Commits on Jul 19, 2022

1. Fix tracking of collected value across pathfinding iterations …

   If we end up "paying" for an `htlc_minimum_msat` with fees, we
   increment `already_collected_value_msat` by more than the amount
   of the path that we collected (who's `value_contribution_msat` is
   higher than the total payment amount, despite having been reduced
   down to the payment amount).
   
   This throws off our total value collection target, though in the
   coming commit(s) it would also throw off our path selection
   calculations.

   

   TheBlueMatt committed Jul 19, 2022
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2. Make route path selection optimize strictly for cost / amount …

   Currently, after we've selected a number of candidate paths, we
   construct a route from a random set of paths repeatedly, and then
   select the route with the lowest total cost. In the vast majority
   of cases this ends up doing a bunch of additional work in order to
   select the path(s) with the total lowest cost, with some vague
   attempt at randomization that doesn't actually work.
   
   Instead, here, we simply sort available paths by `cost / amount`
   and select the top paths. This ends up in practice having the same
   end result with substantially less complexity. In some rare cases
   it gets a better result, which also would have been achieved
   through more random trials. This implies there may in such cases be
   a potential privacy loss, but not a substantial one, given our path
   selection is ultimately mostly deterministic in many cases (or, if
   it is not, then privacy is achieved through randomization at the
   scorer level).

   

   TheBlueMatt committed Jul 19, 2022
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3. Reduce default max_channel_saturation_power_of_half to 2 (max 1/4) …

   Saturating a channel beyond 1/4 of its capacity seems like a more
   reasonable threshold for avoiding a path than 1/2, especially given
   we should still be willing to send a payment with a lower
   saturation limit if it comes to that.
   
   This requires an (obvious) change to some router tests, but also
   requires a change to the `fake_network_test`, opting to simply
   remove some over-limit test code there - `fake_network_test` was
   our first ever functional test, and while it worked great to ensure
   LDK worked at all on day one, we now have a rather large breadth
   of functional tests, and a broad "does it work at all" test is no
   longer all that useful.

   

   TheBlueMatt committed Jul 19, 2022
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