docs(design): async-compatible channel abstraction for transfer pipeline#3647
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Design note for #1591. Proposes a TransferChannel trait that exposes both sync and async send/recv surfaces, with flume as the recommended backing for sync-async bridge sites and crossbeam retained for the pure-sync hot paths. Documents the five migration sites, backpressure and fairness semantics, cancellation footguns, and follow-up work. Wire-compat impact: zero. Channels are internal coordination only.
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…ine (#3647) Design note for #1591. Proposes a TransferChannel trait that exposes both sync and async send/recv surfaces, with flume as the recommended backing for sync-async bridge sites and crossbeam retained for the pure-sync hot paths. Documents the five migration sites, backpressure and fairness semantics, cancellation footguns, and follow-up work. Wire-compat impact: zero. Channels are internal coordination only.
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Summary
Design note for #1591 covering an async-compatible channel abstraction for the transfer pipeline. Proposes a thin
TransferChanneltrait (sync + async send/recv on the same channel) and a default implementation choice (flume for sync-async bridges, crossbeam stays in pure-sync hot paths).Key sections
crates/engine/src/concurrent_delta/work_queue/bounded.rs,crates/transfer/src/pipeline/spsc.rs,crates/transfer/src/pipeline/async_dispatch.rs, andcrates/daemon/src/daemon/async_session/listener.rs.Sender/Receivertypes, synctry_send/send_blocking/recv_blockingplus asyncsend_async/recv_async.Wire-compat invariants
Zero impact. Channels are internal coordination only - no protocol bytes, no capability negotiation, no multiplex frame changes.
Test plan
bounded.rs:8,89-104,listener.rs:25,216-249,async_dispatch.rs:10,consumer.rs:47,Cargo.toml:188,199-200.