Hi all!

        We've had increasing numbers of c-lightning users get upset they
can't open multiple channels, so I guess we're most motivated to allow
splicing of existing channels.  Hence this rough proposal.

For simplicity, I've chosen to only allow a single splice at a time.
It's still complex :(

Feedback welcome!
Splice Negotiation:

1. type: 40 (`splice_add_input`) (`option_splice`)
2. data:
   * [`32`:`channel_id`]
   * [`8`: `satoshis`]
   * [`32`: `prevtxid`]
   * [`4`: `prevtxoutnum`]
   * [`2`: `scriptlen`]
   * [`scriptlen`: `scriptpubkey`]

1. type: 41 (`splice_add_output`) (`option_splice`)
2. data:
   * [`32`:`channel_id`]
   * [`8`: `satoshis`]
   * [`2`: `scriptlen`]
   * [`scriptlen`: `outscript`]

1. type: 42 (`splice_all_added`) (`option_splice`)
2. data:
   * [`32`:`channel_id`]
   * [`4`:`feerate_per_kw`]
   * [`4`:`minimum_depth`]

Each side sends 0 or more `splice_add_input` and 0 or more
`splice_add_output` followed by `spice_all_added` to complete the splice
proposal.  This is done either to initiate a splice, or to respond to a
`splice_*` from the other party.

`splice_add_input` is checked for the following:
- must not be during a current splice
- scriptpubkey is empty, or of form 'HASH160 <20-byte-script-hash> EQUAL'
- `satoshis` doesn't wrap on addition.
- MAY check that it matches outpoint specified (sig will simply be
  invalid if so), and that outpoint is segwit.

`splice_add_output` is checked for the following:
- must not be during a current splice
- `satoshis` is less than or equal to amount owing to proposer, minus
  current reserve, and greater than or equal to `dust_limit_satoshis` we
  sent in our open_channel/accept_channel ,sg.
- script is one of the approved forms as it is for `shutdown`.

FIXME: Do we disallow splice-out if they specified

`splice_all_added` is checked for the following:
- average of `feerate_per_kw` by both sides (round down) is sufficient.
- average of `feerate_per_kw` by both sides not grossly excessive, if we're
  paying some of the fees (see below!)
- both sides can afford the fees from their post-splice funds (see
  Verification Changes below)
- maximum of the two `minimum_depth` is not grossly excessive.
- There is at least one splice_add_input or splice_add_output.

Splice negotiation, like closing negotiation, does not have persistent
state.  Reconnecting forgets previous negotiation.

Splice Signing

Once `splice_all_added` is both sent and received, we need to create and
sign both the splice tx itself, and the first commitment transaction
which spends it (but not in that order!).

1. One input spends the current funding tx output.
2. There is one additional input for each splice_add_input.
3. One output creates the new funding tx.
4. There is one additional output for each splice_add_output.
5. The entire transaction is sorted into BIP69 order.
6. The feerate is the sum of the two `feerate_per_kw` divided by 2,
   rounded down.

1. type: 43 (`splice_commitment_signature`) (`option_splice`)
2. data:
   * [`32`:`channel_id`]
   * [`64`:`commitment_signature`]
   * [`2`:`num_htlcs`]
   * [`num_htlcs*64`:`htlc_signature`]

1. type: 44 (`splice_signature`) (`option_splice`)
2. data:
   * [`32`:`channel_id`]
   * [`64`:`splice_signature`]

1. type: 45 (`splice_witness`) (`option_splice`)
2. data:
   * [`32`:`channel_id`]
   * [`2`: `num_witness_elements`
   * [`2`:`len`]
   * [`len`:`witnesses`]

`witnesses` itself is serialized as `num_witness_elements` of:
* `2`:`len`
* `len`: `witness_element`

Each side sends `splice_commitment_signature` and waits to receive and
verify the other side's `splice_commitment_signature` before sending
`splice_signature` and `splice_witness` for each `splice_add_input` it
proposed, in BIP69 input order.

Once a node has sent `splice_commitment_signature` it should remember
the splice proposal across reconnects.  Once it has both sent
`splice_signature`, the splice is locked in.

Splice Announcement

We have to tell the network about the new channel, otherwise there will
be a distruption when it sees the old funding transaction spent.  This
is inevitable for older nodes who won't understand splicing anyway.

