Sandbox: CWD was not being created for workspaces

The code was creating the cwd folder but when the workspace was
mounted in to the buildroot it was hiding the folder created in it
behind the bind mounted workspace.

However by using the bubblewarp `--dir` directive to ensure that cwd
exists we can cover both workspace and non workspace situations with
the same method.

buildstream/sandbox/_sandboxbwrap.py

@@ -108,9 +108,6 @@ class SandboxBwrap(Sandbox):
             bwrap_command += ['--unshare-uts', '--hostname', 'buildstream']
             bwrap_command += ['--unshare-ipc']
 
-        if cwd is not None:
-            bwrap_command += ['--chdir', cwd]
-
         # Give it a proc and tmpfs
         bwrap_command += [
             '--proc', '/proc',
@@ -151,6 +148,10 @@ class SandboxBwrap(Sandbox):
         if flags & SandboxFlags.ROOT_READ_ONLY:
             bwrap_command += ["--remount-ro", "/"]
 
+        if cwd is not None:
+            bwrap_command += ['--dir', cwd]
+            bwrap_command += ['--chdir', cwd]
+
         # Set UID and GUI
         if self.user_ns_available:
             bwrap_command += ['--unshare-user']
@@ -179,11 +180,6 @@ class SandboxBwrap(Sandbox):
         with ExitStack() as stack:
             stack.enter_context(mount_map.mounted(self))
 
-            # Ensure the cwd exists
-            if cwd is not None:
-                workdir = os.path.join(root_mount_source, cwd.lstrip(os.sep))
-                os.makedirs(workdir, exist_ok=True)
-
             # If we're interactive, we want to inherit our stdin,
             # otherwise redirect to /dev/null, ensuring process
             # disconnected from terminal.

buildstream/sandbox/_sandboxchroot.py

@@ -100,9 +100,8 @@ class SandboxChroot(Sandbox):
 
             # Ensure the cwd exists
             if cwd is not None:
-                workdir = os.path.join(root_mount_source, cwd.lstrip(os.sep))
+                workdir = os.path.join(rootfs, cwd.lstrip(os.sep))
                 os.makedirs(workdir, exist_ok=True)
-
             status = self.chroot(rootfs, command, stdin, stdout,
                                  stderr, cwd, env, flags)
 

buildstream/sandbox/sandbox.py

@@ -223,7 +223,9 @@ class Sandbox():
         .. note::
 
            The optional *cwd* argument will default to the value set with
-           :func:`~buildstream.sandbox.Sandbox.set_work_directory`
+           :func:`~buildstream.sandbox.Sandbox.set_work_directory` and this
+           function must make sure the directory will be created if it does
+           not exist yet, even if a workspace is being used.
         """
         raise ImplError("Sandbox of type '{}' does not implement run()"
                         .format(type(self).__name__))