Score payment paths from events in BackgroundProcessor

[valentinewallace]

Opening this as a draft before I finish testing+supporting in async to get ACKs on the patch.

It uses a higher-ranked trait bound, and works in the sample. The real solution is GATs, which doesn't fit our MSRV.

We want to move scoring here to migrate it out of the InvoicePayer as part of moving payment retries to ChannelManager.

Partially addresses #1932.

[TheBlueMatt]

No, I think an HRTB is the correct solution here, not a GAT - we aren't looking for a specific type but rather a range of types. Sadly we've had issues with that in the bindings due to rustc limitations in resolving HRTBs, but we'll see if it works this time.

[valentinewallace]

No, I think an HRTB is the correct solution here, not a GAT - we aren't looking for a specific type but rather a range of types.

If we had GATs, we could remove the lifetime parameter from WriteableScore, by removing it from LockableScore:

pub trait LockableScore {
	/// The locked [`Score`] type.
	type Locked<'a>: 'a + Score where Self: 'a;

	/// Returns the locked scorer.
	fn lock(&self) -> Self::Locked<'_>;
}

[TheBlueMatt]

Ah, right.

[valentinewallace]

Sadly we've had issues with that in the bindings due to rustc limitations in resolving HRTBs, but we'll see if it works this time.

Hmm, that seems like a big issue if it breaks bindings. Approach-wise, I wonder if that means the scoring needs to happen elsewhere?

[TheBlueMatt]

No, let's do it The Right Way first, and if it breaks in bindings we can figure it out later.

[codecov-commenter]

Codecov Report

Base: 90.91% // Head: 90.82% // Decreases project coverage by -0.09% ⚠️

Coverage data is based on head (2f9c3e5) compared to base (d4de913).
Patch coverage: 70.76% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1996      +/-   ##
==========================================
- Coverage   90.91%   90.82%   -0.09%     
==========================================
  Files          99       99              
  Lines       52505    52690     +185     
  Branches    52505    52690     +185     
==========================================
+ Hits        47735    47858     +123     
- Misses       4770     4832      +62     

Impacted Files	Coverage Δ
lightning-background-processor/src/lib.rs	86.34% <70.68%> (-9.13%)	⬇️
lightning/src/ln/channelmanager.rs	87.26% <75.00%> (-0.02%)	⬇️
lightning-invoice/src/utils.rs	96.94% <0.00%> (-0.68%)	⬇️
lightning-net-tokio/src/lib.rs	77.48% <0.00%> (-0.30%)	⬇️
lightning/src/ln/functional_tests.rs	96.87% <0.00%> (-0.20%)	⬇️
lightning/src/chain/channelmonitor.rs	91.02% <0.00%> (-0.10%)	⬇️
lightning/src/routing/router.rs	91.11% <0.00%> (-0.04%)	⬇️
lightning/src/ln/peer_channel_encryptor.rs	92.18% <0.00%> (+0.23%)	⬆️
lightning/src/routing/scoring.rs	97.35% <0.00%> (+1.43%)	⬆️

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[TheBlueMatt]

LGTM, feel free to squash.