Module undo 

Undo transactions and batched writes.

Every library write goes through a few functions (register_item, forget_item, save_metadata, save_tags, save_filters). While a transaction is open they record what they replace: the first pre-edit copy of each item, the ids of items that did not exist before, and the metadata, tags and saved filters as last saved. Library::revert puts that state back and returns the inverse transaction, for redo.

In WriteMode::Batched the same functions only mark what is dirty, and Library::flush writes each dirty file once. The bytes are the same as WriteMode::Immediate would have left: every writer serialises the whole in-memory value, so the last write wins either way.

Structs

FlushReport

What a Library::flush wrote.

Transaction

A group of edits that can be reverted together.

Enums

WriteMode

When library writes reach the disk.