input: add new ScriptIsOpReturn helper func

input/fuzz_script_is_op_return_test.go

@@ -0,0 +1,51 @@
+package input
+
+import (
+	"testing"
+
+	"github.com/btcsuite/btcd/chaincfg"
+	"github.com/btcsuite/btcd/txscript"
+	"github.com/stretchr/testify/require"
+)
+
+// FuzzScriptIsOpReturn fuzzes the ScriptIsOpReturn function.
+func FuzzScriptIsOpReturn(f *testing.F) {
+	// Seed the corpus with some representative inputs.
+	f.Add([]byte{})
+	f.Add([]byte{txscript.OP_RETURN})
+
+	// Use our canonical ScriptBuilder to produce a valid OP_RETURN script.
+	builder := txscript.NewScriptBuilder()
+	builder.AddOp(txscript.OP_RETURN)
+	builder.AddData([]byte("valid data"))
+	script, err := builder.Script()
+	require.NoError(f, err)
+	f.Add(script)
+
+	// An example of a script that does not start with OP_RETURN.
+	f.Add([]byte{txscript.OP_DUP, txscript.OP_RETURN})
+
+	f.Fuzz(func(t *testing.T, pkScript []byte) {
+		result := ScriptIsOpReturn(pkScript)
+
+		var expected bool
+		if len(script) == 0 || script[0] != txscript.OP_RETURN {
+			expected = false
+		} else {
+			scriptClass, _, _, err := txscript.ExtractPkScriptAddrs(
+				pkScript, &chaincfg.MainNetParams,
+			)
+			if err != nil {
+				t.Fatalf("unable to extract pk script "+
+					"addresses: %v", err)
+			}
+
+			expected = scriptClass == txscript.NullDataTy
+		}
+
+		if result != expected {
+			t.Fatalf("for script %x, expected ScriptIsOpReturn=%v;"+
+				" got %v", script, expected, result)
+		}
+	})
+}

input/script_utils.go

@@ -3239,3 +3239,36 @@ func ComputeCommitmentPoint(commitSecret []byte) *btcec.PublicKey {
 	_, pubKey := btcec.PrivKeyFromBytes(commitSecret)
 	return pubKey
 }
+
+// ScriptIsOpReturn returns true if the passed script is an OP_RETURN script.
+//
+// Lifted from the txscript package:
+// https://github.com/btcsuite/btcd/blob/cc26860b40265e1332cca8748c5dbaf3c81cc094/txscript/standard.go#L493-L526.
+//
+//nolint:ll
+func ScriptIsOpReturn(script []byte) bool {
+	// A null script is of the form:
+	//  OP_RETURN <optional data>
+	//
+	// Thus, it can either be a single OP_RETURN or an OP_RETURN followed by
+	// a data push up to MaxDataCarrierSize bytes.
+
+	// The script can't possibly be a null data script if it doesn't start
+	// with OP_RETURN.  Fail fast to avoid more work below.
+	if len(script) < 1 || script[0] != txscript.OP_RETURN {
+		return false
+	}
+
+	// Single OP_RETURN.
+	if len(script) == 1 {
+		return true
+	}
+
+	// OP_RETURN followed by data push up to MaxDataCarrierSize bytes.
+	tokenizer := txscript.MakeScriptTokenizer(0, script[1:])
+
+	return tokenizer.Next() && tokenizer.Done() &&
+		(txscript.IsSmallInt(tokenizer.Opcode()) ||
+			tokenizer.Opcode() <= txscript.OP_PUSHDATA4) &&
+		len(tokenizer.Data()) <= txscript.MaxDataCarrierSize
+}