consensus/ethash: relax requirements when determining future-blocks

consensus/ethash/consensus.go

@@ -39,6 +39,7 @@ 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
+	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
 		}
 	}