This is the root cause of the problem worked around in 50949b7b9c
"askrene: hack in some padding so we don't overflow capacities."
When adding fees to flows, we didn't recheck the boundary conditions: in
renepay this is done by routebuilder.
Fortunately, we can use our "reservations" infrastructure to temporarily
use capacity as we process flows, so we handle the cases where they are
not independent correclty.
My assumption is that the resulting errors are small, so we divide
them between the remaining flows based on highest-to-least
probability.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Simply calculate it when we need it, which means we don't have to keep it
up-to-date as we tweak the flow.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
In general, we should be using tmpctx unless there's a specific reason not to.
It's clear, and simplifies the code somewhat.
If tmpctx is not cleaned often enough, we can look at a per-MCF context, but this
seems like premature optimization.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
