Remove pathfinding "second chance" logic

[joostjager]

When a routing node has to fail a payment because the fee is insufficient, this may be caused by the sender not having the latest channel policy. The routing node may have increased its fees recently. If this failure happens, an up to date channel update is returned to the sender and they can choose to retry.

See also the explanation in code here:

lnd/routing/missioncontrol.go

Line 24 in 9d04b0c

// If nodes return a channel policy related failure, they may get a

The goal of this issue to discuss how desirable lnd's so called "second chance" logic still is.

Advantages:

• After senders apply the updated channel policy, the previously chosen route may still be the best option. If they'd immediately penalize the node for returning fee_insufficient, that best route won't be chosen for the next attempt.

• Routing nodes can adjust their fees upwards without losing traffic during the window where senders haven't received the update yet.

Disadvantages:

• Because routing nodes can adjust their fees without being penalized, they may do this often. This leads to extra gossip traffic and more (first try) payment failures.

• By always advertising zero fees, senders' pathfinders are attracted to that part of the graph and will explore it. Previously successful edges will have a high success probability for future payments and pull pathfinding even more towards that area. By always returning a fee_insufficient at first, nodes can still collect the non-zero fee that they really want to charge. They can keep broadcasting zero fee channel updates to "reset" senders' graphs so that they keep going to the area.

If the second chance logic were to be removed, an attached channel update would still be applied to the sender's graph, but the same channel wouldn't be retried immediately. After the failure has decayed, the sender will still use that updated policy if it leads to the best route. So even in the case where the channel update really doesn't reach the sender via gossip, they'll be able to use the channel with the new policy eventually.

[joostjager]

In the context of this issue, I think it could make sense to re-evaluate lnd's stopping criterium for payment attempts. Currently the cycle is stopped when the probability of the best route dips below a threshold. With the removal of the 2nd chance logic, an outdated policy and not having alternative routes may cause this to happen. The payment fails, where as previously another attempt would be made.

Perhaps the stopping criterium should be to stop when pathfinding returns a route (and fees) that has been tried before already.