_casbaseddirectory.py: add _save() function.

buildstream/storage/_casbaseddirectory.py

@@ -561,3 +561,20 @@ class CasBasedDirectory(Directory):
         throw an exception. """
         raise VirtualDirectoryError("_get_underlying_directory was called on a CAS-backed directory," +
                                     " which has no underlying directory.")
+
+    def _save(self, name):
+        """Saves this directory into the content cache as a named ref. Used
+        by remote execution to make references for source directories so they
+        can be pushed to a remote artifact server.
+
+        """
+        self._recalculate_recursing_up()
+        self._recalculate_recursing_down()
+        (rel_refpath, refname) = os.path.split(name)
+        refdir = os.path.join(self.cas_directory, 'refs', 'heads', rel_refpath)
+        refname = os.path.join(refdir, refname)
+
+        if not os.path.exists(refdir):
+            os.makedirs(refdir)
+        with open(refname, "wb") as f:
+            f.write(self.ref.SerializeToString())