This cause of cascading failure was pointed out by @t-bast: if fees spike and
you don't timeout an outgoing onchain HTLC, you should nonetheless fail the incoming htlc
because otherwise the incoming peer will close on you.
Of course, there's a risk of losing funds, but this only happens if you weren't going to get the HTLC spend in time anyway. And it would also catch any other reason that the downstream onchain goes wrong, containing the damage.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Reported-by: @t-bast
Changelog-Fixed: Protocol: We will close incoming HTLCs early if the outgoing HTLC is stuck onchain long enough, to avoid cascating failure.