[Sync] Sync task

[Voxelot]

The sync task receives information about the block height of peers via a stream from P2P. It primarily filters out previously seen block heights and then updates a mutexed shared state with the import task. It will never generate back pressure because it can always update the state and notify import without awaiting.

flowchart LR
	H{Heatbeat} --> |BlockHeight|S
	S --> |height > best_height|F[stream::filter ]
	F -.->|Update| ST{{State}}
	F -.->|Notify| Import{Import}

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Shared State:

/// State that is shared between the sync and import tasks
struct State {
	/// The highest seen height so far.
  best_seen_height: Option<Height>,
	/// The range of heights that are currently being imported.
  in_flight_heights: Option<Range<Height>>,
}

// Empty state means we have not observed any heights and
// we have not imported any headers.
State {
	best_seen_height: None,
	in_flight_height: None,
}

// There is a new height that has been observed but not imported.
State {
	best_seen_height: Some(5),
	in_flight_height: None,
}

// Currently importing headers 0..=5.
State {
	best_seen_height: Some(5),
	in_flight_height: Some(0..=5),
}

// Have imported headers 0..=3 but still need to import 4..=5.
State {
	best_seen_height: Some(5),
	in_flight_height: Some(3..=5),
}

// A newer height has been seen while the stream is importing headers.
State {
	best_seen_height: Some(7),
	in_flight_height: Some(4..=5),
}

// All caught up with the current best seen.
State {
	best_seen_height: Some(7),
	in_flight_height: Some(7..=7),
}

Ports

trait P2P {
    /// Get a stream of block heights reported by peers
    async fn block_heights(&self) -> BoxStream<BlockHeight>;
}