[ZODB-Dev] Overwriting existing files in rename_or_copy_blob

[Thomas Lotze]

ZODB.blob.rename_or_copy_blob may silently overwrite existing blob files
since it uses os.rename without checking for the target file first. This
may affect committed data although we remove write permissions after the
commit.

This seems questionable to us, so we wonder whether it is actually desired
behaviour or a bug. We think raising an error would be appropriate.

-- 
Thomas