ContractTest Analysis¶

Note: This document is a historical record from our test data validation work. The root cause was identified as a test fixture data issue, not a bug in the gas calculation code.

Date: 2025-11-16
Status: ✅ Root Cause Identified
Finding: Gas calculation code is CORRECT - Issue was in test fixture data

Summary¶

Investigation confirmed that the EVM gas metering implementation is correct and matches Ethereum reference implementations (Besu, core-geth). The test was failing due to inconsistent test fixture data where accounts had incorrect codeHash values.

Key Finding: The gas calculation correctly charges 21,272 gas for calling an account with no code, which is exactly what the Ethereum specification requires.

Investigation Timeline¶

1. Initial Hypothesis: Gas Calculation Bug¶

Transaction in block 1 expected to use 47,834 gas
Actual gas reported: 21,272 gas
Difference: 26,562 gas (55% reduction)

2. Gas Calculation Verification¶

Transaction Details: - Target: 0x247d9c1a8560acfef96bbc6b4e4740a05e976395 - Data: 0xd6960697 (4 bytes, function selector) - Value: 3.125 ETH - Gas limit: 3,144,590

Intrinsic Gas Calculation:

Base transaction cost:     21,000 gas
Data (4 non-zero bytes):   4 × 68 = 272 gas
Total Intrinsic:           21,272 gas ✓

Expected Execution Gas:

Total expected:            47,834 gas
Intrinsic:                 21,272 gas
Expected EVM execution:    26,562 gas

3. Comparison with Reference Implementations¶

Verified against ethereum/tests standards and reference implementations:

Implementation	TX_BASE_COST	TX_DATA_NONZERO	Our Implementation
Besu (Java)	21,000	68	✓ Matches
core-geth (Go)	21,000	68	✓ Matches
ethereum/tests	21,000	68	✓ Matches

Conclusion: Our intrinsic gas calculation is CORRECT.

4. Keccak-256 vs SHA3 Investigation¶

Verified that the codebase correctly uses Keccak-256 (not NIST SHA-3): - crypto.kec256 uses KeccakDigest(256) from BouncyCastle ✓ - Address hashing for state trie: kec256(addr.toArray) ✓ - This is NOT the issue.

5. VM Execution Analysis¶

Added debug logging to trace VM execution:

[VM] Executing contract at 0x247d9c1a8560acfef96bbc6b4e4740a05e976395
[VM]   Account exists: true
[VM]   Account codeHash: c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470
[VM]   Code size: 0 bytes, startGas=3123318
[VM] Contract execution completed: gasRemaining=3123318, error=None

KEY FINDING: The account exists but has 0 bytes of code!

6. CodeHash Analysis¶

The account's codeHash is c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470.

This is the Keccak-256 hash of empty bytes, which Ethereum uses as a marker for accounts with NO CODE.

>>> import hashlib
>>> hashlib.sha3_256(b'').hexdigest()
# Note: sha3_256 is NIST SHA-3, not Keccak-256
>>> # For Keccak-256:
'c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470'

7. Fixture Data Investigation¶

Contract code EXISTS in fixture: - File: src/it/resources/txExecTest/purchaseContract/evmCode.txt - Code hash: de3565a1f31ab3ea98fd46e0293d5ef7c05ea05b58e3314807e2293d8bfcb060 - Code: 1738 bytes of Solidity bytecode

Account data in fixture: - File: src/it/resources/txExecTest/purchaseContract/stateTree.txt - Account: 0x247d9c1a8560acfef96bbc6b4e4740a05e976395 - CodeHash: c5d2460186f7... (WRONG - should be de3565a1f31ab3...)

Problem: The account's codeHash field points to empty code when it should point to the actual contract code.

Root Cause¶

The test fixture has inconsistent data: 1. Contract code exists in evmCode.txt (hash: de3565a1f31ab3...) 2. Account in stateTree.txt has codeHash = c5d2460186f7... (empty code marker) 3. When FixtureProvider loads the fixtures, it only saves code for accounts with non-empty codeHash 4. Result: The code is in the fixture file but never gets loaded into the test database

Why Gas is 21,272 (Correct Behavior)¶

When a transaction calls an account with no code: 1. Intrinsic gas is charged: 21,272 ✓ 2. VM executes with empty code: uses 0 gas ✓ 3. Total gas used: 21,272 ✓

This is CORRECT behavior per Ethereum specification.

Why Fixture Expects 47,834 (Incorrect Expectation)¶

The fixture was probably generated from a working blockchain where: 1. The account HAD contract code 2. The transaction executed the contract code 3. Total gas used was 47,834 (21,272 intrinsic + 26,562 execution)

But the fixture's state tree data got corrupted, losing the correct codeHash value.

Solution Implemented¶

Added code to FixtureProvider.prepareStorages() to pre-load ALL EVM code from fixtures:

// Pre-load ALL EVM code from fixtures into storage
// This is necessary because some fixtures have account codeHash values that don't match
// the actual code hash (they may have empty codeHash when they should have the real hash)
fixtures.evmCode.foreach { case (codeHash, code) =>
  storages.evmCodeStorage.put(codeHash, code).commit()
}

Note: This is a partial workaround. The code gets loaded into storage, but the account still has the wrong codeHash, so world.getCode(address) still returns empty.

Complete Fix Options¶

Option 1: Regenerate Fixture (RECOMMENDED)¶

Regenerate the purchaseContract test fixture with correct account codeHash values.

Pros: - Fixes root cause - Clean solution - No code workarounds needed

Cons: - Requires fixture generation tools/process - May affect other tests using same fixture

Option 2: Implement Account Patching (COMPLEX)¶

Modify FixtureProvider to detect and fix incorrect codeHash values when loading.

Approach: 1. For each account with codeHash = emptyEvm 2. Check if any code in evmCode.txt should belong to this address 3. Update the account's codeHash in the state trie 4. Re-save the modified account

Pros: - Handles corrupted fixtures automatically - No fixture regeneration needed

Cons: - Complex implementation (requires state trie manipulation) - Needs mapping of addresses to code hashes - May hide real data integrity issues

Option 3: Skip Test Temporarily¶

Mark ContractTest as @Ignore with detailed comment.

Pros: - Simple immediate fix - Allows other tests to proceed

Cons: - Loses test coverage - Temporary workaround only

Verification Summary¶

Code verified as correct ✓ - src/main/scala/com/chipprbots/ethereum/vm/EvmConfig.scala - Intrinsic gas calculation - src/main/scala/com/chipprbots/ethereum/ledger/BlockPreparator.scala - Gas refund calculation - src/main/scala/com/chipprbots/ethereum/vm/VM.scala - VM execution and gas tracking - src/main/scala/com/chipprbots/ethereum/domain/Address.scala - Keccak-256 hashing - crypto/src/main/scala/com/chipprbots/ethereum/crypto/package.scala - Crypto functions

Fixture issue identified: - src/it/resources/txExecTest/purchaseContract/stateTree.txt - Account codeHash needed update