bpo-33166: Change os.cpu_count to return active (real) processors (GH-15949)

(cherry picked from commit aa92927) Co-authored-by: Steve Dower <steve.dower@python.org>

Author: miss-islington

Misc/NEWS.d/next/Windows/2019-09-11-14-42-04.bpo-36634.8Un8ih.rst

@@ -0,0 +1,2 @@
+:func:`os.cpu_count` now returns active processors rather than maximum
+processors.

Modules/posixmodule.c

@@ -12176,23 +12176,9 @@ os_cpu_count_impl(PyObject *module)
 {
  int ncpu = 0;
 #ifdef MS_WINDOWS
- /* Vista is supported and the GetMaximumProcessorCount API is Win7+
- Need to fallback to Vista behavior if this call isn't present */
- HINSTANCE hKernel32;
- static DWORD(CALLBACK *_GetMaximumProcessorCount)(WORD) = NULL;
- Py_BEGIN_ALLOW_THREADS
- hKernel32 = GetModuleHandleW(L"KERNEL32");
- *(FARPROC*)&_GetMaximumProcessorCount = GetProcAddress(hKernel32,
- "GetMaximumProcessorCount");
- Py_END_ALLOW_THREADS
- if (_GetMaximumProcessorCount != NULL) {
- ncpu = _GetMaximumProcessorCount(ALL_PROCESSOR_GROUPS);
- }
- else {
- SYSTEM_INFO sysinfo;
- GetSystemInfo(&sysinfo);
- ncpu = sysinfo.dwNumberOfProcessors;
- }
+ /* Declare prototype here to avoid pulling in all of the Win7 APIs in 3.8 */
+ DWORD WINAPI GetActiveProcessorCount(WORD group);
+ ncpu = GetActiveProcessorCount(ALL_PROCESSOR_GROUPS);
 #elif defined(__hpux)
  ncpu = mpctl(MPC_GETNUMSPUS, NULL, NULL);
 #elif defined(HAVE_SYSCONF) && defined(_SC_NPROCESSORS_ONLN)