feat(ethflow-watcher): decode CoWSwapEthFlow OrderPlacement (BLEU-832)#9
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\`on_event(Event::Logs)\` matches each log against
\`CoWSwapOnchainOrders.OrderPlacement\` and keeps the four
fields BLEU-833's submission path will consume:
(sender, order, signature, data)
where \`order\` is the 12-field \`GPv2OrderData\` the
settlement contract verifies, and \`signature\` is the typed
\`OnchainSignature { scheme, data }\` pair (EIP-1271 or
PreSign).
Guardrails: \`decode_order_placement\` rejects the log when
the contract address is not one of the canonical EthFlow
deployments (production or staging — both share the same
address on every chain). topic0 must match the event
signature hash and the body must round-trip through
\`SolEvent::decode_raw_log\`. The decoder is on plain slices so
the seven host-free tests cover the happy path, the alternate
staging address, an unrelated contract, a wrong topic, a
truncated address, a truncated body, and an empty topic list.
\`DecodedPlacement\` boxes \`GPv2OrderData\` (~300 bytes); the
struct is kept private and \`#[allow(dead_code)]\` until
BLEU-833 wires the submit path.
\`cargo build --target wasm32-wasip2 --release -p
ethflow-watcher\` -> 96 KB .wasm (was 67 KB skeleton; the
\`CoWSwapOnchainOrders\` ABI + GPv2OrderData decode pull in
~30 KB of alloy sol-types runtime).
Linear: BLEU-832. Ref ADR-0006.
This was referenced Jun 15, 2026
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Summary
`on_event(Event::Logs)` matches each log against `CoWSwapOnchainOrders.OrderPlacement` and keeps the four fields BLEU-833's submission path consumes: `sender`, `order` (`GPv2OrderData`), `signature` (`OnchainSignature { scheme, data }`), and `data`.
Guardrails
`decode_order_placement` rejects a log when:
The decoder is on plain slices, so the host-free tests cover all branches without wit-bindgen scaffolding.
`DecodedPlacement` boxes `GPv2OrderData` (~300 bytes) for cache friendliness; the struct stays private + `#[allow(dead_code)]` until BLEU-833 wires submission.
Stacks on #8 (BLEU-831 skeleton).
Linear: BLEU-832. Ref ADR-0006.
Test plan