eth/tracers: apply block header overrides correctly (#32183)

Fixes #32175.

This fixes the scenario where the blockhash opcode would return 0x0
during RPC simulations when using BlockOverrides with a future block
number. The root cause was that BlockOverrides.Apply() only modified the
vm.BlockContext, but GetHashFn() depends on the actual
types.Header.Number to resolve valid historical block hashes. This
caused a mismatch and resulted in incorrect behavior during trace and
call simulations.

---------

Co-authored-by: shantichanal <158101918+shantichanal@users.noreply.github.com>
Co-authored-by: lightclient <lightclient@protonmail.com>

eth/tracers/api.go

@@ -953,40 +953,53 @@ func (api *API) TraceCall(ctx context.Context, args ethapi.TransactionArgs, bloc
 	}
 	defer release()
 
-	vmctx := core.NewEVMBlockContext(block.Header(), api.chainContext(ctx), nil)
+	h := block.Header()
+	blockContext := core.NewEVMBlockContext(h, api.chainContext(ctx), nil)
+
 	// Apply the customization rules if required.
 	if config != nil {
-		if overrideErr := config.BlockOverrides.Apply(&vmctx); overrideErr != nil {
-			return nil, overrideErr
+		if config.BlockOverrides != nil && config.BlockOverrides.Number.ToInt().Uint64() == h.Number.Uint64()+1 {
+			// Overriding the block number to n+1 is a common way for wallets to
+			// simulate transactions, however without the following fix, a contract
+			// can assert it is being simulated by checking if blockhash(n) == 0x0 and
+			// can behave differently during the simulation. (#32175 for more info)
+			// --
+			// Modify the parent hash and number so that downstream, blockContext's
+			// GetHash function can correctly return n.
+			h.ParentHash = h.Hash()
+			h.Number.Add(h.Number, big.NewInt(1))
 		}
-		rules := api.backend.ChainConfig().Rules(vmctx.BlockNumber, vmctx.Random != nil, vmctx.Time)
-
+		if err := config.BlockOverrides.Apply(&blockContext); err != nil {
+			return nil, err
+		}
+		rules := api.backend.ChainConfig().Rules(blockContext.BlockNumber, blockContext.Random != nil, blockContext.Time)
 		precompiles = vm.ActivePrecompiledContracts(rules)
 		if err := config.StateOverrides.Apply(statedb, precompiles); err != nil {
 			return nil, err
 		}
 	}
-	// Execute the trace
-	if err := args.CallDefaults(api.backend.RPCGasCap(), vmctx.BaseFee, api.backend.ChainConfig().ChainID); err != nil {
+
+	// Execute the trace.
+	if err := args.CallDefaults(api.backend.RPCGasCap(), blockContext.BaseFee, api.backend.ChainConfig().ChainID); err != nil {
 		return nil, err
 	}
 	var (
-		msg         = args.ToMessage(vmctx.BaseFee, true, true)
+		msg         = args.ToMessage(blockContext.BaseFee, true, true)
 		tx          = args.ToTransaction(types.LegacyTxType)
 		traceConfig *TraceConfig
 	)
 	// Lower the basefee to 0 to avoid breaking EVM
 	// invariants (basefee < feecap).
 	if msg.GasPrice.Sign() == 0 {
-		vmctx.BaseFee = new(big.Int)
+		blockContext.BaseFee = new(big.Int)
 	}
 	if msg.BlobGasFeeCap != nil && msg.BlobGasFeeCap.BitLen() == 0 {
-		vmctx.BlobBaseFee = new(big.Int)
+		blockContext.BlobBaseFee = new(big.Int)
 	}
 	if config != nil {
 		traceConfig = &config.TraceConfig
 	}
-	return api.traceTx(ctx, tx, msg, new(Context), vmctx, statedb, traceConfig, precompiles)
+	return api.traceTx(ctx, tx, msg, new(Context), blockContext, statedb, traceConfig, precompiles)
 }
 
 // traceTx configures a new tracer according to the provided configuration, and

eth/tracers/api_test.go

@@ -689,6 +689,7 @@ func TestTracingWithOverrides(t *testing.T) {
 		Failed      bool
 		ReturnValue string
 	}
+
 	var testSuite = []struct {
 		blockNumber rpc.BlockNumber
 		call        ethapi.TransactionArgs
@@ -788,6 +789,25 @@ func TestTracingWithOverrides(t *testing.T) {
 			},
 			want: `{"gas":72666,"failed":false,"returnValue":"0x000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"}`,
 		},
+		{ // Override blocknumber with block n+1 and query a blockhash (resolves issue #32175)
+			blockNumber: rpc.LatestBlockNumber,
+			call: ethapi.TransactionArgs{
+				From: &accounts[0].addr,
+				Input: newRPCBytes([]byte{
+					byte(vm.PUSH1), byte(genBlocks),
+					byte(vm.BLOCKHASH),
+					byte(vm.PUSH1), 0x00,
+					byte(vm.MSTORE),
+					byte(vm.PUSH1), 0x20,
+					byte(vm.PUSH1), 0x00,
+					byte(vm.RETURN),
+				}),
+			},
+			config: &TraceCallConfig{
+				BlockOverrides: &override.BlockOverrides{Number: (*hexutil.Big)(big.NewInt(int64(genBlocks + 1)))},
+			},
+			want: fmt.Sprintf(`{"gas":59590,"failed":false,"returnValue":"%s"}`, backend.chain.GetHeaderByNumber(uint64(genBlocks)).Hash().Hex()),
+		},
 		/*
 			pragma solidity =0.8.12;