Add ChannelContext::get_commitment_stats

[tankyleo]

    Add `ChannelContext::get_commitment_stats`
    
    It can be useful to get the stats on a potential commitment transaction
    without actually building it. Therefore, this commit splits the stats
    calculations from the actual build of a commitment transaction.
    
    This introduces an extra loop over the pending htlcs, but current
    network behavior produces very few concurrent htlcs on channels.
    Furthermore, each iteration of the loop in the stats calculation is very
    cheap.

Requested by @wpaulino in #3641

    Rename `CommitmentTransaction::htlcs` to `nondust_htlcs`

    Remove unnecessary clones of the HTLC-HTLCSource table

[ldk-reviews-bot]

👋 Thanks for assigning @TheBlueMatt as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[TheBlueMatt]

Why is this draft?

[tankyleo]

Why is this draft?

@TheBlueMatt I don't feel great about the code duplication between get_commitment_stats and build_commitment_transaction. Both functions filter for the non-dust HTLCs, but they do different things with them.

Let me know what you think.

[tankyleo]

I'm working on further cleanups now.

[tankyleo]

@wpaulino the PR is in a better spot now, let me know what you think.

[codecov]

Codecov Report

Attention: Patch coverage is 98.00995% with 4 lines in your changes missing coverage. Please review.

Project coverage is 89.16%. Comparing base (2e435de) to head (71df614).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/chan_utils.rs	86.66%	2 Missing ⚠️
lightning/src/ln/channel.rs	98.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3682      +/-   ##
==========================================
- Coverage   89.18%   89.16%   -0.03%     
==========================================
  Files         155      155              
  Lines      120796   120861      +65     
  Branches   120796   120861      +65     
==========================================
+ Hits       107731   107760      +29     
- Misses      10415    10446      +31     
- Partials     2650     2655       +5     

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[tankyleo]

@TheBlueMatt take a look when you can thank you!

[TheBlueMatt]

Heh, if its just for logging I think we can #[derive(Debug)] :)

[tankyleo]

Debug would log all their inner contents too - don't think that's what we want here ?

I would have to derive Debug in a bunch more places.

[tankyleo]

Actually I'll just move that function to an impl fmt::Debug for InboundHTLCState :)

[tankyleo]

Went with an impl fmt::Display here, seems this corresponds most to what we want.

[TheBlueMatt]

The fn name here doesn't really communicate what it does/is used for. Maybe inbound_htlcs_in_commitment? It might also be more Rust-y to return an impl Iterator and let the callsite do the for, rather than doing it as a callback.

[tankyleo]

We initially had something like this is it closer to what you have in mind?

                for htlc in self.pending_inbound_htlcs.iter() {
                        if htlc.state.included_in_commitment(generated_by_local) {
                               ..
                        } else {
                               ..
                        }
                }

some other options for that specific function's return value:

1. one iterator yielding (included, htlc)
2. two iterators each yielding htlc, one for included, one for excluded.

[tankyleo]

Went with the very first option above.