Add OutputSweeper utility persisting and sweeping spendable outputs

We add a `OutputSweeper` utility that allows to track the state of
spendable output descriptors as emitted by `Event::SpendableOutputs`.

To this end, the `OutputSweeper` persists the necessary information in
our `KVStore` and regularly tries to sweep the spendable outputs,
removing them after reaching threshold confirmations, i.e.,
`ANTI_REORG_DELAY`.

Author: tnull

lightning/src/chain/chaininterface.rs

@@ -124,7 +124,16 @@ pub enum ConfirmationTarget {
 	///
 	/// [`ChannelManager::close_channel_with_feerate_and_script`]: crate::ln::channelmanager::ChannelManager::close_channel_with_feerate_and_script
 	ChannelCloseMinimum,
-	/// XXX
+	/// The feerate [`OutputSweeper`] will use on transactions spending
+	/// [`SpendableOutputDescriptor`]s after a channel closure.
+	///
+	/// Generally spending these outputs is safe as long as they eventually confirm, so a value
+	/// (slightly above) the mempool minimum should suffice. However, as this value will influence
+	/// how long funds will be unavailable after channel closure, [`FeeEstimator`] implementors
+	/// might want to choose a higher feerate to regain control over funds faster.
+	///
+	/// [`OutputSweeper`]: crate::util::sweep::OutputSweeper
+	/// [`SpendableOutputDescriptor`]: crate::sign::SpendableOutputDescriptor
 	OutputSpendingFee,
 }
 

lightning/src/chain/mod.rs

@@ -20,6 +20,7 @@ use crate::chain::channelmonitor::{ChannelMonitor, ChannelMonitorUpdate, Monitor
 use crate::ln::ChannelId;
 use crate::sign::ecdsa::WriteableEcdsaChannelSigner;
 use crate::chain::transaction::{OutPoint, TransactionData};
+use crate::impl_writeable_tlv_based;
 
 #[allow(unused_imports)]
 use crate::prelude::*;
@@ -56,6 +57,11 @@ impl BestBlock {
 	}
 }
 
+impl_writeable_tlv_based!(BestBlock, {
+	(0, block_hash, required),
+	(2, height, required),
+});
+
 
 /// The `Listen` trait is used to notify when blocks have been connected or disconnected from the
 /// chain.

lightning/src/util/mod.rs

@@ -22,6 +22,7 @@ pub mod invoice;
 pub mod persist;
 pub mod scid_utils;
 pub mod string;
+pub mod sweep;
 pub mod wakers;
 #[cfg(fuzzing)]
 pub mod base32;

lightning/src/util/persist.rs

@@ -70,6 +70,20 @@ pub const SCORER_PERSISTENCE_SECONDARY_NAMESPACE: &str = "";
 /// The key under which the [`WriteableScore`] will be persisted.
 pub const SCORER_PERSISTENCE_KEY: &str = "scorer";
 
+/// The primary namespace under which [`OutputSweeper`] state will be persisted.
+///
+/// [`OutputSweeper`]: crate::util::sweep::OutputSweeper
+pub const OUTPUT_SWEEPER_PERSISTENCE_PRIMARY_NAMESPACE: &str = "";
+/// The secondary namespace under which [`OutputSweeper`] state will be persisted.
+///
+/// [`OutputSweeper`]: crate::util::sweep::OutputSweeper
+pub const OUTPUT_SWEEPER_PERSISTENCE_SECONDARY_NAMESPACE: &str = "";
+/// The secondary namespace under which [`OutputSweeper`] state will be persisted.
+/// The key under which [`OutputSweeper`] state will be persisted.
+///
+/// [`OutputSweeper`]: crate::util::sweep::OutputSweeper
+pub const OUTPUT_SWEEPER_PERSISTENCE_KEY: &str = "output_sweeper";
+
 /// A sentinel value to be prepended to monitors persisted by the [`MonitorUpdatingPersister`].
 ///
 /// This serves to prevent someone from accidentally loading such monitors (which may need