Sweeper async change destination source fetching #3734

joostjager · 2025-04-14T11:30:11Z

This PR converts OutputSweeper to take an async ChangeDestinationSource implementation. This allows a (remote) address fetch call to run without blocking chain notifications.

Furthermore the changes demonstrates how LDK could be written in a natively async way, and still allow usage from a sync context using wrappers.

Part of #3540

ldk-reviews-bot · 2025-04-14T11:30:15Z

👋 Thanks for assigning @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

lightning/src/sign/mod.rs

joostjager · 2025-04-14T11:32:13Z

lightning/src/util/sweep.rs

 		}
-
-		Ok(())
+		self.persist_state(&*state_lock).map_err(|e| {


No more sweeping in this method, all moved to a timer.

joostjager · 2025-04-14T11:32:31Z

lightning/src/util/sweep.rs

-		if let Some(spending_tx) = spending_tx_opt {
-			self.broadcaster.broadcast_transactions(&[&spending_tx]);
-		}
+		let _ = self.persist_state(&*state_lock).map_err(|e| {


No more sweeping in this event handler.

lightning/src/util/sweep.rs

lightning-background-processor/src/lib.rs

joostjager · 2025-04-14T19:11:20Z

lightning/src/util/sweep.rs

-		if respend_descriptors.is_empty() {
-			// Nothing to do.
-			return None;
+			let change_destination_script = change_destination_script_result?;


Todo: address risk of getting a tx with a new address every block.

Todo: investigate what BDK does here?

Traced through BDK a bit. It seems that there is only inflation if we actually use the address in a tx. It won't blindly regenerate addresses when called. But will double check this.

It seems that there is only inflation if we actually use the address in a tx. It won't blindly regenerate addresses when called. But will double check this.

This mostly depends on the API used.

lightning-background-processor/src/lib.rs

lightning/src/util/sweep.rs

lightning/src/lib.rs

lightning/src/util/sweep.rs

joostjager · 2025-04-16T16:33:56Z

lightning/src/util/sweep.rs

+	sweeper: OutputSweeper<B, D, E, F, K, L, O>,
+}
+
+impl<B: Deref, D: Deref, E: Deref, F: Deref, K: Deref, L: Deref, O: Deref>


What might be good about this wrapper is that it eliminates the possibility of someone implementing async logic in combination with future poll/ready checking. This wrapper only accepts a sync trait.

lightning/src/util/sweep.rs

codecov · 2025-04-21T09:07:47Z

Codecov Report

Attention: Patch coverage is 81.65939% with 42 lines in your changes missing coverage. Please review.

Project coverage is 90.61%. Comparing base (f507778) to head (1a98158).
Report is 68 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/util/sweep.rs	77.16%	25 Missing and 12 partials ⚠️
lightning/src/util/async_poll.rs	57.14%	3 Missing ⚠️
lightning-background-processor/src/lib.rs	96.22%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3734      +/-   ##
==========================================
+ Coverage   89.13%   90.61%   +1.47%     
==========================================
  Files         157      157              
  Lines      123851   134826   +10975     
  Branches   123851   134826   +10975     
==========================================
+ Hits       110395   122170   +11775     
+ Misses      10779    10016     -763     
+ Partials     2677     2640      -37

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ldk-reviews-bot · 2025-05-03T12:33:22Z

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ldk-reviews-bot · 2025-05-05T12:33:45Z

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ldk-reviews-bot · 2025-05-07T12:34:08Z

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ldk-reviews-bot · 2025-05-10T00:00:41Z

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ldk-reviews-bot · 2025-05-12T00:01:12Z

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tnull

Excuse the delay here!

Mostly looks good from my side I think, just a few minor comments/nits. Will to another final thorough pass after.

lightning/src/util/sweep.rs

tnull · 2025-05-12T13:49:35Z

lightning/src/util/sweep.rs


+use crate::sync::Arc;


nit: Let's move this to the other crate imports

It's cleaner, but also I have to say I never look at these imports. But may be different for others. Moved.

tnull · 2025-05-12T13:52:36Z

lightning/src/util/sweep.rs

@@ -567,16 +623,15 @@ where
 	}

 	fn spend_outputs(
-		&self, sweeper_state: &SweeperState, descriptors: Vec<&SpendableOutputDescriptor>,
+		&self, sweeper_state: &SweeperState, descriptors: &Vec<&SpendableOutputDescriptor>,


nit: Mhh, &Vec<&..> looks funny. I guess the signature from spend_spendable_outputs would be preferable: &[&SpendableOutputDescriptor].

Out of curiosity, what is funny about it? Or how is &[&..] less funny? Fixed.

