Serialize lamports per signature

[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

To avoid the same issue as #25181, we manually serialize it as an extra field at the end of the snapshot. The issue with the previous solution was that we serialized it as part of the bank which caused problems as the accounts db fields were serialized after it.

Fixes #23545

[codecov]

Codecov Report

Merging #25364 (5baba2c) into master (f9f6b94) will increase coverage by 0.0%.
The diff coverage is 78.9%.

@@            Coverage Diff            @@
##           master   #25364     +/-   ##
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  Coverage    82.0%    82.0%             
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  Files         655      620     -35     
  Lines      171822   170582   -1240     
  Branches      335        0    -335     
=========================================
- Hits       140972   140027    -945     
+ Misses      30734    30555    -179     
+ Partials      116        0    -116     

[brooksprumo]

Overall looks good! Sorry about the long review delay...

[brooksprumo]

Thanks for the previous replies; this PR looks good.

One last question is on the decision to put lamports_per_signature as a new field in the returned tuple from serialize_bank_and_storage():

solana/runtime/src/serde_snapshot/newer.rs

Lines 208 to 211 in 5baba2c

	// Additional fields, we manually store the lamps per signature here so that
	// we can grab it on restart.
	// TODO: if we do a snapshot version bump, consider moving this out.
	lamports_per_signature,

Instead, would it be possible to put lamports_per_signature at the end of DeserializableVersionedBank with the serde default_on_eof?

    #[serde(deserialize_with = "default_on_eof")]
    lamports_per_signature: u64,

solana/runtime/src/serde_snapshot/newer.rs

Lines 29 to 63 in 5baba2c

	struct DeserializableVersionedBank {
	blockhash_queue: BlockhashQueue,
	ancestors: AncestorsForSerialization,
	hash: Hash,
	parent_hash: Hash,
	parent_slot: Slot,
	hard_forks: HardForks,
	transaction_count: u64,
	tick_height: u64,
	signature_count: u64,
	capitalization: u64,
	max_tick_height: u64,
	hashes_per_tick: Option<u64>,
	ticks_per_slot: u64,
	ns_per_slot: u128,
	genesis_creation_time: UnixTimestamp,
	slots_per_year: f64,
	accounts_data_len: u64,
	slot: Slot,
	epoch: Epoch,
	block_height: u64,
	collector_id: Pubkey,
	collector_fees: u64,
	fee_calculator: FeeCalculator,
	fee_rate_governor: FeeRateGovernor,
	collected_rent: u64,
	rent_collector: RentCollector,
	epoch_schedule: EpochSchedule,
	inflation: Inflation,
	stakes: Stakes<Delegation>,
	#[allow(dead_code)]
	unused_accounts: UnusedAccounts,
	epoch_stakes: HashMap<Epoch, EpochStakes>,
	is_delta: bool,
	}

That's how historical_roots is handled for AccountsDbFields:

solana/runtime/src/serde_snapshot.rs

Lines 67 to 78 in 5baba2c

	struct AccountsDbFields<T>(
	HashMap<Slot, Vec<T>>,
	StoredMetaWriteVersion,
	Slot,
	BankHashInfo,
	/// all slots that were roots within the last epoch
	#[serde(deserialize_with = "default_on_eof")]
	Vec<Slot>,
	/// slots that were roots within the last epoch for which we care about the hash value
	#[serde(deserialize_with = "default_on_eof")]
	Vec<(Slot, Hash)>,
	);

That may make the impl simpler. Sorry if you already tried doing this but I missed it!

[AshwinSekar]

That was actually my initial approach see here (https://github.com/solana-labs/solana/pull/25181/files). Turns out it doesn't
work because both the bank and accounts db are serialized in the same file with the bank being serialized first

https://github.com/solana-labs/solana/blob/master/runtime/src/serde_snapshot/newer.rs#L199

Because of this default_on_eof doesn't work as it ends up picking up part of the accounts db data.

[brooksprumo]

Lgtm