Constant DELTA_PARENT_DIGEST_KEY 

pub const DELTA_PARENT_DIGEST_KEY: &'static str;

Metadata custom key that records a delta checkpoint’s parent.

A checkpoint produced via Checkpoint::delta_from sets this key to the string returned by Checkpoint::content_digest of the base checkpoint it was diffed against. Checkpoint::applied_with_delta (and anyone reading the delta) can use it to validate the delta is being applied on top of the correct base.