upb will fail to import this proto:

enum Foo { BAR = 0; }

To fix this bug, upb_table needs to be modified to not use "0" as its "end-of-chain" marker. Hopefully there is some int32 that cannot be a valid enum value. If not, the fix might have to be more intrusive.

This is fixed in commit: https://github.com/haberman/upb/commit/6117730c85e5d64239337f0e8514109054202f5a