Support async signing of interactive-tx initial commitment signatures #4355
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Previously, we'd emit a FundingTransactionReadyForSigning event once the initial commitment_signed is exchanged for a splicing/dual-funding attempt and require users to call back with their signed inputs using ChannelManager::funding_transaction_signed. While this approach worked in practice, it prevents us from abandoning a splice if we cannot or no longer wish to sign as the splice has already been committed to by this point. This commit reworks the API such that this is now possible. After exchanging tx_complete, we will no longer immediately send our initial commitment_signed. We will now emit the FundingTransactionReadyForSigning event and wait for the user to call back before releasing both our initial commitment_signed and our tx_signatures. As a result, the event is now persisted, as there is only one possible path in which it is generated. Note that we continue to only emit the event if a local contribution to negotiated transaction was made. Future work will expose a cancellation API such that we can abandon splice attempts safely (we can just force close the channel with dual-funding).
This is crucial to enable the splice cancellation use case. When we process the initial commitment signed from our counterparty, we queue a monitor update that cannot be undone. To give the user a chance to abort the splice negotiation before it's committed to, we buffer the message until a successful call to `Channel::funding_transaction_signed` and process it then. Note that this is currently only done for splice and RBF attempts, as if we want to abort a dual funding negotiation, we can just force close the channel as it hasn't been funded yet.
Now that we require users to first call `ChannelManager::funding_transaction_signed` before releasing any signatures, it's possible that it is called before we receive the initial commitment signed from our counterparty, which would transition the channel to funded. Because of this, we need to support the API call while the channel is still in the unfunded phase. Note that this commit is mostly a code move of `FundedChannel::funding_transaction_signed` to `Channel::funding_transaction_signed` that doesn't alter the signing logic.
This commit allows for an async signer to immediately return upon a call to `EcdsaChannelSigner::sign_counterparty_commitment` for the initial commitment signatures of an interactively funded transaction, such that they can call back in via `ChannelManager::signer_unblocked` once the signatures are ready. This is done for both splices and dual-funded channels, though note that the latter still require more work to be integrated. Since `tx_signatures` must be sent only after exchanging `commitment_signed`, we make sure to hold them back if they're ready to be sent until our `commitment_signed` is also ready.
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Codecov Report❌ Patch coverage is Additional details and impacted files@@ Coverage Diff @@
## main #4355 +/- ##
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- Coverage 86.53% 86.08% -0.46%
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Files 158 156 -2
Lines 103190 102620 -570
Branches 103190 102620 -570
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- Hits 89300 88340 -960
- Misses 11469 11775 +306
- Partials 2421 2505 +84
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Depends on #4336.