Revert "[lit] Use os.cpu_count() to cleanup TODO"

A bot owner contacted me.  I will re-land after confirming that this
doesn't break anyone (since it's low priority).

This reverts commit 9946b169c3.

llvm/utils/lit/lit/cl_arguments.py

@@ -23,7 +23,7 @@ def parse_args():
             metavar="N",
             help="Number of workers used for testing",
             type=_positive_int,
-            default=lit.util.usable_core_count())
+            default=lit.util.detectCPUs())
     parser.add_argument("--config-prefix",
             dest="configPrefix",
             metavar="NAME",

llvm/utils/lit/lit/run.py

@@ -110,7 +110,7 @@ class Run(object):
     # threads counts toward the current process limit. Try to raise the (soft)
     # process limit so that tests don't fail due to resource exhaustion.
     def _increase_process_limit(self):
-        ncpus = lit.util.usable_core_count()
+        ncpus = lit.util.detectCPUs()
         desired_limit = self.workers * ncpus * 2 # the 2 is a safety factor
 
         # Importing the resource module will likely fail on Windows.

llvm/utils/lit/lit/util.py

@@ -109,23 +109,32 @@ def to_unicode(s):
     return s
 
 
-def usable_core_count():
-    """Return the number of cores the current process can use, if supported.
-    Otherwise, return the total number of cores (like `os.cpu_count()`).
-    Default to 1 if undetermined.
+# TODO(yln): multiprocessing.cpu_count()
+# TODO(python3): len(os.sched_getaffinity(0)) and os.cpu_count()
+def detectCPUs():
+    """Detects the number of CPUs on a system.
+
+    Cribbed from pp.
 
     """
-    try:
-        n = len(os.sched_getaffinity(0))
-    except AttributeError:
-        n = os.cpu_count() or 1
-
-    # On Windows, with more than 32 processes, process creation often fails with
+    # Linux, Unix and MacOS:
+    if hasattr(os, 'sysconf'):
+        if 'SC_NPROCESSORS_ONLN' in os.sysconf_names:
+            # Linux & Unix:
+            ncpus = os.sysconf('SC_NPROCESSORS_ONLN')
+            if isinstance(ncpus, int) and ncpus > 0:
+                return ncpus
+        else:  # OSX:
+            return int(subprocess.check_output(['sysctl', '-n', 'hw.ncpu'],
+                                               stderr=subprocess.STDOUT))
+    # Windows:
+    if 'NUMBER_OF_PROCESSORS' in os.environ:
+        ncpus = int(os.environ['NUMBER_OF_PROCESSORS'])
+        if ncpus > 0:
+            # With more than 32 processes, process creation often fails with
             # "Too many open files".  FIXME: Check if there's a better fix.
-    if platform.system() == 'Windows':
-        return min(n, 32)
-
-    return n
+            return min(ncpus, 32)
+    return 1  # Default
 
 
 def mkdir(path):