Usually we try to create the least-strict API if there is no reason to. In this case, the prior API would enforce a reference to an allocated Vec, while, if we just require the slice in the API, we might (easier) be able to drop the allocation if it's not necessary.

tnull

LGTM, mod one question and a few final nits.

lightning/src/util/sweep.rs

tnull · 2025-05-13T08:20:27Z

lightning/src/util/sweep.rs

+				},
+				Err(e) => {
+					log_error!(self.logger, "Error spending outputs: {:?}", e);
+					return Ok(());


Why are we ignoring the error here instead of propagating it up? (If we'd want to ignore it for some reason, can we just have spend_outputs not return a Result and move this match / logging into it?)

I think it was pre-existing to not do anything with that error other than log:

rust-lightning/lightning/src/util/sweep.rs

Line 507 in 5316257

Err(e) => {

I am not sure if the caller can do something more with the error. I do see that there non-happy flow error cases in spend_spendable_outputs, so might be better to at least give the users the choice. Added map_err.

lightning/src/util/sweep.rs

tnull

LGTM, feel free to squash.

One outstanding nit, non-blocking though. If you want, feel free to address while squashing also.

lightning/src/util/sweep.rs

TheBlueMatt

A new comments but they can all happen in followups too.

TheBlueMatt · 2025-05-13T13:39:25Z

lightning-background-processor/src/lib.rs

@@ -677,14 +703,18 @@ use futures_util::{dummy_waker, OptionalSelector, Selector, SelectorOutput};
 /// #     persister: Arc<Store>,
 /// #     logger: Arc<Logger>,
 /// #     scorer: Arc<Scorer>,
+/// # 	  sweeper: Arc<OutputSweeper<Arc<B>, Arc<D>, Arc<FE>, Arc<F>, Arc<K>, Arc<Logger>, Arc<O>>>,


Here and above at least on the D line, you have inconsistent spaces/tabs with the existing code.

Unfortunate that github doesn't show it. Convert all tabs in this block to spaces, so that should have caught them all.

Ironically I can see it in github's diff snippet that they included in the emails about these comments!

lightning-background-processor/src/lib.rs

TheBlueMatt · 2025-05-13T14:25:07Z

lightning/src/util/sweep.rs

+				},
+			};
+
+			if state_lock.outputs.iter().find(|o| o.descriptor == output_info.descriptor).is_some()


For move-only/indentation-change changes, please do the move or indentation change and then rustfmt in a separate commit.

This isn't great indeed. Also seems an unfortunate diff by github, because in vscode it detects a lot more whitespace only line changes.

-b does somewhat better, which I imagine is what vscode is doing. But git show --color-moved would be much happier if the formatting were a separate commit.

Suppressing rustfmt in a commit for diff optimization. Wasn't prepared for that 😅

TheBlueMatt · 2025-05-13T14:35:31Z

lightning/src/util/sweep.rs

-		let cur_height = sweeper_state.best_block.height;
-		let cur_hash = sweeper_state.best_block.block_hash;
-		let filter_fn = |o: &TrackedSpendableOutput| {
+	/// Regenerates and broadcasts the spending transaction for any outputs that are pending. This method will be a


Would be nice to explain the return value (specifically that it will return an Err only due to failures calling user code - failures persisting or failures signing a spending transaction).

TheBlueMatt · 2025-05-13T14:41:22Z

lightning/src/util/sweep.rs

+		{
+			let mut sweeper_state = self.sweeper_state.lock().unwrap();
+
+			let change_destination_script = change_destination_script_result?;


Not sure I buy that we need to do ? in the state lock :)

Left over from when I didn't have the atomic bool, good catch.

To prepare for asynchronous processing of the sweep, we need to decouple the spending from the chain notifications. These notifications run in a sync context and wouldn't allow calls into an async trait. Instead we now periodically call into the sweeper, to open up the possibility to do so from an async context if desired.

synchronous wrappers for usage in a sync context.

joostjager · 2025-05-13T19:50:16Z

@TheBlueMatt addressed comments. Didn't do fixup commits, the github compare button shows the changes.

joostjager commented Apr 14, 2025

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TheBlueMatt mentioned this pull request May 13, 2025

Move from bounds on low-level objects to BP Send bounds #3776

Merged

joostjager added 3 commits May 13, 2025 21:47

Prepare sweeper filter fn for reuse

42d3fb5

Convert sweeper to use an async ChangeDestinationSource and provide

1a98158

synchronous wrappers for usage in a sync context.

joostjager dismissed stale reviews from TheBlueMatt and tnull via 1a98158 May 13, 2025 19:48

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joostjager requested review from TheBlueMatt and tnull May 13, 2025 19:49

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tnull merged commit 361ff01 into lightningdevkit:main May 14, 2025
27 of 28 checks passed

joostjager mentioned this pull request May 14, 2025

Drop background processor futures feature #3777

Merged

Sweeper async change destination source fetching #3734

Sweeper async change destination source fetching #3734

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