Suppose you have a multiple-output derivation, and one of its output is already present in the store.

When re-building the derivation, the already-present output is not present.

This leads to an assertion failure at "assert(!S_ISDIR(st.st_mode));" in canonicalisePathMetaData_ because the owner of the already-present output is `root', not the build user.

A possible fix would be to remove any already-present outputs before rebuilding the derivation.