Datastores are preloaded only for content principals. The preloading is triggered as soon as possible to lower the chance of blocking the main thread in content process. If there is no physical database on disk for given origin, datastore is not created. Preloaded datastores are kept alive for 20 seconds.
Storage events are fired either directly after getting response from synchronous SetItem call or through observers. When a new onstorage event listener is added, we sycnhronously register an observer in the parent process. There's always only one observer actor per content process.
PBackgroundLSDatabase is now managed by a new PBackgroundLSObject protocol. PBackgroundLSObject is needed to eliminate the need to pass the principal info and document URI everytime a write operation occurs.
Preparation of an observer shares some states with preparation of a datastore, so common stuff now lives in LSRequestBase and preparation of a datastore now implements a nested state machine.
This patch was enhanced by asuth to drop observers only when the last storage listener is removed.
EventListenerRemoved is invoked on any removal, not just the final removal, so we need to make sure it's the final removal before dropping observer.
The implementation is based on a cache (datastore) living in the parent process and sync IPC calls initiated from content processes.
IPC communication is done using per principal/origin database actors which connect to the datastore.
The synchronous blocking of the main thread is done by creating a nested event target and spinning the event loop.
