Offers compat fixes

[rustyrussell]

This addresses all but one (sending to a blinded path which starts at ourselves) of the issues reported by @t-bast in compat testing with Eclair in #5823

Unfortunately, it breaks compatibility again :( But omelette, eggs and engineering time...

[t-bast]

Neat, I'll do another round of compat' testing based on that branch soon!

[t-bast]

You mentioned that cln would create blinded hops when its local channels are private, but that doesn't seem to work.

With a basic setup eclair -> cln and a single private channel between the two nodes, cln still doesn't create a blinded hop and doesn't create a dummy blinded hop either, it only provides a payload for itself:

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

{
   "type": "bolt12 invoice",
   "offer_id": "78362da27528af131bb63c9af89c544d1b94206040de3a87bf1f05b43b4c2d72",
   "offer_chains": [
      "06226e46111a0b59caaf126043eb5bbf28c34f3a5e332a1fc7b2b73cf188910f"
   ],
   "offer_description": "yolo",
   "offer_node_id": "038d002917b4fd6ef1eb9921c6ebbe2a8c5f4e68d3bd8d25aecc41a2cc30fd5cc6",
   "invreq_metadata": "289c7e54f247254f5b7b1df9620acfae",
   "invreq_payer_id": "03f948f3207c564c4e6e9b183a7dea96b86b35dbde6c43d3b1a9c39f69172d1389",
   "invreq_chain": "06226e46111a0b59caaf126043eb5bbf28c34f3a5e332a1fc7b2b73cf188910f",
   "invreq_amount_msat": "10000000msat",
   "invoice_paths": [
      {
         "first_node_id": "038d002917b4fd6ef1eb9921c6ebbe2a8c5f4e68d3bd8d25aecc41a2cc30fd5cc6",
         "blinding": "02287e43bb71703bfba0b36b35873190463382dfd46c6586ffdc641e054b197a2e",
         "payinfo": {
            "fee_base_msat": "0msat",
            "fee_proportional_millionths": 0,
            "cltv_expiry_delta": 10,
            "features": ""
         },
         "path": [
            {
               "blinded_node_id": "03ef431ec7dc8b5c2984ec6b5b398723e4337bc0474f82a0885eff2b860e15fe2e",
               "encrypted_recipient_data": "537590574a4491a157dc340cc5cce9a3f307f425bbf7ef29a3e465afd0f3d619eb68fe237154af0419560a4b2c5ab0c01e3a"
            }
         ]
      }
   ],
   "invoice_created_at": 1674036434,
   "invoice_relative_expiry": 7200,
   "invoice_payment_hash": "76916a57b17a43bda34d91702189a1857d1c5e706d89c3d6504b5e330aec7c04",
   "invoice_amount_msat": "10000000msat",
   "invoice_node_id": "038d002917b4fd6ef1eb9921c6ebbe2a8c5f4e68d3bd8d25aecc41a2cc30fd5cc6",
   "signature": "460c84db5b9a712b56e4a4e9e201c5e32ba2fbf09865981205dc0319c2039f1564602b0ff0f8f8e20522be42d386ecfc3c75241e0c9422233123b31dd2582c24",
   "valid": true
}

There are some test cases I'm thus unable to run, is there an API argument I'm missing that would unblock this?

Another issue I found is that cln doesn't seem to support splitting an outgoing payment across blinded paths?
I'm creating this graph:

                     100 000 sat   +-----+   25 000 sat
            +----------------------| Bob |-----------------------+
            |                      +-----+                       |
 50 000 sat |                                                    | 125 000 sat
            |                                                    |
        +-------+                                             +------+
        | Alice |                                             | Dave |
        +-------+                                             +------+
            |                                                    |
 50 000 sat |                                                    | 125 000 sat
            |                     +-------+                      |
            +---------------------| Carol |----------------------+
                    100 000 sat   +-------+   25 000 sat

Alice then creates a bolt 12 invoice for 150 000 sats with two blinded paths:

• Bob -> Alice with capacity 100 000 sats
• Carol -> Alice with capacity 100 000 sats

Dave immediately fails paying it with "no path found"...

Apart from that, the issues highlighted in #5823 seem to be fixed 🎉

[rustyrussell]

Ah, this gets overridden by the fact that CLN knows the Eclair node is a dead-end, so won't create a hop (this shares the logic we use for route hints, where it makes more sense). If there's another node attached to the Eclair node, it will work! (with a public channel, obv)

[carlaKC]

Ah, this gets overridden by the fact that CLN knows the Eclair node is a dead-end, so won't create a hop (this shares the logic we use for route hints, where it makes more sense). If there's another node attached to the Eclair node, it will work! (with a public channel, obv)

Running into a different issue trying to fetch an offer invoice with a blinded route on 12761c3:
Alice --- Bob --- Carol --(private)---Dave

lightning-cli-dave offer 1000 test
lightnig-cli-alice fetchinvoice lno...

Dave has dev-fast-gossip enabled, and listchannels has an up to date network view.
I can retrieve the invoice but it doesn't have a blinded path:

2023-02-01T14:18:45.096Z **BROKEN** plugin-offers: Could not parse listincoming: Parsing '{id:%,incoming_capacity_msat:%,htlc_min_msat:%,htlc_max_msat:%,fee_base_msat:%': json_to_u32 could not parse \"1000msat\"
2023-02-01T14:18:45.096Z UNUSUAL plugin-offers: No incoming channel for 1000msat, so no blinded path