Import sandbox subclass when instantiating sandbox

Sandbox: Ensure that we only import the sandbox subclasses when instantiate a sandbox

This patch was required due to not being able to execute `bst` on
a Linux machine without Fuse or Bubblewrap available

buildstream/_platform/darwin.py

@@ -19,7 +19,7 @@ import os
 import resource
 
 from .._exceptions import PlatformError
-from ..sandbox import SandboxChroot, SandboxDummy
+from ..sandbox import SandboxDummy
 
 from . import Platform
 

buildstream/_platform/linux.py

@@ -23,7 +23,7 @@ import subprocess
 from .. import _site
 from .. import utils
 from .._message import Message, MessageType
-from ..sandbox import SandboxBwrap, SandboxDummy
+from ..sandbox import SandboxDummy
 
 from . import Platform
 
@@ -48,6 +48,7 @@ class Linux(Platform):
         if not self._local_sandbox_available():
             return SandboxDummy(*args, **kwargs)
         else:
+            from ..sandbox._sandboxbwrap import SandboxBwrap
             # 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

buildstream/_platform/unix.py

@@ -20,7 +20,6 @@
 import os
 
 from .._exceptions import PlatformError
-from ..sandbox import SandboxChroot
 
 from . import Platform
 
@@ -39,6 +38,7 @@ class Unix(Platform):
             raise PlatformError("Root privileges are required to run without bubblewrap.")
 
     def create_sandbox(self, *args, **kwargs):
+        from ..sandbox._sandboxchroot import SandboxChroot
         return SandboxChroot(*args, **kwargs)
 
     def check_sandbox_config(self, config):

buildstream/sandbox/__init__.py

@@ -18,7 +18,5 @@
 #        Tristan Maat <tristan.maat@codethink.co.uk>
 
 from .sandbox import Sandbox, SandboxFlags
-from ._sandboxchroot import SandboxChroot
-from ._sandboxbwrap import SandboxBwrap
 from ._sandboxremote import SandboxRemote
 from ._sandboxdummy import SandboxDummy