From e81c728ff63c9df04fb93a388f953beeff234a06 Mon Sep 17 00:00:00 2001
From: HaoyangLiu <lhy.0318@163.com>
Date: Fri, 11 Sep 2020 22:22:11 +0800
Subject: [PATCH] R4R: add max gas allwance calculation (#36)

* calculate max gas allwance

* return error for missing from
---
 internal/ethapi/api.go | 37 +++++++++++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/internal/ethapi/api.go b/internal/ethapi/api.go
index 7b9f9dafa..b0641c251 100644
--- a/internal/ethapi/api.go
+++ b/internal/ethapi/api.go
@@ -887,6 +887,11 @@ func DoEstimateGas(ctx context.Context, b Backend, args CallArgs, blockNrOrHash
 		hi  uint64
 		cap uint64
 	)
+	// Use zero address if sender unspecified.
+	if args.From == nil {
+		return 0, fmt.Errorf("missing from address")
+	}
+	// Determine the highest gas limit can be used during the estimation.
 	if args.Gas != nil && uint64(*args.Gas) >= params.TxGas {
 		hi = uint64(*args.Gas)
 	} else {
@@ -897,16 +902,40 @@ func DoEstimateGas(ctx context.Context, b Backend, args CallArgs, blockNrOrHash
 		}
 		hi = block.GasLimit()
 	}
+	// Recap the highest gas limit with account's available balance.
+	if args.GasPrice != nil && args.GasPrice.ToInt().BitLen() != 0 {
+		state, _, err := b.StateAndHeaderByNumberOrHash(ctx, blockNrOrHash)
+		if err != nil {
+			return 0, err
+		}
+		balance := state.GetBalance(*args.From) // from can't be nil
+		available := new(big.Int).Set(balance)
+		if args.Value != nil {
+			if args.Value.ToInt().Cmp(available) >= 0 {
+				return 0, errors.New("insufficient funds for transfer")
+			}
+			available.Sub(available, args.Value.ToInt())
+		}
+		allowance := new(big.Int).Div(available, args.GasPrice.ToInt())
+
+		// If the allowance is larger than maximum uint64, skip checking
+		if allowance.IsUint64() && hi > allowance.Uint64() {
+			transfer := args.Value
+			if transfer == nil {
+				transfer = new(hexutil.Big)
+			}
+			log.Warn("Gas estimation capped by limited funds", "original", hi, "balance", balance,
+				"sent", transfer.ToInt(), "gasprice", args.GasPrice.ToInt(), "fundable", allowance)
+			hi = allowance.Uint64()
+		}
+	}
+	// Recap the highest gas allowance with specified gascap.
 	if gasCap != nil && hi > gasCap.Uint64() {
 		log.Warn("Caller gas above allowance, capping", "requested", hi, "cap", gasCap)
 		hi = gasCap.Uint64()
 	}
 	cap = hi
 
-	// Use zero address if sender unspecified.
-	if args.From == nil {
-		args.From = new(common.Address)
-	}
 	// Create a helper to check if a gas allowance results in an executable transaction
 	executable := func(gas uint64) bool {
 		args.Gas = (*hexutil.Uint64)(&gas)