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Zhengchao An
6559248f55
The legacy erasure-decode overlap path drove the speculative next-stripe read and the current-stripe emit with `tokio::join!`, which runs both futures to completion. When the current stripe's emit terminated the loop — a client disconnect or any emit error — the join still waited for the prefetch read, so a `Stop` could stall for a full shard-read deadline on a slow or wedged remote shard before the GET could fail. Drive the two futures with a biased `select!` instead and, the moment emit reports `Stop`, drop the in-flight read future. Because the entire read pipeline is structured async (a `FuturesUnordered` of `read_shard` futures inside `ParallelReader::read`/`read_lockstep`, with no `tokio::spawn`), dropping the read future is a real cancellation: it drops every in-flight shard read and propagates cancellation down to the RemoteDisk/HTTP reader, leaving no background read behind. The `select!` is scoped so both pinned futures drop before `reader`/`shards` are reused, which is what performs the cancellation in the `Stop` case. This only affects the overlap-enabled path. The default remains OFF (`prefetch_count == 1` and bitrot-decode overlap disabled), and the strictly-serial read -> reconstruct -> emit default branch is untouched and byte-for-byte identical. The `Continue` path preserves offset, short-tail, buffer recycle, and bitrot/reconstruction error ordering exactly as before. Scope: cancel-safety only. The rollout decision (whether to enable overlap by default) still requires the Linux multi-node high-RTT three-size A/B from https://github.com/rustfs/backlog/issues/1310 and is deferred; this change does not flip the default or introduce any behavior that A/B must adjudicate. White-box test `test_legacy_prefetch_cancels_next_read_on_emit_failure` drives the real `Erasure::decode` path with overlap enabled, a writer that fails emit, and shards that serve the first stripe then stall the next-stripe read far beyond the assertion window. Under the paused clock the read future is dropped and decode returns at virtual t~=0; reverting cancel-safety makes it wait out the shard-read timeout, so the test fails closed. Refs: https://github.com/rustfs/backlog/issues/1310