Convert uses of external_directory to get_underlying_directory()

buildstream/sandbox/_mount.py

@@ -32,8 +32,10 @@ from .._fuse import SafeHardlinks
 class Mount():
     def __init__(self, sandbox, mount_point, safe_hardlinks):
         scratch_directory = sandbox._get_scratch_directory()
-        # Getting external_directory here is acceptable as we're part of the sandbox code.
-        root_directory = sandbox.get_virtual_directory().external_directory
+        # Getting get_underlying_directory() here is acceptable as
+        # we're part of the sandbox code. This will fail if our
+        # directory is CAS-based.
+        root_directory = sandbox.get_virtual_directory().get_underlying_directory()
 
         self.mount_point = mount_point
         self.safe_hardlinks = safe_hardlinks

buildstream/sandbox/_sandboxbwrap.py

@@ -58,7 +58,7 @@ class SandboxBwrap(Sandbox):
         stdout, stderr = self._get_output()
 
         # Allowable access to underlying storage as we're part of the sandbox
-        root_directory = self.get_virtual_directory().external_directory
+        root_directory = self.get_virtual_directory().get_underlying_directory()
 
         # Fallback to the sandbox default settings for
         # the cwd and env.

buildstream/storage/_filebaseddirectory.py

@@ -214,3 +214,8 @@ class FileBasedDirectory(Directory):
         # which exposes the sandbox directory; we will have to assume for the time being
         # that people will not abuse __str__.
         return self.external_directory
+
+    def get_underlying_directory(self) -> str:
+        """ Returns the underlying (real) file system directory this
+        object refers to. """
+        return self.external_directory

buildstream/storage/directory.py

@@ -153,3 +153,9 @@ class Directory():
 
         """
         raise NotImplementedError()
+
+    def get_underlying_directory(self) -> str:
+        """ Returns the underlying (real) file system directory this
+        object refers to. This will throw an exception if there isn't
+        a real directory behind the object. """
+        raise NotImplementedError()