Adding Dummy sandbox

This is to allow platforms that do not support sandboxing
To error is a controlled/known way

buildstream/_platform/linux.py

@@ -23,7 +23,7 @@ from .. import _site
 from .. import utils
 from .._artifactcache.cascache import CASCache
 from .._message import Message, MessageType
-from ..sandbox import SandboxBwrap
+from ..sandbox import SandboxBwrap, SandboxDummy
 
 from . import Platform
 
@@ -48,10 +48,13 @@ class Linux(Platform):
         return self._artifact_cache
 
     def create_sandbox(self, *args, **kwargs):
-        # Inform the bubblewrap sandbox as to whether it can use user namespaces or not
-        kwargs['user_ns_available'] = self._user_ns_available
-        kwargs['die_with_parent_available'] = self._die_with_parent_available
-        return SandboxBwrap(*args, **kwargs)
+        if not self._local_sandbox_available():
+            return SandboxDummy(*args, **kwargs)
+        else:
+            # Inform the bubblewrap sandbox as to whether it can use user namespaces or not
+            kwargs['user_ns_available'] = self._user_ns_available
+            kwargs['die_with_parent_available'] = self._die_with_parent_available
+            return SandboxBwrap(*args, **kwargs)
 
     def create_artifact_cache(self, context, *, enable_push):
         return super().create_artifact_cache(context=context, enable_push=self._user_ns_available)

buildstream/sandbox/__init__.py

@@ -21,3 +21,4 @@ from .sandbox import Sandbox, SandboxFlags
 from ._sandboxchroot import SandboxChroot
 from ._sandboxbwrap import SandboxBwrap
 from ._sandboxremote import SandboxRemote
+from ._sandboxdummy import SandboxDummy

buildstream/sandbox/_sandboxdummy.py

+#
+#  Copyright (C) 2017 Codethink Limited
+#
+#  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/>.
+#
+#  Authors:
+
+from .._exceptions import SandboxError
+from . import Sandbox
+
+
+class SandboxDummy(Sandbox):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+    def run(self, command, flags, *, cwd=None, env=None):
+
+        # Fallback to the sandbox default settings for
+        # the cwd and env.
+        #
+        cwd = self._get_work_directory(cwd=cwd)
+        env = self._get_environment(cwd=cwd, env=env)
+
+        if not self._has_command(command[0], env):
+            raise SandboxError("Staged artifacts do not provide command "
+                               "'{}'".format(command[0]),
+                               reason='missing-command')
+
+        raise SandboxError("This platform does not support local builds")