router: Use aggregate capacity for first-hop fragmentation check #4331
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The routing fragmentation mitigation heuristic requires each channel to contribute at least `payment_amount / max_path_count` to avoid excessive path splitting. This makes sense for network paths where each additional path incurs base fees and increases failure probability. However, for first hops this is overly restrictive. Multiple channels to the same peer converge immediately at the first hop - there's no actual network-level fragmentation. A 2M sat channel and a 48M sat channel to the same peer should be usable together for a 50M sat payment, but currently the small channel gets rejected (threshold would be 5M sat with default max_path_count=10), leaving only 48M available. This change checks the aggregate capacity across all first-hop channels to a peer. If the aggregate meets the contribution threshold, individual channels are allowed regardless of their size.
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Codecov Report❌ Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## main #4331 +/- ##
==========================================
- Coverage 86.53% 85.84% -0.70%
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Files 158 156 -2
Lines 103188 102869 -319
Branches 103188 102869 -319
==========================================
- Hits 89292 88303 -989
- Misses 11471 12000 +529
- Partials 2425 2566 +141
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The routing fragmentation mitigation heuristic requires each channel to contribute at least
payment_amount / max_path_countto avoid excessive path splitting. This makes sense for network paths where each additional path incurs base fees and increases failure probability.However, for first hops this is overly restrictive. Multiple channels to the same peer converge immediately at the first hop - there's no actual network-level fragmentation. A 2M sat channel and a 48M sat channel to the same peer should be usable together for a 50M sat payment, but currently the small channel gets rejected (threshold would be 5M sat with default max_path_count=10), leaving only 48M available.
This change checks the aggregate capacity across all first-hop channels to a peer. If the aggregate meets the contribution threshold, individual channels are allowed regardless of their size.