[router] Use the invoice value for last-hop hint channel capacity

[TheBlueMatt]

If an invoice contains route hints, we should assume the channel
capacity is sufficient to route the invoice's value.

[codecov]

Codecov Report

Merging #986 (9169dfb) into main (84967fa) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #986   +/-   ##
=======================================
  Coverage   90.73%   90.73%           
=======================================
  Files          60       60           
  Lines       30650    30650           
=======================================
  Hits        27811    27811           
  Misses       2839     2839           

Impacted Files	Coverage Δ
lightning/src/routing/router.rs	96.06% <100.00%> (ø)

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[naumenkogs]

If an invoice contains route hints, we should assume the channel capacity is sufficient to route the invoice's value.

It seems like this patch changes nothing at all (at least for now)?

---

We can also add asserts e.g. htlc_maximum_msat >= final_value_msat.

[TheBlueMatt]

It seems like this patch changes nothing at all (at least for now)?

No - if you go to send a payment from an invoice that has route hints on it, we'll currently assume that the last-hop channel can only route 0.0025 BTC, even if we're trying to pay more than that. This causes us to fail to find a route, since we assume that each last-hop channel can only route 0.0025 BTC. Instead, here, we assume that each last-hop can handle the full value of the payment, which is maybe also not true, but better than just entirely failing.

[naumenkogs]

ACK 9169dfb

[ariard]

ACK 9169dfb