Fix line length formatting to comply with linter

Author: MaxResnick

proposals/0298-parent-bank-hash.md

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+---
+simd: '0298'
+title: Add Parent Bank Hash to Block Header
+authors:
+  - Max Resnick
+category: Standard
+type: Core
+status: Idea
+created: 2024-03-26
+---
+
+## Summary
+
+This proposal adds the parent bank hash to the block header and introduces validity
+constraints to ensure the parent bank hash matches the expected value. This change
+enhances the security and integrity of the blockchain by providing an additional
+verification layer for block validity.
+
+## Motivation
+
+The current block header structure does not include the parent bank hash, which
+means validators cannot verify that the parent block's bank state matches their
+expectations. By adding this field and enforcing its validity, we can:
+
+1. Provide stronger guarantees about the chain's state consistency
+2. Enable more robust fork detection
+3. Improve the security of the consensus mechanism
+4. Help validators detect potential state divergence earlier
+
+## New Terminology
+
+- **Parent Bank Hash**: The hash of the bank state from the parent block
+- **Bank State**: The complete state of all accounts, programs, and system state
+  at a given slot
+
+## Detailed Design
+
+### Block Header Changes
+
+The block header structure will be extended to include a new field:
+
+```rust
+pub struct BlockHeader {
+    // ... existing fields ...
+    pub parent_bank_hash: Hash,  // Hash of the parent block's bank state
+}
+```
+
+### Validity Constraints
+
+A block will be considered invalid if:
+
+1. The parent_bank_hash does not match the hash of the parent block's bank state
+2. The parent_bank_hash is missing (for blocks after the feature is activated)
+
+### Implementation Details
+
+1. The bank hash will be computed at the end of processing each block
+2. The hash will be stored in the block metadata
+3. Validators will verify the parent_bank_hash matches their computed hash of the
+   parent block's state
+4. The feature will be gated behind a feature flag for smooth activation
+
+### Feature Activation
+
+This change will be implemented behind a feature flag and activated through the
+feature activation program. The activation will require:
+
+1. Implementation of the new block header structure
+2. Updates to block validation logic
+3. Updates to block production logic
+4. Updates to block replay logic
+
+## Alternatives Considered
+
+1. **Full State Hash**: Instead of just the parent bank hash, we could include a
+   full state hash. This was rejected as it would be more computationally
+   expensive and the parent bank hash provides sufficient security guarantees.
+
+2. **Optional Field**: Making the parent_bank_hash optional was considered but
+   rejected as it would weaken the security guarantees we're trying to achieve.
+
+3. **Delayed Verification**: Verifying the parent bank hash after block
+   processing was considered but rejected as it would allow invalid blocks to be
+   processed before being rejected.
+
+## Impact
+
+### Validators
+
+- Will need to compute and store bank hashes
+- Will need to verify parent bank hashes
+- May see slightly increased computational overhead
+
+### Core Contributors
+
+- Need to implement the new block header structure
+- Need to update block validation logic
+- Need to implement feature activation
+
+### DApp Developers
+
+- No direct impact on dapp developers
+- May benefit from increased chain security
+
+## Security Considerations
+
+1. The parent bank hash provides an additional layer of security by ensuring
+   state consistency
+2. Malicious validators cannot produce blocks with incorrect parent bank hashes
+3. The change helps detect state divergence earlier in the consensus process
+4. The feature should be activated carefully to ensure smooth transition
+
+## Backwards Compatibility
+
+This change is not backwards compatible and requires a feature flag for
+activation. All validators must upgrade to support the new block header format
+and validation rules before the feature is activated.
+
+## Implementation Plan
+
+1. Create feature flag for parent bank hash
+2. Implement block header changes
+3. Update block validation logic
+4. Update block production logic
+5. Add tests for new validation rules
+6. Deploy to testnet
+7. Activate feature on mainnet-beta