multi: support MPP and AMP records in QueryRoutes, BuildRoute and Sen…

[murraystewart96]

…dToRoute

Closes #9952

• Add payment_addr field to QueryRoutesRequest to allow callers to include the invoice payment secret. The MPP record is injected into the final hop of the returned route.
• Add amp_record field to QueryRoutesRequest and BuildRouteRequest.
• Enforce that SendToRouteV2 and BuildRoute require a payment_addr, as payment_secret is now mandatory per the BOLT 11 spec.
• Add AMP record support to BuildRoute, threading it through finalHopParams into newRoute with a feature check against the destination node.

Change Description

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Pull Request Checklist

Testing

• Your PR passes all CI checks.
• [ x] Tests covering the positive and negative (error paths) are included.
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[gemini-code-assist]

Summary of Changes

Hello, 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 introduces support for Multi-Path Payment (MPP) and Atomic Multi-Path (AMP) records within the routing layer. By updating the RPC definitions and internal route-building logic, the changes allow callers to provide necessary payment secrets and AMP records, which are then correctly injected into the final hop of a route. These updates ensure the system remains compliant with current BOLT 11 standards while providing robust validation for new payment types.

Highlights

• RPC Interface Updates: Added payment_addr and amp_record fields to QueryRoutesRequest and BuildRouteRequest to support advanced payment features.
• Validation and Enforcement: Enforced mandatory payment_addr in SendToRouteV2 and BuildRoute to ensure compliance with BOLT 11 specifications.
• AMP Record Integration: Implemented logic to thread AMP records through the route-building process and inject them into the final hop of a payment route.
• Testing and Verification: Added comprehensive test coverage for both positive and negative paths regarding MPP and AMP record injection.

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[gemini-code-assist]

Code Review

This pull request adds support for Multi-Path Payments (MPP) and Atomic Multi-Path (AMP) records across the routing and RPC layers by introducing payment_addr and amp_record fields to QueryRoutesRequest and BuildRouteRequest. These records are now injected into the final hop of generated routes, and SendToRouteV2 has been updated to enforce mandatory MPP records. Review feedback highlights a critical logic error in BuildRoute that prevents AMP record processing, identifies missing length validations for payment_addr which could lead to panics, and notes a style guide violation in unit tests where manual error checks should be replaced with the require library.

[github-actions]

PR Severity: HIGH. Files: lnrpc/lightning.proto, lnrpc/lightning.swagger.json, lnrpc/routerrpc/router.proto, lnrpc/routerrpc/router.swagger.json, lnrpc/routerrpc/router_backend.go, lnrpc/routerrpc/router_server.go (all HIGH via lnrpc/), routing/pathfind.go, routing/router.go (HIGH via routing/). 9 non-excluded files, ~143 non-excluded lines - no bump. <!-- pr-severity-bot -->