7 +37,6 @@
 
 ext_suffix = os.environ.get("SETUPTOOLS_EXT_SUFFIX")
 if ext_suffix:
-    import sysconfig
     # setuptools._distutils.command.build_ext doesn't use
     # SETUPTOOLS_EXT_SUFFIX like setuptools.command.build_ext does.
     # Work around the issue so that cross-compilation can work
@@ -55,22 +55,27 @@
     if py39compat:
         py39compat.add_ext_suffix = lambda vars: None
 
-try:
-    import cffi
-
-    # PyPy (and possibly other distros) have CFFI distributed as part of
-    # them.
-    cffi_version = LooseVersion(cffi.__version__)
-    if cffi_version < LooseVersion(MINIMUM_CFFI_VERSION):
-        print(
-            "CFFI %s or newer required (%s found); "
-            "not building CFFI backend" % (MINIMUM_CFFI_VERSION, cffi_version),
-            file=sys.stderr,
-        )
-        cffi = None
-
-except ImportError:
+if bool(sysconfig.get_config_var("Py_GIL_DISABLED")):
+    # cffi does not yet support the free-threaded build so we
+    # disable the cffi backend
     cffi = None
+else:
+    try:
+        import cffi
+
+        # PyPy (and possibly other distros) have CFFI distributed as part of
+        # them.
+        cffi_version = LooseVersion(cffi.__version__)
+        if cffi_version < LooseVersion(MINIMUM_CFFI_VERSION):
+            print(
+                "CFFI %s or newer required (%s found); "
+                "not building CFFI backend" % (MINIMUM_CFFI_VERSION, cffi_version),
+                file=sys.stderr,
+            )
+            cffi = None
+
+    except ImportError:
+        cffi = None
 
 sys.path.insert(0, ".")