Adding darwin.py (MacOS) platform

Adding functionality to recognise Darwin as a platform in plaform.py

buildstream/_platform/darwin.py

+#
+#  Copyright (C) 2017 Codethink Limited
+#  Copyright (C) 2018 Bloomberg Finance LP
+#
+#  This program is free software; you can redistribute it and/or
+#  modify it under the terms of the GNU Lesser General Public
+#  License as published by the Free Software Foundation; either
+#  version 2 of the License, or (at your option) any later version.
+#
+#  This library is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU
+#  Lesser General Public License for more details.
+#
+#  You should have received a copy of the GNU Lesser General Public
+#  License along with this library. If not, see <http://www.gnu.org/licenses/>.
+
+import os
+import resource
+
+from .._exceptions import PlatformError
+from ..sandbox import SandboxChroot, SandboxDummy
+
+from . import Platform
+
+
+class Darwin(Platform):
+
+    # This value comes from OPEN_MAX in syslimits.h
+    OPEN_MAX = 10240
+
+    def __init__(self, context):
+
+        super().__init__(context)
+
+    def create_sandbox(self, *args, **kwargs):
+        return SandboxDummy(*args, **kwargs)
+
+    def check_sandbox_config(self, config):
+        # Accept all sandbox configs as it's irrelevant with the dummy sandbox (no Sandbox.run).
+        return True
+
+    def get_cpu_count(self, cap=None):
+        if cap < os.cpu_count():
+            return cap
+        else:
+            return os.cpu_count()
+
+    def set_resource_limits(self, soft_limit=OPEN_MAX, hard_limit=None):
+        super().set_resource_limits(soft_limit)

buildstream/_platform/platform.py

@@ -37,19 +37,22 @@ class Platform():
 
     @classmethod
     def _create_instance(cls):
-        if sys.platform.startswith('linux'):
-            backend = 'linux'
-        else:
-            backend = 'unix'
-
         # Meant for testing purposes and therefore hidden in the
         # deepest corners of the source code. Try not to abuse this,
         # please?
         if os.getenv('BST_FORCE_BACKEND'):
             backend = os.getenv('BST_FORCE_BACKEND')
+        elif sys.platform.startswith('linux'):
+            backend = 'linux'
+        elif sys.platform.startswith('darwin'):
+            backend = 'darwin'
+        else:
+            backend = 'unix'
 
         if backend == 'linux':
             from .linux import Linux as PlatformImpl
+        elif backend == 'darwin':
+            from .darwin import Darwin as PlatformImpl
         elif backend == 'unix':
             from .unix import Unix as PlatformImpl
         else: