swig: fix Py_NewRef compatibility with Python < 3.10

SWIG >= 4.1 generates calls to Py_NewRef() in its runtime boilerplate.
Py_NewRef was introduced in Python 3.10, causing an undefined symbol
error on older distributions (e.g. SLES 15.6/15.7 with Python 3.6).

The shim must live in a %begin block so it precedes SWIG's own runtime
in the generated C file. Since Python.h is not yet included at that
point, PyObject is unavailable, so a macro is used instead of an inline
function. A -DSWIG_COMPAT_PY_NEWREF flag, set by meson when Py_NewRef
is absent, gates the macro so it never conflicts with Python 3.10+'s own
definition.

Signed-off-by: Martin Belanger <[email protected]>
Assisted-by: Claude Sonnet 4.6 <[email protected]>

Author: Martin Belanger

libnvme/libnvme/meson.build

@@ -14,6 +14,12 @@ if want_python
         swig_cmd = [swig, '-python', '-o', '@OUTPUT1@', '@INPUT0@']
     endif
 
+    py_compat_c_args = []
+    if not cc.has_header_symbol('Python.h', 'Py_NewRef', dependencies: py3_dep)
+        warning('Python < 3.10 detected: providing Py_NewRef compatibility shim in nvme.i')
+        py_compat_c_args = ['-DSWIG_COMPAT_PY_NEWREF']
+    endif
+
     pymod_swig = custom_target(
         'nvme.py',
         input:   ['nvme.i'],
@@ -28,6 +34,7 @@ if want_python
     pynvme_clib = python3.extension_module(
         '_nvme',
         nvme_wrap_c,
+        c_args: py_compat_c_args,
         dependencies: [config_dep, ccan_dep, libnvme_dep, py3_dep],
         install: true,
         subdir: 'libnvme',

libnvme/libnvme/nvme.i

@@ -10,6 +10,16 @@
                clashes with the same macro defined in Python.h.
  */
 #undef fallthrough
+
+/* WORKAROUND: Py_NewRef() was introduced in Python 3.10. SWIG >= 4.1 generates
+               calls to it in its runtime boilerplate. On older distributions,
+               meson detects the absence of Py_NewRef and passes
+               -DSWIG_COMPAT_PY_NEWREF so we can provide a macro shim here,
+               before Python.h is included.
+ */
+#ifdef SWIG_COMPAT_PY_NEWREF
+#define Py_NewRef(obj) (Py_INCREF(obj), (obj))
+#endif
 %}
 
 %module(docstring="Python bindings for libnvme") nvme