txscript: Add null data script creator

This adds a new function named NullDataScript to the txscript package that returns a provably-pruneable  OP_RETURN script with the provided data.  The function will return an error if the provided data is larger than the maximum allowed length for a nulldata script to be be considered standard.

txscript/standard.go

@@ -351,6 +351,16 @@ func PayToAddrScript(addr btcutil.Address) ([]byte, error) {
 	return nil, ErrUnsupportedAddress
 }
 
+// NullDataScript creates a provably prunable script
+// containing OP_RETURN followed by the passed data.
+func NullDataScript(data []byte) ([]byte, error) {
+	if len(data) > MaxDataCarrierSize {
+		return nil, ErrStackLongScript
+	}
+
+	return NewScriptBuilder().AddOp(OP_RETURN).AddData(data).Script()
+}
+
 // MultiSigScript returns a valid script for a multisignature redemption where
 // nrequired of the keys in pubkeys are required to have signed the transaction
 // for success.  An ErrBadNumRequired will be returned if nrequired is larger

txscript/standard_test.go

@@ -1028,3 +1028,89 @@ func TestStringifyClass(t *testing.T) {
 		}
 	}
 }
+
+// TestNullDataScript tests whether NullDataScript returns a valid script.
+func TestNullDataScript(t *testing.T) {
+	tiny := hexToBytes("01")
+	short := hexToBytes("0102030405060708090a0b0c0d0e0f1" +
+		"01112131415161718")
+	shortExp := hexToBytes("6a180102030405060708090a0b0c0d0e0f1" +
+		"01112131415161718")
+	justRight := hexToBytes("000102030405060708090a0b0c0d0e0f" +
+		"101112131415161718191a1b1c1d1e1f20212223242526272829" +
+		"2a2b2c2d2e2f303132333435363738393a3b3c3d3e3f40414243" +
+		"4445464748494a4b4c4d4e4f")
+	tooLong := hexToBytes("000102030405060708090a0b0c0d0e0f" +
+		"101112131415161718191a1b1c1d1e1f20212223242526272829" +
+		"2a2b2c2d2e2f303132333435363738393a3b3c3d3e3f40414243" +
+		"4445464748494a4b4c4d4e4f50")
+
+	tests := []struct {
+		name     string
+		data     []byte
+		valid    bool
+		expected []byte
+	}{
+		{
+			name:     "small data",
+			data:     tiny,
+			valid:    true,
+			expected: []byte{0x6a, 0x51},
+		},
+		{
+			name:     "data of size before OP_PUSHDATA1 is needed",
+			data:     short,
+			valid:    true,
+			expected: shortExp,
+		},
+		{
+			name:  "just right",
+			data:  justRight,
+			valid: true,
+			expected: append([]byte{
+				OP_RETURN, OP_PUSHDATA1, 80},
+				justRight...),
+		},
+		{
+			name:     "too big",
+			data:     tooLong,
+			valid:    false,
+			expected: nil,
+		},
+	}
+
+	for _, test := range tests {
+		script, err := NullDataScript(test.data)
+
+		if !test.valid {
+			if err == nil {
+				t.Errorf("NullDataScript test %v: expected error but none was returned. ",
+					test.name)
+			}
+
+			continue
+		}
+
+		if err != nil {
+			t.Errorf("NullDataScript test %v: Unexpected error returned %v ",
+				test.name, err)
+
+			continue
+		}
+
+		// Check that the expected result was returned.
+		if !bytes.Equal(script, test.expected) {
+			t.Errorf("NullDataScript test %v: Expected %x, got %x",
+				test.name, test.expected, script)
+
+			continue
+		}
+
+		// Check that the script has the correct type.
+		scriptType := GetScriptClass(script)
+		if scriptType != NullDataTy {
+			t.Errorf("NullDataScript test %v: Expected NullDataTy, got %v",
+				test.name, scriptType)
+		}
+	}
+}