worker: restore old countcpus code (issue4869)

This is a backout of d29859cfcfc2. The stdlib implementation of
multiprocessing.cpu_count() attempts to invoke a process on BSD and
Darwin platforms (at least on 2.7). Under certain conditions (such as
cwd being removed) this could raise. Our old code was silently catching
the exception.

The old code was more robust, so restore it.

mercurial/worker.py

@@ -8,7 +8,6 b''
 from __future__ import absolute_import
 
 import errno
-import multiprocessing
 import os
 import signal
 import sys
@@ -19,10 +18,24 b' from . import util'
 
 def countcpus():
     '''try to count the number of CPUs on the system'''
+
+    # posix
     try:
-        return multiprocessing.cpu_count()
-    except NotImplementedError:
-        return 1
+        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
 
 def _numworkers(ui):
     s = ui.config('worker', 'numcpus')