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

❌ Patch coverage is 71.06599% with 57 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.73%. Comparing base (d4de913) to head (2f9c3e5).
⚠️ Report is 6594 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning-background-processor/src/lib.rs	70.68%	56 Missing ⚠️
lightning/src/ln/channelmanager.rs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1996      +/-   ##
==========================================
+ Coverage   90.91%   91.73%   +0.82%     
==========================================
  Files          99       99              
  Lines       52505    59263    +6758     
  Branches    52505    59263    +6758     
==========================================
+ Hits        47735    54366    +6631     
- Misses       4770     4897     +127     

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[TheBlueMatt]

LGTM, feel free to squash.