htlcswitch: fix error misclassification for on-chain resolutions

[calvinrzachman]

Change Description

This PR demonstrates a subtle bug in the htlcswitch that causes an incorrect error to be returned for on-chain HTLC resolutions. On the introduction of the switchrpc service in #9489, this bug can be escalated, leading to an ambiguous scenario where the Switch reports a payment result which has no forwarding error, pre-image, or encrypted error for the RPC client.

Problem

When the contractcourt resolves a dust HTLC on-chain, it sends a ResolutionMsg to the switch that results in an UpdateFailHTLC with a nil error Reason.

The core issue stems from a a subtle artifact in the networkResult store's persistence logic:

1. A nil htlc.Reason slice is serialized to disk as a zero-length byte array.
2. Upon deserialization, this is read back as a non-nil, empty byte slice ([]byte{}).

This nil -> []byte{} transformation causes the check for on-chain resolutions to fail. As a result, this code path, which is intended to return FailPermanentChannelFailure to signal the channel is being resolved on-chain, is never executed for results read from disk.

Instead, the code falls through to the default case. The Switch attempts to decrypt the empty slice, which fails, causing parseFailedPayment to return ErrUnreadableFailureMessage.

The impact on master is a misclassification of the error. The ChannelRouter receives a generic, imprecise error instead of the correct FailPermanentChannelFailure, hindering its ability to accurately understand the payment's terminal state.

Solution

The fix is to make the check in parseFailedPayment robust to the serialization artifact by checking the length of the reason instead of whether it's nil.

--- a/htlcswitch/switch.go
+++ b/htlcswitch/switch.go
-       case isResolution && htlc.Reason == nil:
+       case isResolution && len(htlc.Reason) == 0:

This change ensures that on-chain resolutions are correctly identified and processed, whether the Reason is nil (in-memory) or an empty slice (post-deserialization). This makes the intended code path reachable and allows the switch to return the precise error to the ChannelRouter.

Steps to Test

• Run this to demo the nil -> []byte{} transformation during a disk round-trip:
  • go test -v -timeout 30s --tags dev -run ^TestNetworkResultSerialization$ github.com/lightningnetwork/lnd/htlcswitch
• Then run this more realistic TestExtractResult, which simulates the disk I/O, both with and without the final commit which provides the fix:
  • go test -v -timeout 30s -run ^TestExtractResult$ github.com/lightningnetwork/lnd/htlcswitch

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

[gemini-code-assist]

Summary of Changes

Hello @calvinrzachman, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical bug within the htlcswitch component that caused on-chain HTLC resolutions to be miscategorized. The problem arose from how nil error reasons were transformed into empty byte slices during disk serialization and deserialization, preventing the system from correctly identifying these resolutions. By adjusting the logic to check for an empty reason length instead of a nil value, the fix ensures accurate error reporting, particularly for FailPermanentChannelFailure, which is crucial for the ChannelRouter to understand payment outcomes and for RPC clients to receive precise forwarding errors.

Highlights

• Bug Fix for On-Chain HTLC Resolutions: Corrects an issue where on-chain HTLC resolutions were misclassified due to a nil error reason transforming into an empty byte slice during serialization, leading to imprecise error reporting.
• Robust Error Identification: The condition for identifying on-chain resolutions in htlcswitch/switch.go was updated from checking for a nil htlc.Reason to checking if len(htlc.Reason) is 0, making it resilient to the serialization behavior.
• Enhanced Testing: New test cases and a dedicated TestExtractResult in htlcswitch/switch_test.go have been added to validate the fix and ensure correct processing of payment results under various conditions, including simulating disk persistence.
• Mock Error Decryptor: A mockErrorDecryptor was introduced in htlcswitch/mock.go to facilitate comprehensive testing of error decryption scenarios.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a subtle bug in htlcswitch that led to misclassifying on-chain HTLC resolution errors. The fix of checking len(htlc.Reason) == 0 instead of htlc.Reason == nil is robust and handles the serialization artifact well. The addition of the TestExtractResult unit test is excellent, as it thoroughly covers various scenarios and prevents regressions. I have a couple of minor suggestions to improve comment clarity and fix a typo in the new test code.

[ziggie1984]

so this test fails with the old nil check ?

[calvinrzachman]

Yea, it fails since we fall into the default switch case within parseFailedPayment which attempts to perform error decryption.

[ziggie1984]

not super high prio because the local router still flags it as a failed payment but I think we can add the fix if you polish this PR.

[calvinrzachman]

Basically, this should improve the functioning of parseFailedPayment such that it can handle this aspect of its godoc comment as it seems was intended.

//  2. A resolution from the chain arbitrator, which possibly has no failure
//     reason attached.