Expose sync progress via an onSyncProgress callback

[BigTava]

Problem

Consumers have no way to observe warp-sync progress.

Proposed API

A new optional callback on addChain:

export type SyncProgress =
  | { type: "warp-sync"; verified: number }
  | { type: "chain-sync"; current: number; target: number }
  | { type: "in-sync" };

chain = await client.addChain({
  chainSpec,
  onSyncProgress: (p: SyncProgress) => void,
});

[BigTava]

Is there an existing way to approximate this?

As far as I see we have:

• system_health with boolean isSyncing
• system_peers with max(bestNumber) across peers as a "network head" reference
• chain_subscribeFinalizedHeads with local finalized head as it advances

Should we do?

localFinalized / max(networkBest, localFinalized)

[lexnv]

Whats the use-case for exposing the sync progress?

[skunert]

@lexnv We have discussed in element to go with a RPC approach.

Currently all user-facing interaction is using RPC and we should not add a smoldot-specific callback here, its harder to integrate into tooling too. There is a draft here, but we need to iterate on the format. AFAIK it currently respects no warpsync fragments and we need to think whether it should be a RPC that requires polling or we go with a WS subscription. The latter makes currently more sense to me.