binaryHasher.updateAccount computed codeLen from len(account.Code.Code),
which is only non-zero when the code itself was modified in the current
block. For balance- or nonce-only updates account.Code is nil and the
computed codeLen was 0, silently overwriting the code_size field packed
into the bintrie BasicData leaf (EIP-7864 bytes 5-7) with zero every
time a contract was touched without a code write.
The TODO(rjl493456442) on updateAccount acknowledged this. Fix it by
adding a CodeSize field to AccountMut and having the caller at
StateDB.IntermediateRoot populate it via stateObject.CodeSize(), which
returns len(obj.code) when the bytes are loaded, otherwise falls back
to a code-size lookup via the reader. The binary hasher then passes
account.CodeSize straight to BinaryTrie.UpdateAccount as its codeLen
argument, and the TODO is removed.
Rationale for placing CodeSize on AccountMut rather than Account:
AccountMut already carries Code *CodeMut — the new bytecode, which is
not a field of Account — because code is write-time data that is not
persisted in the flat-state format (SlimAccountRLP). CodeSize has the
identical lifecycle: it is not in SlimAccountRLP, it is not populated
by any reader, and it is only consumed by the hasher at write time.
Mirroring Code's placement keeps the read-side/write-side split honest
(Account models the persisted flat-state record; AccountMut adds the
code-related write-time parameters). If the bintrie flat-state format
is later extended to carry code_size, CodeSize can be promoted onto
Account at that time.
merkleHasher is unaffected: StateTrie.UpdateAccount ignores its codeLen
parameter, so the wrapTrie.UpdateAccount shim continues to pass 0 and
no state-root divergence is introduced on the MPT path.
Regression test TestVerkleCodeSizePreserved verifies that the state
root produced by "create contract, commit, reload, modify balance,
commit" matches the root of a single-step construction of the same
final state. Before the fix the roots diverge:
path A (reload + balance): 1a675599...
path B (fresh, same state): de0cfb03...
