Report data loss to script

[inexorabletash]

Imported from https://www.w3.org/Bugs/Public/show_bug.cgi?id=22370

from dgrogan@chromium:

It would be nice to be able to notify script that some data may have been lost due to disk corruption or other failure.

Previous discussion:
http://lists.w3.org/Archives/Public/public-webapps/2013OctDec/0108.html

[inexorabletash]

Chrome has shipped two additional properties:

enum IDBDataLossAmount { "none", "total" };
partial interface IDBVersionChangeEvent {
  readonly attribute IDBDataLossAmount dataLoss;
  readonly attribute DOMString dataLossMessage; 
};

With the speclet: if backing store corruption is detected when a database is opened, the backing store will be deleted and thus the open request will be treated as an initial open/upgrade; dataLoss will be set to "total" indicating that the database was deleted. This allows sites to reconcile out-of-band data (cookies, localStorage).

This isn't great but it works.

Another idea that we thought of is firing an event of some type. Possibly at the connection (IDBDatabase), or the open request (IDBOpenDBRequest), or maybe even at the factory (IDBFactory) when a backing store error is encountered.

[inexorabletash]

TPAC 2019 Web Apps Indexed DB triage notes:

This is being explored more generically over in:

https://github.com/wanderview/storage-corruption-reporting