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

Makefile.pre.in

@@ -1817,6 +1817,7 @@ Python/interpconfig.o: $(srcdir)/Python/interpconfig.c $(srcdir)/Python/config_c
 Python/dynload_shlib.o: $(srcdir)/Python/dynload_shlib.c Makefile
 	$(CC) -c $(PY_CORE_CFLAGS) \
 		-DSOABI='"$(SOABI)"' \
+		$(MULTIARCH_CPPFLAGS) \
 		-o $@ $(srcdir)/Python/dynload_shlib.c
 
 Python/dynload_hpux.o: $(srcdir)/Python/dynload_hpux.c Makefile

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

@@ -45,6 +45,9 @@ const char *_PyImport_DynLoadFiletab[] = {
     "." ALT_SOABI ".so",
 #endif
     ".abi" PYTHON_ABI_STRING ".so",
+#ifdef MULTIARCH
+    ".abi" PYTHON_ABI_STRING "-" MULTIARCH ".so",
+#endif
     ".so",
 #endif  /* __CYGWIN__ */
     NULL,