Commits on Nov 7, 2022

1. Allow nodes to overshoot the MPP total_msat when paying …

   When a node retires a failed path as part of a larger MPP payment,
   the node may wish to use a path which is constrained by an
   `htlc_minimum_msat` value. In this case, the node is forced to
   overpay, likely overshooting the `total_msat` it set in the earlier
   onions for the same MPP payment.
   
   There are two possible solutions to this - either allow the
   `total_msat` value to change in later HTLCs or allow the node to
   (slightly) overshoot the `total_msat` value.
   
   Allowing `total_msat` to change across HTLCs is nontrivial to
   implement - HTLCs may arrive out-of-order, causing the receiving
   node to have to track all seen `total_msat` values and accept a
   set of HTLCs which meet any of the seen `total_msat` values.
   
   Instead, this commit changes the MPP logic to simply allow a sender
   to overshoot the stated `total_msat`.
   
   Sadly the backwards-compatibility story for this is not great.
   There doesn't seem to be a good way to resolve this issue in a
   backwards-compatible way. Instead we just bite the bullet and make
   the incompatible change, hoping the overshooting is rare enough
   that it's not a major issue.

   

   TheBlueMatt committed Nov 7, 2022
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