blockchain: export CheckSerializedHeight, reduce allocs for cb height

parsing

Some callers only want to check the coinbase height rather than be given
the height.

blockchain/validate.go

@@ -5,6 +5,7 @@
 package blockchain
 
 import (
+	"bytes"
 	"encoding/binary"
 	"fmt"
 	"math"
@@ -41,6 +42,10 @@ const (
 	// baseSubsidy is the starting subsidy amount for mined blocks.  This
 	// value is halved every SubsidyHalvingInterval blocks.
 	baseSubsidy = 50 * btcutil.SatoshiPerBitcoin
+
+	// coinbaseHeightAllocSize is the amount of bytes that the
+	// ScriptBuilder will allocate when validating the coinbase height.
+	coinbaseHeightAllocSize = 5
 )
 
 var (
@@ -610,16 +615,25 @@ func ExtractCoinbaseHeight(coinbaseTx *btcutil.Tx) (int32, error) {
 		return 0, ruleError(ErrMissingCoinbaseHeight, str)
 	}
 
-	serializedHeightBytes := make([]byte, 8)
-	copy(serializedHeightBytes, sigScript[1:serializedLen+1])
-	serializedHeight := binary.LittleEndian.Uint64(serializedHeightBytes)
+	// We use 4 bytes here since it saves us allocations. We use a stack
+	// allocation rather than a heap allocation here.
+	var serializedHeightBytes [4]byte
+	copy(serializedHeightBytes[:], sigScript[1:serializedLen+1])
+
+	serializedHeight := int32(
+		binary.LittleEndian.Uint32(serializedHeightBytes[:]),
+	)
+
+	if err := compareScript(serializedHeight, sigScript); err != nil {
+		return 0, err
+	}
 
-	return int32(serializedHeight), nil
+	return serializedHeight, nil
 }
 
-// checkSerializedHeight checks if the signature script in the passed
+// CheckSerializedHeight checks if the signature script in the passed
 // transaction starts with the serialized block height of wantHeight.
-func checkSerializedHeight(coinbaseTx *btcutil.Tx, wantHeight int32) error {
+func CheckSerializedHeight(coinbaseTx *btcutil.Tx, wantHeight int32) error {
 	serializedHeight, err := ExtractCoinbaseHeight(coinbaseTx)
 	if err != nil {
 		return err
@@ -634,6 +648,26 @@ func checkSerializedHeight(coinbaseTx *btcutil.Tx, wantHeight int32) error {
 	return nil
 }
 
+func compareScript(height int32, script []byte) error {
+	scriptBuilder := txscript.NewScriptBuilder(
+		txscript.WithScriptAllocSize(coinbaseHeightAllocSize),
+	)
+	scriptHeight, err := scriptBuilder.AddInt64(
+		int64(height),
+	).Script()
+	if err != nil {
+		return err
+	}
+
+	if !bytes.HasPrefix(script, scriptHeight) {
+		str := fmt.Sprintf("the coinbase signature script does not "+
+			"minimally encode the height %d", height)
+		return ruleError(ErrBadCoinbaseHeight, str)
+	}
+
+	return nil
+}
+
 // CheckBlockHeaderContext performs several validation checks on the block header
 // which depend on its position within the block chain.
 //
@@ -787,7 +821,7 @@ func (b *BlockChain) checkBlockContext(block *btcutil.Block, prevNode *blockNode
 			blockHeight >= b.chainParams.BIP0034Height {
 
 			coinbaseTx := block.Transactions()[0]
-			err := checkSerializedHeight(coinbaseTx, blockHeight)
+			err := CheckSerializedHeight(coinbaseTx, blockHeight)
 			if err != nil {
 				return err
 			}

blockchain/validate_test.go

@@ -169,7 +169,7 @@ func TestCheckBlockSanity(t *testing.T) {
 	}
 }
 
-// TestCheckSerializedHeight tests the checkSerializedHeight function with
+// TestCheckSerializedHeight tests the CheckSerializedHeight function with
 // various serialized heights and also does negative tests to ensure errors
 // and handled properly.
 func TestCheckSerializedHeight(t *testing.T) {
@@ -215,17 +215,17 @@ func TestCheckSerializedHeight(t *testing.T) {
 		msgTx.TxIn[0].SignatureScript = test.sigScript
 		tx := btcutil.NewTx(msgTx)
 
-		err := checkSerializedHeight(tx, test.wantHeight)
+		err := CheckSerializedHeight(tx, test.wantHeight)
 		if reflect.TypeOf(err) != reflect.TypeOf(test.err) {
-			t.Errorf("checkSerializedHeight #%d wrong error type "+
+			t.Errorf("CheckSerializedHeight #%d wrong error type "+
 				"got: %v <%T>, want: %T", i, err, err, test.err)
 			continue
 		}
 
 		if rerr, ok := err.(RuleError); ok {
 			trerr := test.err.(RuleError)
 			if rerr.ErrorCode != trerr.ErrorCode {
-				t.Errorf("checkSerializedHeight #%d wrong "+
+				t.Errorf("CheckSerializedHeight #%d wrong "+
 					"error code got: %v, want: %v", i,
 					rerr.ErrorCode, trerr.ErrorCode)
 				continue