Serialize lamports per signature in snapshots

[AshwinSekar]

Problem

The FeeRateGovernor in a bank does not serialize its current lamports_per_signature on snapshot. This causes issues if the block that we start a snapshot from is lined up with a fee increase, it is possible for this lamports_per_signature to be invalid.

Summary of Changes

Serialize and deserialize this field by accessing it from the FeeRateGovernor

Fixes #23545

[codecov]

Codecov Report

Merging #25181 (857e0ba) into master (69a0ff9) will increase coverage by 0.0%.
The diff coverage is 76.6%.

❗ Current head 857e0ba differs from pull request most recent head b60ac35. Consider uploading reports for the commit b60ac35 to get more accurate results

@@            Coverage Diff            @@
##           master   #25181     +/-   ##
=========================================
  Coverage    82.0%    82.1%             
=========================================
  Files         598      610     +12     
  Lines      165882   168250   +2368     
=========================================
+ Hits       136125   138143   +2018     
- Misses      29757    30107    +350     

[brooksprumo]

Looks good. One nit/comment below, and then also posted over in the main issue about the direction for this fix.

[brooksprumo]

Minor nits/suggestions here:

• How about building off of a self reference instead of consuming the old governor?
• Should be able to use the .. syntax to construct the new governor
• Rename the fn to use clone instead of copy (since the type itself doesn't implement Copy)

Suggested change

	pub fn copy_with_new_lamports_per_signature(
	old: FeeRateGovernor,
	lamports_per_signature: u64,
	) -> FeeRateGovernor {
	FeeRateGovernor {
	lamports_per_signature,
	target_lamports_per_signature: old.target_lamports_per_signature,
	target_signatures_per_slot: old.target_signatures_per_slot,
	min_lamports_per_signature: old.min_lamports_per_signature,
	max_lamports_per_signature: old.max_lamports_per_signature,
	burn_percent: old.burn_percent,
	}
	}
	pub fn clone_with_lamports_per_signature(
	&self,
	lamports_per_signature: u64,
	) -> Self {
	Self {
	lamports_per_signature,
	..self
	}
	}

(Note: I haven't tried compiling this to ensure it works heh 😅)

[AshwinSekar]

Spot on 😄 , just had to dereference that last self

[brooksprumo]

@AshwinSekar It'll probably be valuable to add tests to ensure compatibility between snapshots with and without the new lamports_per_signature field.