build: enable building with the limited API

Cython 3.1 makes it possible to build with the limited API. This adds
the necessary build configuration so that python-flint can be built with
the limited API which in principle makes it possible to have abi3 wheels
that can work with a range of CPython versions.

Although this builds the tests crash pretty quickly in a segfault so
more work would be needed to make these builds useful. For now we just
enable this as an experimental build mode. When it is working it would
be worth running some timings to see if it is slower than a normal
build.

Author: oscarbenjamin

meson.build

@@ -2,7 +2,8 @@ project(
   'python-flint',
   'cython',
   'c',
-  meson_version : '>=1.1',
+  meson_version : '>=1.3.0',
+  default_options: ['python.allow_limited_api=false'],
 )
 #
 # The minimum versions are because we know that it will not work with earlier
@@ -83,15 +84,20 @@ add_project_arguments(
     language : 'cython'
 )
 
+# Only used if python.allow_limited_api is true (not the default)
+limited_api_version = get_option('limited_api_version')
+
 # Enable free-threading if Cython is new enough. The check should be
 # >= 3.1.0a1 but meson gets confused by the a1 alpha release suffix.
 # so we go with >= 3.1 (which will be correct once 3.1 is released).
 cy = meson.get_compiler('cython')
-if cy.version().version_compare('>=3.1')
+if get_option('python.allow_limited_api')
+  message('Using Python limited API version ' + limited_api_version)
+elif cy.version().version_compare('>=3.1')
   message('Enabling freethreading')
   add_project_arguments('-Xfreethreading_compatible=true', language : 'cython')
 else
-  message('Disabling freethreading')
+  message('Normal build (no freethreading and no limited API)')
 endif
 
 if get_option('coverage')

meson.options

@@ -1,3 +1,4 @@
 option('coverage', type : 'boolean', value : false, description : 'enable coverage build')
 option('add_flint_rpath', type : 'boolean', value : false)
 option('flint_version_check', type: 'boolean', value : true)
+option('limited_api_version', type: 'string', value : '3.12')

pyproject.toml

@@ -44,6 +44,10 @@ content-type = "text/markdown"
 requires = ["meson-python >= 0.18", "cython >=3.1,<3.2"]
 build-backend = "mesonpy"
 
+[tool.meson-python]
+
+limited-api = true
+
 [tool.cython-lint]
 # E129 visually indented line with same indent as next logical line
 # Reasoning: this rule is a little controversial

src/flint/flint_base/meson.build

@@ -23,5 +23,6 @@ foreach ext : exts
     dependencies: pyflint_deps,
     install: true,
     subdir: pkgdir,
+    limited_api: limited_api_version,
   )
 endforeach

src/flint/functions/meson.build

@@ -19,5 +19,6 @@ foreach ext : exts
     dependencies: pyflint_deps,
     install: true,
     subdir: 'flint/functions',
+    limited_api: limited_api_version,
   )
 endforeach

src/flint/meson.build

@@ -31,6 +31,7 @@ foreach ext : exts
     dependencies: pyflint_deps,
     install: true,
     subdir: thisdir,
+    limited_api: limited_api_version,
   )
 endforeach
 

src/flint/types/acb_poly.pyx

@@ -1,4 +1,4 @@
-from cpython.list cimport PyList_GET_SIZE
+from cpython.list cimport PyList_Size as PyList_GET_SIZE
 from flint.utils.typecheck cimport typecheck
 from flint.flint_base.flint_context cimport getprec
 from flint.flint_base.flint_base cimport flint_poly

src/flint/types/arb.pyx

@@ -1,4 +1,4 @@
-from cpython.float cimport PyFloat_AS_DOUBLE
+from cpython.float cimport PyFloat_AsDouble
 from cpython.long cimport PyLong_Check
 
 from flint.flint_base.flint_context cimport getprec
@@ -101,7 +101,7 @@ cdef int arb_set_python(arb_t x, obj, bint allow_conversion) except -1:
         return 1
 
     if typecheck(obj, float):
-        arf_set_d(arb_midref(x), PyFloat_AS_DOUBLE(obj))
+        arf_set_d(arb_midref(x), PyFloat_AsDouble(obj))
         mag_zero(arb_radref(x))
         return 1
 

src/flint/types/arb_poly.pyx

@@ -1,4 +1,4 @@
-from cpython.list cimport PyList_GET_SIZE
+from cpython.list cimport PyList_Size as PyList_GET_SIZE
 from flint.utils.typecheck cimport typecheck
 from flint.flint_base.flint_context cimport getprec
 from flint.flint_base.flint_base cimport flint_poly

src/flint/types/arf.pyx

@@ -1,4 +1,4 @@
-from cpython.float cimport PyFloat_AS_DOUBLE
+from cpython.float cimport PyFloat_AsDouble
 from flint.flint_base.flint_context cimport getprec
 from flint.flint_base.flint_context cimport thectx
 from flint.utils.typecheck cimport typecheck
@@ -44,7 +44,7 @@ cdef class arf:
             elif typecheck(val, arf):
                 arf_set(self.val, (<arf>val).val)
             elif typecheck(val, float):
-                arf_set_d(self.val, PyFloat_AS_DOUBLE(val))
+                arf_set_d(self.val, PyFloat_AsDouble(val))
             elif typecheck(val, tuple):
                 man = any_as_fmpz(val[0])
                 exp = any_as_fmpz(val[1])