ethclient/gethclient: fix bugs in override object encoding (#25616)

This fixes a bug where contract code would be overridden to empty code ("0x")
when the Code field of OverrideAccount was left nil. The change also cleans up
the encoding of overrides to only send necessary fields, and improves documentation.

Fixes #25615

Co-authored-by: Felix Lange <fjl@twurst.com>
Co-authored-by: Martin Holst Swende <martin@swende.se>

ethclient/gethclient/gethclient.go

@@ -19,6 +19,7 @@ package gethclient
 
 import (
 	"context"
+	"encoding/json"
 	"math/big"
 	"runtime"
 	"runtime/debug"
@@ -118,15 +119,6 @@ func (ec *Client) GetProof(ctx context.Context, account common.Address, keys []s
 	return &result, err
 }
 
-// OverrideAccount specifies the state of an account to be overridden.
-type OverrideAccount struct {
-	Nonce     uint64                      `json:"nonce"`
-	Code      []byte                      `json:"code"`
-	Balance   *big.Int                    `json:"balance"`
-	State     map[common.Hash]common.Hash `json:"state"`
-	StateDiff map[common.Hash]common.Hash `json:"stateDiff"`
-}
-
 // CallContract executes a message call transaction, which is directly executed in the VM
 // of the node, but never mined into the blockchain.
 //
@@ -141,7 +133,7 @@ func (ec *Client) CallContract(ctx context.Context, msg ethereum.CallMsg, blockN
 	var hex hexutil.Bytes
 	err := ec.c.CallContext(
 		ctx, &hex, "eth_call", toCallArg(msg),
-		toBlockNumArg(blockNumber), toOverrideMap(overrides),
+		toBlockNumArg(blockNumber), overrides,
 	)
 	return hex, err
 }
@@ -218,26 +210,48 @@ func toCallArg(msg ethereum.CallMsg) interface{} {
 	return arg
 }
 
-func toOverrideMap(overrides *map[common.Address]OverrideAccount) interface{} {
-	if overrides == nil {
-		return nil
+// OverrideAccount specifies the state of an account to be overridden.
+type OverrideAccount struct {
+	// Nonce sets nonce of the account. Note: the nonce override will only
+	// be applied when it is set to a non-zero value.
+	Nonce uint64
+
+	// Code sets the contract code. The override will be applied
+	// when the code is non-nil, i.e. setting empty code is possible
+	// using an empty slice.
+	Code []byte
+
+	// Balance sets the account balance.
+	Balance *big.Int
+
+	// State sets the complete storage. The override will be applied
+	// when the given map is non-nil. Using an empty map wipes the
+	// entire contract storage during the call.
+	State map[common.Hash]common.Hash
+
+	// StateDiff allows overriding individual storage slots.
+	StateDiff map[common.Hash]common.Hash
 }
-	type overrideAccount struct {
-		Nonce     hexutil.Uint64              `json:"nonce"`
-		Code      hexutil.Bytes               `json:"code"`
-		Balance   *hexutil.Big                `json:"balance"`
-		State     map[common.Hash]common.Hash `json:"state"`
-		StateDiff map[common.Hash]common.Hash `json:"stateDiff"`
+
+func (a OverrideAccount) MarshalJSON() ([]byte, error) {
+	type acc struct {
+		Nonce     hexutil.Uint64              `json:"nonce,omitempty"`
+		Code      string                      `json:"code,omitempty"`
+		Balance   *hexutil.Big                `json:"balance,omitempty"`
+		State     interface{}                 `json:"state,omitempty"`
+		StateDiff map[common.Hash]common.Hash `json:"stateDiff,omitempty"`
+	}
+
+	output := acc{
+		Nonce:     hexutil.Uint64(a.Nonce),
+		Balance:   (*hexutil.Big)(a.Balance),
+		StateDiff: a.StateDiff,
 	}
-	result := make(map[common.Address]overrideAccount)
-	for addr, override := range *overrides {
-		result[addr] = overrideAccount{
-			Nonce:     hexutil.Uint64(override.Nonce),
-			Code:      override.Code,
-			Balance:   (*hexutil.Big)(override.Balance),
-			State:     override.State,
-			StateDiff: override.StateDiff,
+	if a.Code != nil {
+		output.Code = hexutil.Encode(a.Code)
 	}
+	if a.State != nil {
+		output.State = a.State
 	}
-	return &result
+	return json.Marshal(output)
 }

ethclient/gethclient/gethclient_test.go

@@ -19,6 +19,7 @@ package gethclient
 import (
 	"bytes"
 	"context"
+	"encoding/json"
 	"math/big"
 	"testing"
 
@@ -322,3 +323,53 @@ func testCallContract(t *testing.T, client *rpc.Client) {
 		t.Fatalf("unexpected error: %v", err)
 	}
 }
+
+func TestOverrideAccountMarshal(t *testing.T) {
+	om := map[common.Address]OverrideAccount{
+		common.Address{0x11}: OverrideAccount{
+			// Zero-valued nonce is not overriddden, but simply dropped by the encoder.
+			Nonce: 0,
+		},
+		common.Address{0xaa}: OverrideAccount{
+			Nonce: 5,
+		},
+		common.Address{0xbb}: OverrideAccount{
+			Code: []byte{1},
+		},
+		common.Address{0xcc}: OverrideAccount{
+			// 'code', 'balance', 'state' should be set when input is
+			// a non-nil but empty value.
+			Code:    []byte{},
+			Balance: big.NewInt(0),
+			State:   map[common.Hash]common.Hash{},
+			// For 'stateDiff' the behavior is different, empty map
+			// is ignored because it makes no difference.
+			StateDiff: map[common.Hash]common.Hash{},
+		},
+	}
+
+	marshalled, err := json.MarshalIndent(&om, "", "  ")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	expected := `{
+  "0x1100000000000000000000000000000000000000": {},
+  "0xaa00000000000000000000000000000000000000": {
+    "nonce": "0x5"
+  },
+  "0xbb00000000000000000000000000000000000000": {
+    "code": "0x01"
+  },
+  "0xcc00000000000000000000000000000000000000": {
+    "code": "0x",
+    "balance": "0x0",
+    "state": {}
+  }
+}`
+
+	if string(marshalled) != expected {
+		t.Error("wrong output:", string(marshalled))
+		t.Error("want:", expected)
+	}
+}