Cython: emit CYTHON_LIMITED_API

If the user has specified the `limited_api` keyword arg
on an 'extension_module' invocation, then add `CYTHON_LIMITED_API`
to the C and C++ definitions of the target (along with the existing
`Py_LIMITED_API`).

Cython's support for this functionality was added in version 3.0, so
we issue a warning if the detected Cython compiler is not at least this
version.

Two tests have been added: one based on Cython's limited API test which
aims to test that it works in the case of the user's compiler supporting
this function, and another to test that a warning is issued in the opposite
case.

docs/markdown/Cython.md

@@ -60,3 +60,23 @@ py.extension_module(
     dependencies : dep_py,
 )
 ```
+
+## Limited API support
+
+*(New in 1.4)*
+
+Meson version 1.4 extends the support of the `limited_api` keyword to Cython.
+
+```meson
+project('my project', 'cython')
+
+py.extension_module(
+    'foo',
+    'foo.pyx',
+    dependencies : dep_py,
+    limited_api : '3.8'
+)
+```
+
+Cython's support for Python's Limited API was introduced in Cython version 3.0.
+If the user's Cython version is too old, Meson issues a warning but proceeds.

mesonbuild/modules/python.py

@@ -173,14 +173,17 @@ def extension_module_method(self, args: T.Tuple[str, T.List[BuildTargetSource]],
             target_suffix = self.limited_api_suffix
 
             limited_api_version_hex = self._convert_api_version_to_py_version_hex(limited_api_version, pydep.version)
-            limited_api_definition = f'-DPy_LIMITED_API={limited_api_version_hex}'
+            limited_api_definitions = [
+              f'-DPy_LIMITED_API={limited_api_version_hex}',
+              '-DCYTHON_LIMITED_API=1'
+            ]
 
             new_c_args = mesonlib.extract_as_list(kwargs, 'c_args')
-            new_c_args.append(limited_api_definition)
+            new_c_args.extend(limited_api_definitions)
             kwargs['c_args'] = new_c_args
 
             new_cpp_args = mesonlib.extract_as_list(kwargs, 'cpp_args')
-            new_cpp_args.append(limited_api_definition)
+            new_cpp_args.extend(limited_api_definitions)
             kwargs['cpp_args'] = new_cpp_args
 
             # When compiled under MSVC, Python's PC/pyconfig.h forcibly inserts pythonMAJOR.MINOR.lib
@@ -212,6 +215,10 @@ def extension_module_method(self, args: T.Tuple[str, T.List[BuildTargetSource]],
 
                 kwargs['link_args'] = new_link_args
 
+            cython_compiler = next((c for c in compilers.values() if c.get_id() == 'cython'), None)
+            if cython_compiler is not None and mesonlib.version_compare(cython_compiler.version, '< 3.0.0'):
+                mlog.warning(f'Limited API not supported by Cython versions < 3.0.0 (detected: {cython_compiler.version})', location=self.current_node)
+
         kwargs['dependencies'] = new_deps
 
         # msys2's python3 has "-cpython-36m.dll", we have to be clever

test cases/cython/4 limited api/limited.pyx

@@ -0,0 +1,43 @@
+# This is taken from Cython's limited API tests and
+# as a result is under Apache 2.0
+
+import cython
+
+@cython.binding(False)
+def fib(int n):
+    cdef int a, b
+    a, b = 0, 1
+    while b < n:
+        a, b = b, a + b
+    return b
+
+def lsum(values):
+    cdef long result = 0
+    for value in values:
+        result += value
+    if type(values) is list:
+        for value in reversed(<list>values):
+            result += value
+    elif type(values) is tuple:
+        for value in reversed(<tuple>values):
+            result += value
+    return result
+
+@cython.binding(False)
+def raises():
+    raise RuntimeError()
+
+def decode(bytes b, bytearray ba):
+    return b.decode("utf-8") + ba.decode("utf-8")
+
+def cast_float(object o):
+    return float(o)
+
+class C:
+    pass
+
+cdef class D:
+    pass
+
+cdef class E(D):
+    pass

test cases/cython/4 limited api/meson.build

@@ -0,0 +1,19 @@
+project('cython limited api', ['cython', 'c'])
+
+cythonc = meson.get_compiler('cython')
+if cythonc.version() < '3.0'
+  error('MESON_SKIP_TEST: Cython compiler version does not support limited API')
+endif
+
+py3 = import('python').find_installation()
+
+ext_mod = py3.extension_module('limited',
+  'limited.pyx',
+  limited_api: '3.7'
+)
+
+test('limited_api runner',
+  py3,
+  args : files('run.py'),
+  env : ['PYTHONPATH=' + meson.current_build_dir()]
+)

test cases/cython/4 limited api/run.py

@@ -0,0 +1,25 @@
+# This is taken from Cython's limited API tests and
+# as a result is under Apache 2.0
+
+import limited
+
+limited.fib(11)
+
+assert limited.lsum(list(range(10))) == 90
+assert limited.lsum(tuple(range(10))) == 90
+assert limited.lsum(iter(range(10))) == 45
+
+try:
+    limited.raises()
+except RuntimeError:
+    pass
+
+limited.C()
+limited.D()
+limited.E()
+
+assert limited.decode(b'a', bytearray(b'b')) == "ab"
+
+assert limited.cast_float(1) == 1.0
+assert limited.cast_float("2.0") == 2.0
+assert limited.cast_float(bytearray(b"3")) == 3.0