This improves performance by keeping snapshots around for some time if there are no changes done by other processes. If a snapshot is not destroyed immediately after getting into the stable state then there's a chance that it won't have to be synchronously created again when a new opeartion is requested.
This improves performance a lot in cases when multiple operations are invoked by a single JS function (number of sync IPC calls is reduced to a minimum). It also improves correctness since changes are not visible to other content processes until a JS function finishes.
The patch implements core infrastructure, all items are sent to content when a snapshot is initialized and everything is fully working. However, sending of all items at once is not optimal for bigger databases. Support for lazy loading of items is implemented in a following patch.
If a database actor already exists for given origin, reuse it instead of creating a new one. This improves memory footprint a bit and also eliminates some round trips to the parent process.
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.
This adds a new quota client implementation, but only implements ShutdownWorkThreads.
At shutdown we wait for all running operations to finish including database actors which are closed by using an extra IPC message which avoids races between the parent and child.
Databases are dropped on the child side as soon as they are not used (e.g. after unlinking by the cycle collector).
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.
