element.py: Raise an error if the source staging directory is non-empty

This is the correct place for the check, once for each element in
the overall source staging directory.

1 files changed, 4 insertions, 0 deletions

diff --git a/buildstream/element.py b/buildstream/element.py
index c8fd64ee0..eaaf77bf9 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -1382,6 +1382,10 @@ class Element(Plugin):
     #
     def _stage_sources_at(self, directory):
         with self.timed_activity("Staging sources", silent_nested=True):
+
+            if os.path.isdir(directory) and os.listdir(directory):
+                raise ElementError("Staging directory '{}' is not empty".format(directory))
+
             for source in self.__sources:
                 source._stage(directory)