Gate sync peer selection on per-protocol concurrent-request limit#9456
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Skip peers already at MAX_CONCURRENT_REQUESTS for the protocol being requested and break remaining ties at random, instead of sorting by a peer's total in-flight request count across all protocols.
Hoist ActiveRequestsPerPeer out of the per-column loop and account for in-round assignments via columns_to_request_by_peer.
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The map(|_| 1) flag only de-prioritized a peer's second column and is moot since the round batches to one request per peer, matching the by-range path.
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When sync picks a peer for a request, it now skips peers that are already at the per-protocol concurrency limit (MAX_CONCURRENT_REQUESTS) for the protocol being requested, and breaks the remaining ties at random. Previously it sorted by each peer's total in-flight request count summed across all protocols.
The per-protocol cap is the only hard limit the RPC layer actually enforces, so gating on it directly avoids handing a request to a peer that will just queue it, while randomness spreads the rest of the load evenly without maintaining any cross-protocol counters. This applies to all the sync peer-selection paths: blocks-by-range, columns-by-range, block lookups, payload lookups, and custody column lookups.
Picks up the ActiveRequestsPerPeer idea from dapplion#81.