option_will_fund: Proposal doc for proposed changes?

Not exactly the same as the other proposal, as it includes the actual
updates as well?

Author: niftynei

proposals/010-will-fund-for-food.md

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+# Will Fund For Food Proposal
+## Liquidity Advertisements
+
+## Problem Description
+
+Sourcing liquidity on the lightning network is a common problem
+for any node looking to accept inbound payments.
+
+## Overview
+
+This proposal adds a TLV to the `node_announcement`,
+which permits a node to advertise a rate for which they will contribute
+funds to an open channel request. Due to the nature of the protocol, these
+opens are only possible on the v2 channel protocol.
+
+Any advertised liquidity lease is for a duration of 4032 blocks, or
+approximately 28 days.
+
+Any node wishing to take advantage of the offered funding contribution
+may send an `open_channel2` request to the `node_announcement` node,
+and include a `request_funds` tlv.
+
+The accepter responds to the `open_channel2` by including a `will_fund`
+TLV in their `accept_channel2` response, which includes the
+fees they're charging for funding an open, the weight of the bytes they'll
+add to the funding transaction, the maximum channel-fees
+they will charge for moving HTLCs over the channel, and a signature. The
+signature can be used by the opener to prove the accepter did not
+abide by their channel-fee commitment.
+
+An accepter may refuse to fund (but allow the open to succeed without
+their contribution of funds) or decline to participate in a
+channel open (fail the negotiation).
+
+In the case that the accepter does not or can not supply the
+requested funding amount, they may contribute less than the total
+requested amount. They may also offer to contribute more. The lease fee
+is calculated using the total amount contributed by the accepter.
+
+Any contributed funds are subject to a funding lease of 4032 blocks. To enforce
+this lease, the CSV of the accepter's output on the commitment transaction
+is modified to reflect the remaining time left in the lease.
+
+The opener updates the lease expiry by communicating their current blockheight
+to the accepter via `update_blockheight` messages. If the opener fails
+to update the blockheight (and thus the CSV lock) in a timely manner, the
+accepter should fail the channel.
+
+When the funding lease expires, the CSV locks on the accepter's
+commitment transaction output reverts to the normal value, and the
+fund lease is considered concluded.
+
+## Implementation
+
+This proposal is only applicable to dual-funded channels, as
+single-funded channels do not have the capability to permit
+the accepter to contribute funds to a channel.
+
+A new TLV is added to both the `open_channel2` and `accept_channel2`.
+These allow for the negotiation of the terms and funds for the
+contribution.
+
+A new message, `update_blockheight` is added to the Normal Operation of
+a channel's lifetime. As with `update_feerate`, only the channel opener
+may send this message.
+
+The CSV value for the commitment transaction outputs to the accepter
+are modified. They now incorporate the funding lease and time since
+the channel has been opened. As blocks are published, the CSV
+for the accepter's outputs is decremented. Once the funding lease
+has passed, the CSV value outputs return to the 'normal', non-leased values.
+
+The lease fee, which is owed to the accepter from the opener's funding
+transaction inputs, is found as:
+
+	- the `funding_fee_base_sat`, plus
+	- the accepter's `funding_satoshis` times the
+          `funding_fee_proportional_basis` / 1000, plus
+	- the weight charge times the `funding_feerate_perkw` / 1000
+
+The lease fee is added to the accepter's output in the commitment transaction.
+
+## Security
+The opener should consider implementing a rate limit or routing policy
+that curtails the export of the leased funds during the duration of
+the lease. This depends on the goals of the opener node.
+
+## Open Questions
+TODO