Moving `os.sched_getaffinity(0)` logic

Removed `os.sched_getaffinity(0)` from _project.py
Added `os.sched_getaffinity(0)` to platform.py
Added `os.cpu_count()` to darwin.py

buildstream/_platform/darwin.py

@@ -49,3 +49,6 @@ class Darwin(Platform):
 
     def create_sandbox(self, *args, **kwargs):
         return SandboxChroot(*args, **kwargs)
+
+     def get_cpu_count(self):
+        return str(min(len(os.cpu_count())))

buildstream/_platform/platform.py

@@ -96,3 +96,6 @@ class Platform():
     def create_sandbox(self, *args, **kwargs):
         raise ImplError("Platform {platform} does not implement create_sandbox()"
                         .format(platform=type(self).__name__))
+
+    def get_cpu_count(self):
+        return str(min(len(os.sched_getaffinity(0)), 8))

buildstream/_project.py

@@ -19,6 +19,7 @@
 #        Tiago Gomes <tiago.gomes@codethink.co.uk>
 
 import os
+import sys
 from collections import Mapping, OrderedDict
 from pluginbase import PluginBase
 from . import utils
@@ -38,6 +39,7 @@ from ._loader import Loader
 from .element import Element
 from ._message import Message, MessageType
 from ._includes import Includes
+from ._platform import Platform
 
 
 # Project Configuration file
@@ -140,6 +142,7 @@ class Project():
         profile_start(Topics.LOAD_PROJECT, self.directory.replace(os.sep, '-'))
         self._load(parent_loader=parent_loader, tempdir=tempdir)
         profile_end(Topics.LOAD_PROJECT, self.directory.replace(os.sep, '-'))
+        self.platform = Platform.get_platform()
 
         self._partially_loaded = True
 
@@ -594,7 +597,7 @@ class Project():
         # Based on some testing (mainly on AWS), maximum effective
         # max-jobs value seems to be around 8-10 if we have enough cores
         # users should set values based on workload and build infrastructure
-        output.base_variables['max-jobs'] = str(min(len(os.sched_getaffinity(0)), 8))
+        output.base_variables['max-jobs'] = self.platform.get_cpu_count()
 
         # Export options into variables, if that was requested
         output.options.export_variables(output.base_variables)