blockchain: Always check size and merkle root.

This modifies the assumevalid fast add path used during initial sync to
also check the block size and merkle roots.

These checks are now negligible in terms of the overall sync time since
the hashing has been significantly optimized and retaining them prevents
certain classes of potential harassment during initial sync.

Author: davecgh

internal/blockchain/validate.go

@@ -1922,15 +1922,12 @@ func (b *BlockChain) checkMerkleRoots(block *wire.MsgBlock, prevNode *blockNode)
 // The flags modify the behavior of this function as follows:
 //
 // BFFastAdd:
-//   - The max block size is not checked
-//   - The calculated merkle root(s) of the transaction trees are not checked
-//     against the associated entries in the header
 //   - Transactions are not checked to see if they are finalized
 //   - The included votes, revocations, and treasury spend transactions are
 //     not verified to be allowed
 //
-// The flags are also passed to checkBlockHeaderContext.  See its documentation
-// for how the flags modify its behavior.
+// The flags are also passed to [BlockChain.checkBlockHeaderContext].  See its
+// documentation for how the flags modify its behavior.
 func (b *BlockChain) checkBlockContext(block *dcrutil.Block, prevNode *blockNode, flags BehaviorFlags) error {
 	// The genesis block is valid by definition.
 	if prevNode == nil {
@@ -2216,26 +2213,26 @@ func (b *BlockChain) checkBlockContext(block *dcrutil.Block, prevNode *blockNode
 		}
 	}
 
+	// A block must not exceed the maximum allowed size as defined by the
+	// network parameters and the current status of any consensus votes to
+	// change it when serialized.
+	maxBlockSize := b.maxBlockSize(prevNode)
+	serializedSize := int64(block.MsgBlock().Header.Size)
+	if serializedSize > maxBlockSize {
+		str := fmt.Sprintf("serialized block is too big - got %d, max %d",
+			serializedSize, maxBlockSize)
+		return ruleError(ErrBlockTooBig, str)
+	}
+
+	// The calculated merkle root(s) of the transaction trees must match the
+	// associated entries in the header.
+	err = b.checkMerkleRoots(block.MsgBlock(), prevNode)
+	if err != nil {
+		return err
+	}
+
 	fastAdd := flags&BFFastAdd == BFFastAdd
 	if !fastAdd {
-		// A block must not exceed the maximum allowed size as defined by the
-		// network parameters and the current status of any hard fork votes to
-		// change it when serialized.
-		maxBlockSize := b.maxBlockSize(prevNode)
-		serializedSize := int64(block.MsgBlock().Header.Size)
-		if serializedSize > maxBlockSize {
-			str := fmt.Sprintf("serialized block is too big - got %d, max %d",
-				serializedSize, maxBlockSize)
-			return ruleError(ErrBlockTooBig, str)
-		}
-
-		// The calculated merkle root(s) of the transaction trees must match
-		// the associated entries in the header.
-		err = b.checkMerkleRoots(block.MsgBlock(), prevNode)
-		if err != nil {
-			return err
-		}
-
 		// Switch to using the past median time of the block prior to the block
 		// being checked for all checks related to lock times once the stake
 		// vote for the agenda is active.