gh-109649: Add os.process_cpu_count() function (#109907)

* Refactor os_sched_getaffinity_impl(): move variable definitions to
  their first assignment.
* Fix test_posix.test_sched_getaffinity(): restore the old CPU mask
  when the test completes!
* Doc: Specify that os.cpu_count() counts *logicial* CPUs.
* Doc: Specify that os.sched_getaffinity(0) is related to the calling
  thread.

1 files changed, 14 insertions, 0 deletions

diff --git a/Lib/os.py b/Lib/os.py
index d8c9ba4..35842ce 100644
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -1136,3 +1136,17 @@ if name == 'nt':
             cookie,
             nt._remove_dll_directory
         )
+
+
+if _exists('sched_getaffinity'):
+    def process_cpu_count():
+        """
+        Get the number of CPUs of the current process.
+
+        Return the number of logical CPUs usable by the calling thread of the
+        current process. Return None if indeterminable.
+        """
+        return len(sched_getaffinity(0))
+else:
+    # Just an alias to cpu_count() (same docstring)
+    process_cpu_count = cpu_count