PEP 489: Multi-phase extension module initialization

Known limitations of the current implementation:

- documentation changes are incomplete
- there's a reference leak I haven't tracked down yet

The leak is most visible by running:

  ./python -m test -R3:3 test_importlib

However, you can also see it by running:

  ./python -X showrefcount

Importing the array or _testmultiphase modules, and
then deleting them from both sys.modules and the local
namespace shows significant increases in the total
number of active references each cycle. By contrast,
with _testcapi (which continues to use single-phase
initialisation) the global refcounts stabilise after
a couple of cycles.

Python/dynload_shlib.c

@@ -51,8 +51,10 @@ static struct {
 static int nhandles = 0;
 
 
-dl_funcptr _PyImport_GetDynLoadFunc(const char *shortname,
-                                    const char *pathname, FILE *fp)
+dl_funcptr
+_PyImport_FindSharedFuncptr(const char *prefix,
+                            const char *shortname,
+                            const char *pathname, FILE *fp)
 {
     dl_funcptr p;
     void *handle;
@@ -67,7 +69,7 @@ dl_funcptr _PyImport_GetDynLoadFunc(const char *shortname,
     }
 
     PyOS_snprintf(funcname, sizeof(funcname),
-                  LEAD_UNDERSCORE "PyInit_%.200s", shortname);
+                  LEAD_UNDERSCORE "%.20s_%.200s", prefix, shortname);
 
     if (fp != NULL) {
         int i;