1386441 - Address common need of PBackground-managed services to retrieve NS_APP_USER_PROFILE_50_DIR/"ProfD" from directory service

Status: NEW

(Core :: IPC, enhancement, P3)

Description

[Andrew Sutherland [:asuth] (he/him)]

It's a pretty common need to know the profile directory, but nsDirectoryService can only safely be accessed from the main thread.  This stems from its use of a non-threadsafe hashtable and that it's very common for tests to implement nsIDirectoryServiceProviders in JS which may need to be consulted.

Bug 1350637 is moving LocalStorage from PContent to PBackground and is currently addressing this problem by blocking the background thread while it dispatches a runnable to the main thread.  This is an unfortunate side-effect of the `sync Preload` IPDL method.  Other services are able to do this asynchronously, (ex: QuotaManager::CreateRunnable) and may have other main thread-actions they need to undertake, but I think it's worth thinking about letting PBackground-based services avoid potentially avoidable main-thread bounces, blocking or not.

PS: LocalStorage also uses the runnable to ensure the mozStorage service is initialized.  That's another example of something common that logic off-the-main-thread should ideally be able to assume is available.

Comment 1

[Jan Varga [:janv]]

Yeah, agreed.

Comment 2

[Jim Mathies [:jimm]]

I think this should be in toolkit, but we'll keep it here for now.