refactor cpu number autodetection to avoid the regression

CHANGELOG

@@ -1,3 +1,9 @@
+1.13.1
+-------
+
+- fix a regression -n 0 now disables xdist again
+
+
 1.13
 -------------------------
 
@@ -16,7 +22,7 @@
 - new "--max-slave-restart" option that can be used to control maximum
   number of times pytest-xdist can restart slaves due to crashes. Thanks to
   Anatoly Bubenkov for the report and Bruno Oliveira for the PR.
-  
+
 - release as wheel
 
 - "-n" option now can be set to "auto" for automatic detection of number

xdist/plugin.py

@@ -1,12 +1,20 @@
-import multiprocessing
-
 import py
 import pytest
 
+
+def parse_numprocesses(s):
+    if s == 'auto':
+        import multiprocessing
+        return multiprocessing.cpu_count()
+    else:
+        return int(s)
+
+
 def pytest_addoption(parser):
     group = parser.getgroup("xdist", "distributed and subprocess testing")
     group._addoption('-n', dest="numprocesses", metavar="numprocesses",
            action="store",
+           type=parse_numprocesses,
            help="shortcut for '--dist=load --tx=NUM*popen', "
                 "you can use 'auto' here for auto detection CPUs number on "
                 "host system")
@@ -68,13 +76,6 @@ def pytest_configure(config):
 @pytest.mark.tryfirst
 def pytest_cmdline_main(config):
     if config.option.numprocesses:
-        if config.option.numprocesses == 'auto':
-            config.option.numprocesses = multiprocessing.cpu_count()
-        else:
-            try:
-                config.option.numprocesses = int(config.option.numprocesses)
-            except ValueError:
-                config.option.numprocesses = 1
         config.option.dist = "load"
         config.option.tx = ['popen'] * config.option.numprocesses
     if config.option.distload: