Reliable Storage: Investigate persistent storage for Electron

[jryans]

Electron devs claim all origins should have "durable" storage. However, we have observed IndexedDB being pruned by Electron just as it can be by browsers.

Investigate the details of Electron's behaviour here and try to ensure it always persists.

[jryans]

Affected Users

Anecdotally, I have historically seen debug logs come in where Electron users are seeing the crypto store re-create itself, which tends to suggest that data has been evicted in the same way that it can happen in normal browsers. We haven't seen more such logs arrive since we added more logging about storage state, so it's currently unclear if Electron uses are actively affected in the field.

We are also now tracking storage state in analytics, so we can watch that over time to see if there are any events from Electron users.

Code Analysis

• Electron applies a SpecialStoragePolicy to pages from any origin
  • This applies certain settings, such as:
    • Durable: true: This means storage will stay around even when space is low
    • Unlimited: true: This means storage can use as much space as the drive allows
  • This policy was added on 2015-10-13
• Electron also applies AtomPermissionManager which grants all permission based APIs and seems to have been added in a similar ~2015 time range

To summarise, it appears Electron should allow IndexedDB to use as much space as it likes and should never delete it arbitrarily.

Lab Simulation

I forced Riot to store its profile on a small drive inside a VM to observe what happens. The Electron policy seems to work as designed. I was able to store data until the drive had no space left. IndexedDB survived multiple Riot restarts.

No Space Left

When your entire drive runs out of space, IndexedDB does still explode in ways we don't yet catch, including spinners that block using the app. However, since Riot only supports place its config on your primary drive, presumably you'd notice you are out of space in other ways like OS warnings, etc. Riot should recover with a restart once there's more space.

Summary

It appears from both code inspection and lab testing that Electron should persisting data as desired, so there's no code change to make here. We should keep watching analytics to see if the issue still affects Electron users and investigate again if it does.