routing: don't set custom amount if manager isn't handling #9502

guggero · 2025-02-11T15:30:03Z

Avoids overwriting the HTLC amount if the aux bandwidth manager isn't handling the channel.

coderabbitai · 2025-02-11T15:30:16Z

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ziggie1984 · 2025-02-11T15:33:46Z

routing/bandwidth.go

+			// traffic shaper is not handling the channel. In that
+			// case, we'll just return the original bandwidth and
+			// no custom amount.
+			if auxBandwidth.IsNone() {


I suggest removing the HTLC amount from the bandwidthResult, because if its not a custom payment we just check for the amount which was passed in I think that's what the channel needs to route at a bare minimum anyways, and if its a normal payment we use the real HTLC amount nevertheless.

No, unfortunately we can't do that. If we're trying to pay an invoice over let's say 500k sats, then that amount is passed in as amount. In a channel that only has the capacity of 100k sats, we clearly can't send 500k. And with an asset channel, we're not going to, as we're going to convert to assets and will only really send 354 sats. That's why the bandwidth result overwrites the htlcAmount with 0 (which just means "the minimum amount possible") in case of an asset channel.

What we need to fix here is the behavior for non-asset channels, which is what this commit/PR does.

ahh gottya, so when sending a custom payment the HTLC amount here is not already adjusted for, ok understand.

Correct. This is a first stage to find out what channel(s) are even eligible for carrying the payment. That initial selection, just based on available bandwidth/balance is then used to narrow down the search during the actual pathfinding (no use selecting paths through channels where we know we don't have the required balance).

So the actual conversion of the HTLC into an asset HTLC happens in a next step, after a route has been chosen.
The actual amount that goes on chain is determined here:

lnd/routing/payment_lifecycle.go

Line 773 in 4eea207

result, err := fn.MapOptionZ(

If there are no firstHopCustomRecords, then the initial rt.TotalAmount is returned from the traffic shaper.

GeorgeTsagk

LGTM

should test this against LitD

routing/bandwidth.go

ziggie1984

LGTM

yyforyongyu · 2025-02-11T19:23:07Z

htlcswitch/interfaces.go

+type OptionalBandwidth struct {
+	// IsHandled is true if the external traffic shaper handles the channel.
+	// If this is false, then the bandwidth value is not applicable.
+	IsHandled bool


when does this set to true?

Argh, good catch. Copy/paste mistake. Fixed!
We'll validate all of this with an integration test in litd. Unfortunately we aren't really set up to test these things properly in unit or integration tests with in lnd (but probably worth the investment very soon).

To make it more clear whether the external traffic shaper is handling a channel or not, we return an explicit boolean.

ziggie1984 · 2025-02-12T08:46:59Z

htlcswitch/link.go

-	auxBandwidth.WhenSome(func(bandwidth lnwire.MilliSatoshi) {
-		availableBandwidth = bandwidth
-	})
+	if auxBandwidth.IsHandled && auxBandwidth.Bandwidth.IsSome() {


Do we really need this second arg (auxBandwidth.Bandwidth.IsSome()), if the some is not set we just pass the check below ?

Yeah, this is super-defensive I guess. More for the reader of the code than the compiler. But can remove if you feel it has the opposite effect, reducing clarity.

ahh ok yeah let's keep it then.

lightninglabs-deploy · 2025-02-26T13:56:55Z

@guggero, remember to re-request review from reviewers when ready

guggero · 2025-02-26T14:03:50Z

!lightninglabs-deploy mute

ziggie1984 self-requested a review February 11, 2025 15:30

ziggie1984 reviewed Feb 11, 2025

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GeorgeTsagk approved these changes Feb 11, 2025

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routing/bandwidth.go Outdated Show resolved Hide resolved

guggero force-pushed the bandwidth-manager-fix branch from bdb21bc to 78c8dae Compare February 11, 2025 16:39

ziggie1984 approved these changes Feb 11, 2025

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yyforyongyu reviewed Feb 11, 2025

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guggero added 2 commits February 11, 2025 20:30

htlcswitch+routing: return IsHandled from AuxBandwidth

716db8b

To make it more clear whether the external traffic shaper is handling a channel or not, we return an explicit boolean.

docs: add release notes

e490347

guggero force-pushed the bandwidth-manager-fix branch from 78c8dae to e490347 Compare February 11, 2025 19:31

ziggie1984 reviewed Feb 12, 2025

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routing: don't set custom amount if manager isn't handling #9502

routing: don't set custom amount if manager isn't handling #9502

guggero commented Feb 11, 2025

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guggero commented Feb 26, 2025

routing: don't set custom amount if manager isn't handling #9502

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routing: don't set custom amount if manager isn't handling #9502

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