Copy async docs from async bump transaction objects to sync ones

When we added the async traits and wrapper structs for the
transaction-bumping logic, we didn't bother copying the
documentation to the sync wrappers as we figured the links
sufficed. Sadly, however, this means that our bindings logic will
have no docs but a broken link to an async object that doesn't
exist.

Instead, here, we copy the docs from the async objects to the sync
ones, at least leaving behind a comment noting that both need
updating whenever one gets updated.

Author: TheBlueMatt

lightning/src/events/bump_transaction/mod.rs

@@ -355,6 +355,7 @@ pub struct CoinSelection {
 /// For a synchronous version of this trait, see [`sync::CoinSelectionSourceSync`].
 ///
 /// This is not exported to bindings users as async is only supported in Rust.
+// Note that updates to documentation on this trait should be copied to the synchronous version.
 pub trait CoinSelectionSource {
 	/// Performs coin selection of a set of UTXOs, with at least 1 confirmation each, that are
 	/// available to spend. Implementations are free to pick their coin selection algorithm of
@@ -408,6 +409,7 @@ pub trait CoinSelectionSource {
 /// For a synchronous version of this trait, see [`sync::WalletSourceSync`].
 ///
 /// This is not exported to bindings users as async is only supported in Rust.
+// Note that updates to documentation on this trait should be copied to the synchronous version.
 pub trait WalletSource {
 	/// Returns all UTXOs, with at least 1 confirmation each, that are available to spend.
 	fn list_confirmed_utxos<'a>(&'a self) -> AsyncResult<'a, Vec<Utxo>, ()>;
@@ -430,6 +432,7 @@ pub trait WalletSource {
 /// For a synchronous version of this wrapper, see [`sync::WalletSync`].
 ///
 /// This is not exported to bindings users as async is only supported in Rust.
+// Note that updates to documentation on this struct should be copied to the synchronous version.
 pub struct Wallet<W: Deref + MaybeSync + MaybeSend, L: Deref + MaybeSync + MaybeSend>
 where
 	W::Target: WalletSource + MaybeSend,
@@ -679,6 +682,7 @@ where
 /// This is not exported to bindings users as async is only supported in Rust.
 ///
 /// [`Event::BumpTransaction`]: crate::events::Event::BumpTransaction
+// Note that updates to documentation on this struct should be copied to the synchronous version.
 pub struct BumpTransactionEventHandler<B: Deref, C: Deref, SP: Deref, L: Deref>
 where
 	B::Target: BroadcasterInterface,

lightning/src/events/bump_transaction/sync.rs

@@ -28,13 +28,23 @@ use super::{
 	WalletSource,
 };
 
-/// A synchronous version of the [`WalletSource`] trait.
+/// An alternative to [`CoinSelectionSourceSync`] that can be implemented and used along
+/// [`WalletSync`] to provide a default implementation to [`CoinSelectionSourceSync`].
+///
+/// For an asynchronous version of this trait, see [`WalletSource`].
+// Note that updates to documentation on this trait should be copied to the asynchronous version.
 pub trait WalletSourceSync {
-	/// A synchronous version of [`WalletSource::list_confirmed_utxos`].
+	/// Returns all UTXOs, with at least 1 confirmation each, that are available to spend.
 	fn list_confirmed_utxos(&self) -> Result<Vec<Utxo>, ()>;
-	/// A synchronous version of [`WalletSource::get_change_script`].
+	/// Returns a script to use for change above dust resulting from a successful coin selection
+	/// attempt.
 	fn get_change_script(&self) -> Result<ScriptBuf, ()>;
-	/// A Synchronous version of [`WalletSource::sign_psbt`].
+	/// Signs and provides the full [`TxIn::script_sig`] and [`TxIn::witness`] for all inputs within
+	/// the transaction known to the wallet (i.e., any provided via
+	/// [`WalletSource::list_confirmed_utxos`]).
+	///
+	/// If your wallet does not support signing PSBTs you can call `psbt.extract_tx()` to get the
+	/// unsigned transaction and then sign it with your wallet.
 	fn sign_psbt(&self, psbt: Psbt) -> Result<Transaction, ()>;
 }
 
@@ -74,7 +84,12 @@ where
 	}
 }
 
-/// A synchronous wrapper around [`Wallet`] to be used in contexts where async is not available.
+/// A wrapper over [`WalletSourceSync`] that implements [`CoinSelectionSync`] by preferring UTXOs
+/// that would avoid conflicting double spends. If not enough UTXOs are available to do so,
+/// conflicting double spends may happen.
+///
+/// For an asynchronous version of this wrapper, see [`Wallet`].
+// Note that updates to documentation on this struct should be copied to the asynchronous version.
 pub struct WalletSync<W: Deref + MaybeSync + MaybeSend, L: Deref + MaybeSync + MaybeSend>
 where
 	W::Target: WalletSourceSync + MaybeSend,
@@ -136,15 +151,59 @@ where
 	}
 }
 
