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xpay: don't treat a full HTLC slot table as a capacity shortfall - #9621
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channeld fails a local add with temporary_channel_failure ("Too many
HTLCs") no matter the amount. xpay told askrene the channel was
constrained, which stored amount-1msat, so getroutes retried the same
channel forever and shed dust parts.
A local slot failure now disables that channel for this payment.
Remote temporary_channel_failure is unchanged: the onion does not say
why.
Changelog-Fixed: xpay: a local "Too many HTLCs" failure no longer shrinks the channel by 1 msat and sheds dust parts on every retry.
Fixes ElementsProject#9597.
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Summary
temporary_channel_failurewhose message isToo many HTLCsdoes not depend on the amount.xpaystill told askrene the channel wasconstrained, and askrene stored amount - 1 msat.getroutesthen kept the same channel as the cheapest hop, retried 1 msat smaller, and shed a dust part onto another channel. A slot shortage can never succeed that way. Fixes xpay treats a local "Too many HTLCs" failure as a 1 msat capacity shortfall and floods the route with dust parts. #9597.temporary_channel_failureis unchanged: the onion does not say why, so it can still be a real capacity failure.Test plan
test_xpay_local_too_many_htlcs_not_capacity: fill the forwarder's single slot, thenxpay. OneHTLC slots are full, disabling it for this payment, and noassuming it can't carry.