Add 2618 partial

Author: dunxen

_posts/2023-09-29-#2618.md

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+---
+layout: pr
+date: 2023-09-29
+title: "Include an `outbound_payment` flag in `MaybeTimeoutClaimableHTLC`"
+pr: 2618
+authors: [TheBlueMatt]
+components: ["ChannelMonitor"]
+host: dunxen
+status: upcoming
+commit:
+---
+
+## Notes
+
+* In [PR 2476] the [`AvailableBalances::balance_msat`] field, which caused confusion due to its strange computation, was removed in favour of
+  using `ChannelMonitor::get_claimable_balances` which is more straightforward for most use-cases and indicates balances claimable by the holder
+  after taking on-chain fees into account.
+* In [PR 2333] the [`Balance::claimable_amount_satoshis`] method was added, which excluded balances that we are unsure can be claimed (i.e. returns `0` for those balances).
+  This makes sense on the surface, however, in the case of forwarding an HTLC, we know that we would either be able to claim the timeout balance from
+  downstream or the preimage balance from upstream. Neither of these would be included in this calculation while the forwarded payment was still pending
+  and not timed out.
+* [PR 2618] (this PR) will include the timeout balance (`MaybeTimeoutClaimableHTLC`) but only in the case of forwarded payments. This offers a more accurate
+  view of the claimable balance.
+
+
+## Questions
+1. Did you review the PR? [Concept ACK, approach ACK, tested ACK, or NACK](https://github.com/lightningdevkit/rust-lightning/blob/master/CONTRIBUTING.md#peer-review)?
+1. How was the value of the (now removed) `AvailableBalances::balance_msat` field calculated and what was its intended usage?
+1. What do we use instead of `balance_msat` for the above purpose?
+1. Without the use of `balance_msat` how might a developer using LDK calculate an appropriate balance to show to an end-user? 
+1. What should the `HTLCSource` of an outbound payment be? And for a forwarded payment?
+1. What other fluctuations in balance do we still have after this PR for forwarded payments? How might we resolve these in a future PR?
+
+[PR 2476]: https://github.com/lightningdevkit/rust-lightning/pull/2476
+[PR 2333]: https://github.com/lightningdevkit/rust-lightning/pull/2333
+[PR 2618]: https://github.com/lightningdevkit/rust-lightning/pull/2618
+[`AvailableBalances::balance_msat`]: https://github.com/lightningdevkit/rust-lightning/blob/7a63ab77da7e0be35b1c5d87643e5461cd593b94/lightning/src/ln/channel.rs#L69
+[`Balance::claimable_amount_satoshis`]: https://github.com/lightningdevkit/rust-lightning/blob/6016101ac8ff31275f31f68eef8d33e94b19c0b1/lightning/src/chain/channelmonitor.rs#L658