build: update cffi versions and compatibility for 3.14 and nogil

We need cffi 2.0 for nogil support. cffi 2.0 only implements nogil
support on 3.14+, so we need to disable cffi for nogil on 3.13.

Author: clin1234

docs/news.rst

@@ -12,6 +12,10 @@ Version History
   This fixes a nogil / multi-threaded compile error. (#201, #275)
 * A zstandard commit to fix qsort detection on BSD operating systems
   has been backported. (#272)
+* We now require cffi >= 2.0.0b on Python 3.14. <3.14 still requires 1.17.
+  (#274)
+* The cffi backend is now automatically disabled for nogil builds on Python
+  <3.14, as cffi didn't implement nogil support until the 2.0 release. (#274)
 
 0.24.0 (released 2025-08-17)
 ============================

pyproject.toml

@@ -25,7 +25,8 @@ dependencies = []
 
 [project.optional-dependencies]
 cffi = [
-    'cffi>=1.17 ; python_version >= "3.13" and platform_python_implementation != "PyPy"',
+    "cffi~=1.17; platform_python_implementation != 'PyPy' and python_version < '3.14'",
+    "cffi>=2.0.0b; platform_python_implementation != 'PyPy' and python_version >= '3.14'",
 ]
 
 [project.urls]
@@ -34,7 +35,8 @@ Documentation = "https://python-zstandard.readthedocs.io/en/latest/"
 
 [build-system]
 requires = [
-    "cffi>=1.17.0 ; platform_python_implementation != 'PyPy'",
+    "cffi~=1.17; platform_python_implementation != 'PyPy' and python_version < '3.14'",
+    "cffi>=2.0.0b; platform_python_implementation != 'PyPy' and python_version >= '3.14'",
     "packaging",
     "setuptools",
 ]

setup.py

@@ -10,6 +10,7 @@
 import os
 import platform
 import sys
+import sysconfig
 
 from setuptools import setup
 
@@ -30,7 +31,8 @@
 # garbage collection pitfalls.
 # Require 1.17 everywhere so we don't have to think about supporting older
 # versions.
-MINIMUM_CFFI_VERSION = "1.17"
+# Require 2.0 for Python 3.14+ to add improved free-threading support
+MINIMUM_CFFI_VERSION = "2.0" if sys.version_info[0:2] >= (3, 14) else "1.17"
 
 ext_suffix = os.environ.get("SETUPTOOLS_EXT_SUFFIX")
 if ext_suffix:
@@ -89,6 +91,16 @@
 if platform.python_implementation() == "PyPy":
     C_BACKEND = False
 
+# cffi 2.0 only introduced no-GIL support for 3.14+.
+if sys.version_info[0:2] < (3, 14) and sysconfig.get_config_var(
+    "Py_GIL_DISABLED"
+):
+    print(
+        "cffi backend requires 3.14+ for nogil Python; disabling cffi",
+        file=sys.stderr,
+    )
+    CFFI_BACKEND = False
+
 if "--legacy" in sys.argv:
     SUPPORT_LEGACY = True
     sys.argv.remove("--legacy")