worker: count the number of CPUs

This works on the major platforms, and falls back to a safe guess of
1 elsewhere.

mercurial/worker.py

@@ -0,0 +1,29 @@
+# worker.py - master-slave parallelism support
+#
+# Copyright 2013 Facebook, Inc.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+import os
+
+def countcpus():
+    '''try to count the number of CPUs on the system'''
+
+    # posix
+    try:
+        n = int(os.sysconf('SC_NPROCESSORS_ONLN'))
+        if n > 0:
+            return n
+    except (AttributeError, ValueError):
+        pass
+
+    # windows
+    try:
+        n = int(os.environ['NUMBER_OF_PROCESSORS'])
+        if n > 0:
+            return n
+    except (KeyError, ValueError):
+        pass
+
+    return 1