feat: emit PendingContentReady event

[Frando]

Description

This adds a new LiveEvent variant: PendingContentReady.

It is emitted once, after sync was started, the list of pending content blobs becomes empty. This is the case if either all blobs from this sesion have been downloaded, or failed.

I think this could help with simple examples like https://gist.github.com/dignifiedquire/efbd1a7a1da729494adb088d72f1ceaa#file-main-sync-rs-L84

It should not be used as a guarantee that all content is now available locally, and it should be used with a timeout usually, because it may very legally never be emitted (e.g. if the remote does not have all blobs themselves, or closes the connections, etc).

Breaking Changes

• LiveEvent now has a new variant PendingContentReady. This event is emitted after a sync completed and Self::SyncFinished was emitted. It signals that all currently pending downloads have been completed. Receiving this event does not guarantee that all content in the document is available. If blobs failed to download, this event will still be emitted after all operations completed.

Notes & open questions

Change checklist

• Self-review.
• Documentation updates if relevant.
• Tests if relevant.
• All breaking changes documented.

[dignifiedquire]

2024-05-17T10:38:10.257646Z DEBUG gossip{me=icdmm64o2lmqfyxw}: iroh_gossip::net: handle out_event SendMessage(PublicKey(4dxirwpnfj5gb3av), Message { topic: TopicId(lofyckr47fnbhjaq), message: Gossip(Gossip(Gossip { id: MessageId(nazywnpxm4nuce77), content: <239b>, scope: Swarm(Round(0)) })) })
thread 'sync_restart_node' panicked at iroh\tests\sync.rs:1316:9:
Failed to receive or match all events: Event didn't match any matcher: PendingContentReady

this looks like a real failure?