fix: verify that writeable feepayers are delegated

[bmuddha]

Summary by CodeRabbit

• Bug Fixes
  • Enhanced validation for zero-fee transactions to ensure fee-payer accounts meet eligibility requirements, with early detection of invalid fee-payer states to prevent further processing.

[coderabbitai]

Walkthrough

Fee-payer account validation logic for zero-fee transactions is enhanced. When an initial fee-payer exists, writable fee-payer accounts are now validated to be delegated or privileged; validation failures increment error counters and return InvalidAccountForFee. Fallback to default LoadedTransactionAccount when no initial fee-payer is provided.

Changes

Cohort / File(s)	Summary
Fee-payer validation refactoring src/transaction_processor.rs	Modified zero-fee branch to add early validation: if initial fee-payer account is loaded and writable, validate it is delegated or privileged; reject with InvalidAccountForFee error if not. Explicit fallback to default account when no initial fee-payer exists. Non-zero-fee path unchanged.

Sequence Diagram(s)

sequenceDiagram
    participant Processor as Transaction Processor
    participant ZeroFee as Zero-Fee Path
    participant Validation as Fee-Payer Validation
    participant ErrorCounter as Error Counter

    Processor->>ZeroFee: Check fee_lamports_per_signature == 0
    alt Initial fee-payer loaded
        ZeroFee->>Validation: Validate fee-payer eligibility
        alt Writable & not delegated/privileged
            Validation->>ErrorCounter: Increment invalid_account_for_fee
            Validation-->>ZeroFee: Return InvalidAccountForFee error
            ZeroFee-->>Processor: Reject transaction
        else Valid or read-only
            Validation-->>ZeroFee: Proceed
            ZeroFee-->>Processor: Continue processing
        end
    else No initial fee-payer
        ZeroFee->>ZeroFee: Use default LoadedTransactionAccount
        ZeroFee-->>Processor: Continue processing
    end

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

• Additional focus areas:
  • Logic of the new writable account delegation/privilege check and its correctness
  • Error propagation path when InvalidAccountForFee is returned
  • Edge cases where initial fee-payer is None vs. Some with various account states
  • Interaction between this validation and the non-zero-fee branch behavior

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and accurately summarizes the main change: adding validation to verify that writable fee payers are delegated, which is the core purpose of the code modification.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch bmuddha/fix/gasless-feepayer-check

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Actionable comments posted: 1

📜 Review details

Configuration used: CodeRabbit UI

Review profile: ASSERTIVE

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 11bbaf2 and 4d27862.

📒 Files selected for processing (1)

• src/transaction_processor.rs (1 hunks)

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (1)

• GitHub Check: Run cargo test