routerrpc: FailureDetail enums for invoice/AMP validation failures

[darioAnongba]

Extend routerrpc.FailureDetail with specific values for invoice/AMP validation outcomes and map those results in SubscribeHtlcEvents, so HTLC event subscribers receive stable, explicit failure details instead of UNKNOWN.

Motivation

HTLC interceptor subscribers were dropped when an invoice failure resolution (e.g., external validation failed) had no mapping. This change preserves stability while exposing accurate failure causes.

[gemini-code-assist]

Summary of Changes

Hello @darioAnongba, 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 enhances the router RPC by introducing a more detailed set of failure reasons for invoice and AMP validation processes. By expanding the FailureDetail enum and updating the mapping logic within SubscribeHtlcEvents, the system can now communicate precise failure outcomes. This improvement ensures that applications monitoring HTLC events receive clear, actionable information, thereby preventing scenarios where events might be dropped due to generic or unhandled failure types.

Highlights

• Extended FailureDetail Enum: New specific enum values have been added to routerrpc.FailureDetail to represent various invoice and AMP validation failures, providing more granular error reporting.
• Improved HTLC Event Mapping: The SubscribeHtlcEvents function now maps these new failure resolution results, ensuring that HTLC event subscribers receive explicit and stable failure details instead of generic UNKNOWN errors.
• Enhanced System Stability: This change prevents HTLC interceptor subscribers from being dropped due to previously unmapped invoice failure resolutions, thereby improving the overall stability and reliability of the system.

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[gemini-code-assist]

Code Review

This pull request extends the routerrpc.FailureDetail enum with new values to provide more specific failure reasons related to invoice and AMP validation. The changes correctly propagate these new failure details from the invoices package to the routerrpc package, improving the clarity and stability of HTLC event subscriptions. The changes are well-aligned with the stated motivation of exposing accurate failure causes.

[ziggie1984]

Makes sense to have a complete mapping

LGTM

[GeorgeTsagk]

Should drop a release note as it does affect the behavior of the RPC interface

[ziggie1984]

@gemini-code-assist can you correct this release note, it is not the format we use in LND

[gemini-code-assist]

You are correct, ziggie1984. The current release note entry does not fully adhere to the standard LND release note format. Typically, entries under the "RPC Updates" section are more concise and include the pull request link in parentheses.

Based on the observed format in other LND release notes (e.g., release-notes-0.20.0.md), a more appropriate entry would be:

* HTLC event subscribers now receive specific failure details for invoice/AMP validation outcomes. ([#10520](https://github.com/lightningnetwork/lnd/pull/10520))

I am unable to provide this as a direct code suggestion in the UI because the comment is anchored to lines 95-97 (the ## RPC Updates header), while the new release note entry is on line 98. The suggestion mechanism requires the suggested code to replace the exact lines the comment applies to. Please apply this change manually to line 98.

[ziggie1984]

@gemini-code-assist it should be like this:

Update the complete rpc failure mapping for invoice failure resolutions