Follow-up: bounded concurrent attempts per session (Decision 9 from PR #4005 review)

[piotr-roslaniec]

Context

PR #4005 implements the interactive signing path single-attempt at a time per session. The spec (docs/phase-7-interactive-session-spec-freeze.md §8) reserves bounded n-t+1 concurrent attempts per session for a fast-follow but the implementation is serial today.

A wallet that needs concurrent signing — batch redemption across multiple UTXOs, simultaneous mint + redeem operations — cannot use this layer today; it has to serialize. Against a slashing deadline, this serialization is itself a liveness risk for high-throughput tBTC v2 wallets.

Decision (from gap-inventory.md)

Defer to v1.x. The canary stage 1 wallets operate single-UTXO; the concurrent-signing liveness risk is acceptable for the canary stages.

Acceptance criteria

• Implement bounded n-t+1 concurrent attempts per session in pkg/tbtc/signer/src/engine/interactive.rs.
• The spec already defines the attempt-scoped nonce handle (§4) and attempt-scoped registry keys (§5); wire them through the concurrency layer.
• Concurrent attempts share the wallet key group but maintain independent nonces, attempt contexts, and registry state.
• Hard cap: a session can have at most n-t+1 concurrent attempts; attempts beyond the cap fail closed.
• Update the operator runbook to document the concurrency model and the cap.

Related

• PR feat(tbtc/signer): Rust FROST/ROAST signer (distributed DKG, interactive-only signing, FFI ABI 2.0) #4005 gap-inventory: agent-docs/gap-inventory.md Decision 9
• Spec: docs/phase-7-interactive-session-spec-freeze.md §1, §8

[linear]

ENG-746