We can't send out a `channel_announcement` or `channel_update` for the
new channel until after the new funding transaction has 6 confirmations,
so we append to the existing `channel_update` for the original channel,
using a new `message_flags` field:

| Bit Position  | Name                      | Field                            |
| ------------- | ------------------------- | -------------------------------- |
| 0             | `option_channel_htlc_max` | `htlc_maximum_msat`              |
| 1             | `option_channel_moving`   | `moving_txid                     |

The `channel_update` gains the following field:
    * [`32`: moving_txid`] (option_channel_moving)

If a current `channel_update` for a closing channel contains
`option_channel_moving` a node SHOULD ignore the channel close for at
least 100 blocks iff spent by `moving_txid`.

A node SHOULD immediately forward a `channel_update` it sees containing
`option_channel_moving` if neither previous `channel_update` for the
channel contains `option_channel_moving`.

Each side of the splice can send these unilaterally, and SHOULD allow a
few minutes for propagation (remember, average propagation from old
nodes is still 30 seconds) prior to broadcast of the splice transaction.

Message Changes During Splicing
Once you've sent `splice_commitment_signature` each commitment
transaction is duplicated: one spends the old funding transaction, one
spends the splice transaction:

1. type: 39 (`closing_signed`)
2. data:
   * [`32`:`channel_id`]
   * [`8`:`fee_satoshis`]
   * [`64`:`signature`]
   * [`64`:`splice_signature`] (`option_splice`)

1. type: 132 (`commitment_signed`)
2. data:
   * [`32`:`channel_id`]
   * [`64`:`signature`]
   * [`2`:`num_htlcs`]
   * [`num_htlcs*64`:`htlc_signature`]
   * [`num_htlcs*64`:`htlc_splice_signature`] (`option_splice`)

If a reconnection occurs between between sending and receiving
`splice_commitment_signature`) the peer's status is uncertain (similarly
for closing).  This we have a new field in `channel_reestablish` to flag
that we consider ourselves to be splicing:

1. type: 136 (`channel_reestablish`)
2. data:
   * [`32`:`channel_id`]
   * [`8`:`next_local_commitment_number`]
   * [`8`:`next_remote_revocation_number`]
   * [`32`:`your_last_per_commitment_secret`] (`option_data_loss_protect`)
   * [`33`:`my_current_per_commitment_point`] (`option_data_loss_protect`)
   * [`32`:`splice_txid`] (`option_splice`)

The splice_txid field indicates that this side considers itself to be

The sender:
- if it has sent `splice_commitment_signature` and not sent the corresponding
  `splice_closed`, MUST set `splice_txid` to the txid of the splice tx.
   - Otherwise MUST NOT.

The recipient:
- if it has sent `splice_commitment_signature` and not sent the corresponding
  - if `splice_txid` does not exist or does not match the current splice:
    - SHOULD fail the channel
  - otherwise:
    - MUST retransmit `splice_signature`
- otherwise:
  - if `splice_txid` field exists and is not all zeroes:
    - MUST send `splice_closed`

Validation Changes During Splicing
We track "post-splice" values as well as current values during

The post-splice reserve is 1% of post-splice capcacity (rounded down).

The fees for the splicing transaction itself are divided into parts by
the number of `splice_add_input` plus `splice_add_output`, rounded up.
Each side pays as many parts as it proposed `splice_add_input` plus

(So if Alice proposes two and Bob proposes one, and the total fee is 1000
satoshi, each part is 334 satoshi: Alice pays 668 and Bob pays 334.)

Each side's post-splice funds are debited their `splice_add_output`
amounts, and credited their `splice_add_input` amounts, a debited the
splice tx fees.  If any debiting occurs, the funds must be above the
post-splice reserve (ie. you can have below reserve, but you can't spend
if you're below reserve).

All update_add_htlc must be valid for the *both* the current and
post-splice balances.

Completing Splicing
Once you've seen both side's `minimum_depth` confirmations of the splice
transaction (ie. the maximum of the two `minimum_depth` values), you can
complete the splice by sending:

1. type: 46 (`splice_closed`) (`option_splice`)
2. data:
   * [`32`:`channel_id`]

Once you've sent and received `splice_closed` you can send
`announcement_signatures` for the new channel as per normal rules (ie. 6
confirmations, `announce_channel` bit set).

In addition, you can forget everything about the old channel (including
old HTLCs and revocation requirements).
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