-/// A synchronous version of the [`CoinSelectionSource`] trait.
+/// An abstraction over a bitcoin wallet that can perform coin selection over a set of UTXOs and can
+/// sign for them. The coin selection method aims to mimic Bitcoin Core's `fundrawtransaction` RPC,
+/// which most wallets should be able to satisfy. Otherwise, consider implementing
+/// [`WalletSourceSync`], which can provide a default implementation of this trait when used with
+/// [`WalletSync`].
+///
+/// For an asynchronous version of this trait, see [`CoinSelectionSource`].
+// Note that updates to documentation on this trait should be copied to the asynchronous version.
 pub trait CoinSelectionSourceSync {
-	/// A synchronous version of [`CoinSelectionSource::select_confirmed_utxos`].
+	/// Performs coin selection of a set of UTXOs, with at least 1 confirmation each, that are
+	/// available to spend. Implementations are free to pick their coin selection algorithm of
+	/// choice, as long as the following requirements are met:
+	///
+	/// 1. `must_spend` contains a set of [`Input`]s that must be included in the transaction
+	///    throughout coin selection, but must not be returned as part of the result.
+	/// 2. `must_pay_to` contains a set of [`TxOut`]s that must be included in the transaction
+	///    throughout coin selection. In some cases, like when funding an anchor transaction, this
+	///    set is empty. Implementations should ensure they handle this correctly on their end,
+	///    e.g., Bitcoin Core's `fundrawtransaction` RPC requires at least one output to be
+	///    provided, in which case a zero-value empty OP_RETURN output can be used instead.
+	/// 3. Enough inputs must be selected/contributed for the resulting transaction (including the
+	///    inputs and outputs noted above) to meet `target_feerate_sat_per_1000_weight`.
+	/// 4. The final transaction must have a weight smaller than `max_tx_weight`; if this
+	///    constraint can't be met, return an `Err`. In the case of counterparty-signed HTLC
+	///    transactions, we will remove a chunk of HTLCs and try your algorithm again. As for
+	///    anchor transactions, we will try your coin selection again with the same input-output
+	///    set when you call [`ChannelMonitor::rebroadcast_pending_claims`], as anchor transactions
+	///    cannot be downsized.
+	///
+	/// Implementations must take note that [`Input::satisfaction_weight`] only tracks the weight of
+	/// the input's `script_sig` and `witness`. Some wallets, like Bitcoin Core's, may require
+	/// providing the full input weight. Failing to do so may lead to underestimating fee bumps and
+	/// delaying block inclusion.
+	///
+	/// The `claim_id` must map to the set of external UTXOs assigned to the claim, such that they
+	/// can be re-used within new fee-bumped iterations of the original claiming transaction,
+	/// ensuring that claims don't double spend each other. If a specific `claim_id` has never had a
+	/// transaction associated with it, and all of the available UTXOs have already been assigned to
+	/// other claims, implementations must be willing to double spend their UTXOs. The choice of
+	/// which UTXOs to double spend is left to the implementation, but it must strive to keep the
+	/// set of other claims being double spent to a minimum.
+	///
+	/// [`ChannelMonitor::rebroadcast_pending_claims`]: crate::chain::channelmonitor::ChannelMonitor::rebroadcast_pending_claims
 	fn select_confirmed_utxos(
 		&self, claim_id: ClaimId, must_spend: Vec<Input>, must_pay_to: &[TxOut],
 		target_feerate_sat_per_1000_weight: u32, max_tx_weight: u64,
 	) -> Result<CoinSelection, ()>;
 
-	/// A synchronous version of [`CoinSelectionSource::sign_psbt`].
+	/// Signs and provides the full witness for all inputs within the transaction known to the
+	/// trait (i.e., any provided via [`CoinSelectionSourceSync::select_confirmed_utxos`]).
+	///
+	/// If your wallet does not support signing PSBTs you can call `psbt.extract_tx()` to get the
+	/// unsigned transaction and then sign it with your wallet.
 	fn sign_psbt(&self, psbt: Psbt) -> Result<Transaction, ()>;
 }
 
@@ -188,7 +247,14 @@ where
 	}
 }
 
-/// A synchronous wrapper around [`BumpTransactionEventHandler`] to be used in contexts where async is not available.
+/// A handler for [`Event::BumpTransaction`] events that sources confirmed UTXOs from a
+/// [`CoinSelectionSourceSync`] to fee bump transactions via Child-Pays-For-Parent (CPFP) or
+/// Replace-By-Fee (RBF).
+///
+/// For an asynchronous version of this handler, see [`BumpTransactionEventHandler`].
+///
+/// [`Event::BumpTransaction`]: crate::events::Event::BumpTransaction
+// Note that updates to documentation on this struct should be copied to the synchronous version.
 pub struct BumpTransactionEventHandlerSync<B: Deref, C: Deref, SP: Deref, L: Deref>
 where
 	B::Target: BroadcasterInterface,