[Lightning-dev] Insufficient funder balance for paying fees

[Pierre]

Hi Johan,

That's an interesting corner case. I think it shares some similarities
with the race condition described in BOLT 2 [1], which handling is
specified in BOLT 3 [2].

Note that what matters really is the timing of the
`commit_sig`/`revoke_and_ack` messages, not the `update_add_htlc`s,
because of the acknowledgment logic that excludes remote's unsigned
updates. A side effect is that there can be multiple HTLCs on each
side.

Each party will end up receiving a commitment tx which has
insufficient (possibly zero) fees. At that point according to [2] it
may decide to fail the channel, using its previous commitment (which
it hasn't yet revoked). Currently eclair won't fail the channel, but I
think we probably should, especially if we are the fundee and would
end up with all funds in an unpublishable tx. The funder could face
the same situation if the pending htlcs have a high value (at this
point its main output is zero anyway).

An appropriate choice of channel parameters (`mainly
max_htlc_value_in_flight_msat`, `channel_reserve_satoshis`,
`max_accepted_htlcs`) could probably reduce the probability of this
happening.

Hope that helps,

Pierre

[1] https://github.com/lightningnetwork/lightning-rfc/blob/master/02-peer-protocol.md#updating-fees-update_fee
[2] https://github.com/lightningnetwork/lightning-rfc/blob/master/03-transactions.md#fee-payment