pythongh-109054: configure checks if libatomic is needed

Fix building the _testcapi extension on Linux AArch64 which requires
linking to libatomic when <cpython/pyatomic.h> is used: the
_Py_atomic_or_uint64() function requires libatomic
__atomic_fetch_or_8() on this platform.

The configure script now checks if linking to libatomic is needed and
generates a new LIBATOMIC variable used to build the _testcapi
extension.

Building the _testcapi extension now uses the LIBATOMIC variable in
its LDFLAGS, since Modules/_testcapi/pyatomic.c uses
<cpython/pyatomic.h>.

Makefile.pre.in

@@ -267,6 +267,7 @@ STATIC_LIBPYTHON=	@STATIC_LIBPYTHON@
 
 LIBS=		@LIBS@
 LIBM=		@LIBM@
+LIBATOMIC=	@LIBATOMIC@
 LIBC=		@LIBC@
 SYSLIBS=	$(LIBM) $(LIBC)
 SHLIBS=		@SHLIBS@

Misc/NEWS.d/next/Build/2023-09-07-19-58-05.gh-issue-109054.5r3S3l.rst

@@ -0,0 +1,6 @@
+Fix building the ``_testcapi`` extension on Linux AArch64 which requires
+linking to libatomic when ``<cpython/pyatomic.h>`` is used: the
+``_Py_atomic_or_uint64()`` function requires libatomic
+``__atomic_fetch_or_8()`` on this platform. The configure script now checks
+if linking to libatomic is needed and generates a new LIBATOMIC variable
+used to build the _testcapi extension. Patch by Victor Stinner.

configure.ac

@@ -6962,6 +6962,68 @@ then
             [Define if you want to disable the GIL])
 fi
 
+# gh-109054: Check if -latomic is needed to get <pyatomic.h> atomic functions.
+# On aarch64, GCC may require programs to be linked explicitly to libatomic.
+# Use _Py_atomic_or_uint64() which may require libatomic __atomic_fetch_or_8()
+# on Linux aarch64 (or not depending on the compiler and the compiler flags).
+#
+# Avoid #include <Python.h> or #include <pyport.h>. The <Python.h> header
+# requires <pyconfig.h> header which is only written below by AC_OUTPUT below.
+# If the check is done after AC_OUTPUT, modifying LIBS has no effect anymore.
+# <pyport.h> cannot be included alone, it's designed to be included by
+# <Python.h>: it expects other includes and macros to be defined.
+AC_SUBST([LIBATOMIC])
+LIBATOMIC=""
+
+_SAVE_VAR([CPPFLAGS])
+CPPFLAGS="${BASECPPFLAGS} -I. -I${srcdir}/Include ${CPPFLAGS}"
+
+AC_CACHE_CHECK([whether libatomic is needed by <pyatomic.h>],
+               [ac_cv_libatomic_needed],
+[AC_RUN_IFELSE([AC_LANG_SOURCE([[
+// pyatomic.h needs uint64_t and Py_ssize_t types
+#include <stdint.h>  // int64_t, intptr_t
+#ifdef HAVE_SYS_TYPES_H
+#  include <sys/types.h> // ssize_t
+#endif
+// Code adapted from Include/pyport.h
+#if HAVE_SSIZE_T
+typedef ssize_t         Py_ssize_t;
+#elif SIZEOF_VOID_P == SIZEOF_SIZE_T
+typedef intptr_t Py_ssize_t;
+#else
+#  error "unable to find a type fitting ssize_t"
+#endif
+
+#include "cpython/pyatomic.h"
+
+int main()
+{
+    // use _Py_atomic_or_uint64() which requires libatomic __atomic_fetch_or_8()
+    // on Linux aarch64.
+    uint64_t byte;
+    _Py_atomic_store_uint64(&byte, 2);
+    if (_Py_atomic_or_uint64(&byte, 8) != 2) {
+        return 1; // error
+    }
+    if (_Py_atomic_load_uint64(&byte) != 10) {
+        return 1; // error
+    }
+    return 0; // all good
+}
+]])],[
+  ac_cv_libatomic_needed=no
+],[ac_cv_libatomic_needed=yes],[ac_cv_libatomic_needed=yes])
+])
+
+if test $ac_cv_libatomic_needed = yes
+then
+    LIBATOMIC="-latomic"
+fi
+_RESTORE_VAR([CPPFLAGS])
+
+
+# stdlib
 AC_DEFUN([PY_STDLIB_MOD_SET_NA], [
   m4_foreach([mod], [$@], [
     AS_VAR_SET([py_cv_module_]mod, [n/a])])
@@ -7229,7 +7291,10 @@ PY_STDLIB_MOD([_hashlib], [], [test "$ac_cv_working_openssl_hashlib" = yes],
   [$OPENSSL_INCLUDES], [$OPENSSL_LDFLAGS $OPENSSL_LDFLAGS_RPATH $LIBCRYPTO_LIBS])
 
 dnl test modules
-PY_STDLIB_MOD([_testcapi], [test "$TEST_MODULES" = yes])
+PY_STDLIB_MOD([_testcapi],
+    [test "$TEST_MODULES" = yes],  []
+    dnl Modules/_testcapi/pyatomic.c uses <cpython/pyatomic.h> header
+    [], [], [$LIBATOMIC])
 PY_STDLIB_MOD([_testclinic], [test "$TEST_MODULES" = yes])
 PY_STDLIB_MOD([_testclinic_limited], [test "$TEST_MODULES" = yes])
 PY_STDLIB_MOD([_testinternalcapi], [test "$TEST_MODULES" = yes])
@@ -7262,6 +7327,7 @@ AC_CONFIG_FILES(m4_normalize([
   Modules/Setup.stdlib
 ]))
 AC_CONFIG_FILES([Modules/ld_so_aix], [chmod +x Modules/ld_so_aix])
+# Generate files like pyconfig.h
 AC_OUTPUT
 
 AC_MSG_NOTICE([creating Modules/Setup.local])