Support HTLC interception by source channel by TheBlueMatt · Pull Request #4338 · lightningdevkit/rust-lightning

TheBlueMatt · 2026-01-23T02:21:36Z

Just a few more flags and a small cleanup.

Based on #4300

ldk-reviews-bot · 2026-01-23T02:21:39Z

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ldk-reviews-bot · 2026-01-26T00:00:23Z

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tnull

Please re-request review once #4300 lands and this is rebased.

TheBlueMatt · 2026-01-28T14:11:51Z

Rebased.

tnull

Seems CI is very unhappy as this doesn't build currently.

ldk-reviews-bot · 2026-01-29T09:34:51Z

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We jump through some hoops in order to pass a small list of objects to `forward_htlcs` on a per-channel basis rather than per-HTLC. Then, `forward_htlcs` builds a `PendingAddHTLCInfo` for each HTLC for insertion. Worse, in some `forward_htlcs` callsites we're actually starting with a `PendingAddHTLCInfo`, converting it to a tuple, then back inside `forward_htlcs`. Instead, here we just pass a list of built `PendingAddHTLCInfo`s to `forward_htlcs`, cleaning up a good bit of code and even avoiding an allocation of the HTLCs vec in many cases.

It may be useful in some situations to select HTLCs for interception based on the source channel in addition to the sink. Here we add the ability to do so by adding new flags to `HTLCInterceptionFlags`.

TheBlueMatt · 2026-01-29T15:03:23Z

Oops, silent conflict.

codecov · 2026-01-29T17:57:58Z

Codecov Report

❌ Patch coverage is 87.09677% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.11%. Comparing base (7fe3268) to head (0bf2200).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	87.09%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           main    #4338       +/-   ##
=========================================
+ Coverage      0   86.11%   +86.11%     
=========================================
  Files         0      156      +156     
  Lines         0   102606   +102606     
  Branches      0   102606   +102606     
=========================================
+ Hits          0    88355    +88355     
- Misses        0    11762    +11762     
- Partials      0     2489     +2489

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tests	`86.11% <87.09%> (?)`

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ldk-reviews-bot · 2026-01-31T15:04:02Z

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ldk-reviews-bot · 2026-02-02T15:04:55Z

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elnosh

rustfmt is unhappy but code changes look good to me

elnosh · 2026-02-02T17:27:38Z

lightning/src/ln/channelmanager.rs

-						prev_user_channel_id,
-						forward_info,
-					};
+	fn forward_htlcs<I: IntoIterator<Item = PendingAddHTLCInfo>>(&self, pending_forwards: I) {


now that this method takes PendingAddHTLCInfo, I think this comment: https://github.com/TheBlueMatt/rust-lightning/blob/77d3659d8af798c11750cb5826e45d29691dadd9/lightning/src/ln/channelmanager.rs#L455 can be removed

elnosh · 2026-02-02T17:29:33Z

lightning/src/ln/channelmanager.rs

+						chan.context.get_counterparty_node_id(),
+						chan.context.channel_id(),
+						chan.funding.get_funding_txo().unwrap(),
+						chan.context.get_user_id(),
+						chan.context.config().accept_underpaying_htlcs,
+						chan.context.should_announce(),


nit: this feels slightly more readable

let ( incoming_counterparty_node_id, incoming_channel_id, incoming_funding_txo, incoming_user_channel_id, incoming_accept_underpaying_htlcs, incoming_chan_is_public, ) = match self.do_funded_channel_callback( incoming_scid_alias, |chan: &mut FundedChannel<SP>| { ( chan.context.get_counterparty_node_id(), chan.context.channel_id(), chan.funding.get_funding_txo().unwrap(), chan.context.get_user_id(), chan.context.config().accept_underpaying_htlcs, chan.context.should_announce(), ) }, ) { Some(incoming_channel_details) => incoming_channel_details, None => continue, };

elnosh · 2026-02-02T17:30:21Z

lightning/src/util/config.rs

+	/// public channel will instead generate an [`Event::HTLCIntercepted`] which must be handled
+	/// the same as any other intercepted HTLC.
+	///
+	/// This primarily exists for completeness, and generally interception of of HTLCs between


elnosh · 2026-02-02T17:32:05Z

lightning/src/ln/channelmanager.rs

+					forward_info,
+					prev_outbound_scid_alias: outbound_scid_alias,
+					prev_htlc_id,
+					prev_counterparty_node_id: channel.context.get_counterparty_node_id(),


can use counterparty_node_id from above

ldk-reviews-bot · 2026-02-04T15:05:59Z

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TheBlueMatt requested a review from tnull January 23, 2026 02:21

tnull removed their request for review January 27, 2026 10:22

tnull reviewed Jan 27, 2026

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TheBlueMatt force-pushed the 2026-01-intercept-by-source-v2 branch from b793c1e to 79952e1 Compare January 28, 2026 14:11

TheBlueMatt requested a review from tnull January 28, 2026 14:11

TheBlueMatt force-pushed the 2026-01-intercept-by-source-v2 branch from 79952e1 to 779fce3 Compare January 28, 2026 14:14

tnull reviewed Jan 29, 2026

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TheBlueMatt added 2 commits January 29, 2026 15:03

Allow intercepting HTLCs based on the source channel

0bf2200

It may be useful in some situations to select HTLCs for interception based on the source channel in addition to the sink. Here we add the ability to do so by adding new flags to `HTLCInterceptionFlags`.

TheBlueMatt force-pushed the 2026-01-intercept-by-source-v2 branch from 779fce3 to 0bf2200 Compare January 29, 2026 15:03

TheBlueMatt requested a review from tnull January 29, 2026 15:03

elnosh reviewed Feb 2, 2026

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