core, consensus: pluggable consensus engines (#3817)

This commit adds pluggable consensus engines to go-ethereum. In short, it
introduces a generic consensus interface, and refactors the entire codebase to
use this interface.

consensus/misc/forks.go

@@ -0,0 +1,43 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package misc
+
+import (
+	"fmt"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/params"
+)
+
+// VerifyForkHashes verifies that blocks conforming to network hard-forks do have
+// the correct hashes, to avoid clients going off on different chains. This is an
+// optional feature.
+func VerifyForkHashes(config *params.ChainConfig, header *types.Header, uncle bool) error {
+	// We don't care about uncles
+	if uncle {
+		return nil
+	}
+	// If the homestead reprice hash is set, validate it
+	if config.EIP150Block != nil && config.EIP150Block.Cmp(header.Number) == 0 {
+		if config.EIP150Hash != (common.Hash{}) && config.EIP150Hash != header.Hash() {
+			return fmt.Errorf("homestead gas reprice fork: have 0x%x, want 0x%x", header.Hash(), config.EIP150Hash)
+		}
+	}
+	// All ok, return
+	return nil
+}