Resource.read() and Publication.positions are not safe to be called from the main thread

[jerome65536]

Deadlock in main thread due to EPUBNavigatorViewController.viewDidLoad calling Resource.read via publication.positions with a DefaultHTTPClient-based Fetcher.

Since Resource.read blocks while waiting for a result, and the result can't be processed while the main thread is blocked, this freezes the whole app.

A workaround is to cache the result by calling let _ = publication.positions from a background thread before creating the EPUBNavigatorViewController, presumably preventing later calls of publication.positions from calling Resource.read.

[mickael-menu]

It's probably because the native HTTP APIs are calling synchronously to the main thread. I had a similar (but unrelated to HTTP) issue in this other PR readium/r2-navigator-swift#160.

I think we need to fix two things:

• Printing a warning when calling Resource.read() and Publication.positions from the main thread. These are not safe to be called from there.
• Fixing the navigator to never call them from the main thread.

[mickael-menu]

These APIs are async from 3.0.