gh-122917 Allow stable API extensions to include a multiarch tuple in the filename

This permits stable ABI extensions for multiple architectures to be
co-installed into the same directory, without clashing with each other,
the same way (non-stable ABI) regular extensions can.

It is listed below the current .abi3 suffix because setuptools will
select the first suffix containing .abi3, as the target filename.
We do this to protect older Python versions predating this patch.

Author: stefanor

Doc/whatsnew/3.16.rst

@@ -75,6 +75,14 @@ New features
 Other language changes
 ======================
 
+* Stable ABI extensions may now include a multiarch tuple in the
+  filename, e.g. ``foo.abi3-x86-64-linux-gnu.so``.
+  This permits stable ABI extensions for multiple architectures to be
+  co-installed into the same directory, without clashing with each
+  other, as regular dynamic extensions do. Build tools will not generate
+  these multiarch tagged filenames, by default, while still supporting
+  older Python versions that don't recognize these filenames.
+  (Contributed by Stefano Rivera in :gh:`122931`.)
 
 
 New modules

Misc/NEWS.d/next/C_API/2024-08-12-09-48-04.gh-issue-122931.QwHc2l.rst

@@ -0,0 +1,2 @@
+Allow importing stable ABI C extensions that include a multiarch tuple
+in their filename, e.g. ``foo.abi3-x86-64-linux-gnu.so``.

Python/dynload_shlib.c

@@ -47,8 +47,14 @@ const char *_PyImport_DynLoadFiletab[] = {
 #endif
 #ifndef Py_GIL_DISABLED
     ".abi" PYTHON_ABI_STRING ".so",
+#ifdef SOABI_PLATFORM
+    ".abi" PYTHON_ABI_STRING "-" SOABI_PLATFORM ".so",
+#endif  /* SOABI_PLATFORM */
 #endif  /* Py_GIL_DISABLED */
     ".abi" PYTHON_ABI_STRING "t.so",
+#ifdef SOABI_PLATFORM
+    ".abi" PYTHON_ABI_STRING "t-" SOABI_PLATFORM ".so",
+#endif  /* SOABI_PLATFORM */
     ".so",
 #endif  /* __CYGWIN__ */
     NULL,

configure

@@ -1212,6 +1212,8 @@ AS_CASE([$ac_sys_system],
   [SOABI_PLATFORM=$PLATFORM_TRIPLET]
 )
 
+AC_DEFINE_UNQUOTED([SOABI_PLATFORM], ["${SOABI_PLATFORM}"], [Platform tag, used in binary module extension filenames.])
+
 if test x$MULTIARCH != x; then
   MULTIARCH_CPPFLAGS="-DMULTIARCH=\\\"$MULTIARCH\\\""
 fi

configure.ac

@@ -1890,6 +1890,9 @@
 /* The size of '_Bool', as computed by sizeof. */
 #undef SIZEOF__BOOL
 
+/* Platform tag, used in binary module extension filenames. */
+#undef SOABI_PLATFORM
+
 /* Define to 1 if you have the ANSI C header files. */
 #undef STDC_HEADERS