Implementation Tracking: Recurrence Support for BOLT12 Offers

[shaavan]

This issue tracks the implementation progress for Recurrence.

Spec PR: lightning/bolts#1240
LDK design discussion: #4476

Summary of Design Discussion

Based on the design discussion, Recurrence support will be split across rust-lightning and LDK Node.

rust-lightning will handle the protocol-facing pieces: wire data, a minimal API surface, and the handling logic that does not require access to saved recurrence state.

LDK Node will own the public-facing APIs and outbound recurrence state tracking.

Implementation Plan

rust-lightning

• Introduce recurrence wire fields for BOLT12 messages:
  • Offers
  • Invoice Requests
  • Invoices
• Introduce builder support for creating an InvoiceRequest with an explicit signing pubkey
• Add handler logic:
  • Payee-side InvoiceRequest handling using the recurrence token
  • Payer-side Invoice handling using the BlindedPath MessageContext

LDK Node

• Set up outbound recurrence state tracking, update, and expiry logic
• Introduce recurrence offer builder
• Introduce APIs to pay, re-pay, and cancel recurrence for recurring offers

Spec

• Run interoperability tests
• Review and merge spec