Heuristics in isLSP

[btochi]

When estimating route fees through probing, the isLSP function is called to decided if the destination node is being serviced by a Lightning Service Provider. In such case, the probe is performed to the LSP.

The documentation of such function reads,

// isLSP checks if the route hints indicate an LSP. An LSP is indicated with
// true if the destination hop hint in each route hint has the same node id,
// false otherwise....

Muun Wallet invoices currently show 2 route hints with different node ids. Lightspark seems to have had a similar issue as reported in a comment. Evidently, several LSP use multiple public nodes to route payments to their users. Thus, the requirement that the last node id of each route hint is the same is too demanding. I strongly suggest not returning False in this case.

(since this was the intended behaviour of this function, I'm opening this as a discussion and not a bug report)

[btochi]

@saubyk

[saubyk]

cc: @hieblmi for analysis

The solution can also include the one described in #9994
So basically one pr which addresses the case of:

• two routing hints in an invoice
• the routing hint pointing to a non-existent nodes on the graph

The isLSP function should return true in the first case and false in the second.

Can target a solution with release v0.21 or a minor release if pr is reviewed and merged in time.