Function update 

pub fn update(
    ctx: &Context,
    req: UpdateRequest,
) -> Result<Outcome<PantryContents>, CoreError>

Change an item already in the pantry and write it back.

Only the attributes set on the request are changed; the rest of the item is left as it was. There is no way to clear an attribute — None means “leave it”, not “remove it” — so an item is cleared by removing and adding it.

Returns the pantry as it now stands on disk, and any warnings from parsing what was there before.

Only the entry asked for is touched; see what a write touches.

Errors

• CoreError::PantryEdit if the request sets no attribute at all, since that could only rewrite the file to what it already said; or if there is no such section, or no such item in it. Nothing is written.
• CoreError::MissingConfig if the context carries no pantry, and CoreError::ReadOnlyConfig if it carries one inline.
• As load otherwise, plus CoreError::Io if the file cannot be written.