Feature Gated: remove obsoleted FeeRateGovernor from fee calculation

[tao-stones]

Problem

Understanding lamports_per_signature in blockhash_queue

The lamports_per_signature value stored in blockhash_queue (and also in nonce accounts) is not used for transaction fee calculation. Instead, transaction fees are determined by bank.fee_structure.lamport_per_signature, which is a constant 5_000.

The primary function of lamports_per_signature in the blockhash_queue is to enable the zero_fees_for_test mode—this happens when its value is set to zero.

In tests, "zero-fee" mode is expressed by setting target_lamports_per_signature = 0 in genesis_config (reference).

Current Behavior of fee_rate_governor in Bank wrt blockhash_queue

The bank currently uses fee_rate_governor to adjust lamports_per_signature dynamically for each block based on cluster throughput. This adjustment stays within 50% to 1000% of target_lamports_per_signature.

A key property of fee_rate_governor's adjustment is:

• If target_lamports_per_signature is zero, then all future banks will have lamports_per_signature = 0.
• Otherwise, lamports_per_signature will never be zero.

Tests rely on this behavior to ensure zero transaction fees in specific scenarios.

Proposed Change

The current logic of deriving fee_rate_governor from the parent bank and storing it with blockhash to determine if lamports_per_signature == 0 is unnecessary. Instead, we can:

• Store the target_lamports_per_signature from genesis_config in the blockhash, when creating genesis bank;
• Copy lamports_per_signature from the parent bank when deriving a new bank.

This preserves the existing behavior while simplifying fee tracking and allowing the eventual removal of fee_rate_governor (#3303).

Feature gated

Changing what's stored into RecentBlockHash account changes account-delta-hash, which is part of BankHash for now. #5454 describes a testnet occurrence when divergence happened. So it is necessary to feature gate this change.

Summary of Changes

• Clarify that lamports_per_signature in blockhash_queue and nonce_account is not used for fee calculation but only for determining zero_fees_for_test mode (lamports_per_signature == 0).
• When creating a bank from genesis_config, store its target_lamports_per_signature in the blockhash.
• For non-genesis banks, copy the parent's lamports_per_signature into the blockhash queue.
• Update tests accordingly.
• Feature gated.

Fixes #5782

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.44068% with 8 lines in your changes missing coverage. Please review.

Project coverage is 82.9%. Comparing base (41e9a25) to head (b1a1a77).
Report is 19 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5749   +/-   ##
=======================================
  Coverage    82.9%    82.9%           
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  Files         825      825           
  Lines      375444   375403   -41     
=======================================
- Hits       311541   311531   -10     
+ Misses      63903    63872   -31     

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