consensus/ethash: relax requirements when determining future-blocks

consensus/ethash/consensus.go

@@ -36,9 +36,10 @@ import (
 
 // Ethash proof-of-work protocol constants.
 var (
-	FrontierBlockReward  *big.Int = big.NewInt(5e+18) // Block reward in wei for successfully mining a block
-	ByzantiumBlockReward *big.Int = big.NewInt(3e+18) // Block reward in wei for successfully mining a block upward from Byzantium
-	maxUncles                     = 2                 // Maximum number of uncles allowed in a single block
+	FrontierBlockReward    *big.Int = big.NewInt(5e+18) // Block reward in wei for successfully mining a block
+	ByzantiumBlockReward   *big.Int = big.NewInt(3e+18) // Block reward in wei for successfully mining a block upward from Byzantium
+	maxUncles                       = 2                 // Maximum number of uncles allowed in a single block
+	allowedFutureBlockTime          = 15 * time.Second  // Max time from current time allowed for blocks, before they're considered future blocks
 )
 
 // Various error messages to mark blocks invalid. These should be private to
@@ -231,7 +232,7 @@ func (ethash *Ethash) verifyHeader(chain consensus.ChainReader, header, parent *
 			return errLargeBlockTime
 		}
 	} else {
-		if header.Time.Cmp(big.NewInt(time.Now().Unix())) > 0 {
+		if header.Time.Cmp(big.NewInt(time.Now().Add(allowedFutureBlockTime).Unix())) > 0 {
 			return consensus.ErrFutureBlock
 		}
 	}