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>
I used `amount_msat_eq(x, AMOUNT_MSAT(0))` because I forgot this
function existed. I probably missed it because the name is surprising,
so add "is" in there to make it clear it's a boolean function.
You'll note almost all the places which did use it are Eduardo's and
Lisa's code, so maybe it's just me.
Fix up a few places which I could use it, too.
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>
