swf: Write a non-zero 'index' value for true/false/undefined/null default values

A zero 'index' does not have a subsequent 'kind' field in the
written SWF. The 'index' field is not actually used for
true/false/undefined/null, so it can be anything as long
as it's non-zero.

swf/src/avm2/read.rs

@@ -904,6 +904,24 @@ pub mod tests {
         }
     }
 
+    #[test]
+    fn test_round_trip_default_value() {
+        use crate::avm2::write::Writer;
+
+        let orig_bytes = read_abc_from_file("tests/swfs/Avm2DefaultValue.swf");
+        let mut reader = Reader::new(&orig_bytes[..]);
+        let parsed = reader.read().unwrap();
+
+        let mut out = vec![];
+        let mut writer = Writer::new(&mut out);
+        writer.write(parsed).unwrap();
+
+        assert_eq!(
+            orig_bytes, out,
+            "Incorrectly written Avm2DefaultValue class"
+        );
+    }
+
     #[test]
     fn read_u30() {
         let read = |data: &[u8]| Reader::new(data).read_u30().unwrap();

swf/src/avm2/write.rs

@@ -373,9 +373,9 @@ impl<W: Write> Writer<W> {
             DefaultValue::Private(ref i) => (i.as_u30(), 0x05),
             DefaultValue::Double(ref i) => (i.as_u30(), 0x06),
             DefaultValue::Namespace(ref i) => (i.as_u30(), 0x08),
-            DefaultValue::False => (0, 0x0a),
-            DefaultValue::True => (0, 0x0b),
-            DefaultValue::Null => (0, 0x0c),
+            DefaultValue::False => (0x0a, 0x0a),
+            DefaultValue::True => (0x0b, 0x0b),
+            DefaultValue::Null => (0x0c, 0x0c),
             DefaultValue::Package(ref i) => (i.as_u30(), 0x16),
             DefaultValue::PackageInternal(ref i) => (i.as_u30(), 0x17),
             DefaultValue::Protected(ref i) => (i.as_u30(), 0x18),
@@ -392,16 +392,18 @@ impl<W: Write> Writer<W> {
             None => self.write_u30(0)?,
             Some(ref value) => {
                 let (index, kind) = match *value {
-                    DefaultValue::Undefined => (0, 0x00),
+                    // Just write out a non-zero 'index' field - it's unused,
+                    // so it doesn't matter what it is.
+                    DefaultValue::Undefined => (0x01, 0x00),
                     DefaultValue::String(ref i) => (i.as_u30(), 0x01),
                     DefaultValue::Int(ref i) => (i.as_u30(), 0x03),
                     DefaultValue::Uint(ref i) => (i.as_u30(), 0x04),
                     DefaultValue::Private(ref i) => (i.as_u30(), 0x05),
                     DefaultValue::Double(ref i) => (i.as_u30(), 0x06),
                     DefaultValue::Namespace(ref i) => (i.as_u30(), 0x08),
-                    DefaultValue::False => (0, 0x0a),
-                    DefaultValue::True => (0, 0x0b),
-                    DefaultValue::Null => (0, 0x0c),
+                    DefaultValue::False => (0x0a, 0x0a),
+                    DefaultValue::True => (0x0b, 0x0b),
+                    DefaultValue::Null => (0x0c, 0x0c),
                     DefaultValue::Package(ref i) => (i.as_u30(), 0x16),
                     DefaultValue::PackageInternal(ref i) => (i.as_u30(), 0x17),
                     DefaultValue::Protected(ref i) => (i.as_u30(), 0x18),

swf/tests/swfs/Avm2DefaultValue.as

@@ -0,0 +1,8 @@
+package {
+	public class Avm2DefaultValue {
+		public static var defaultTrue:Boolean = true;
+		public static var defaultFalse:Boolean = false;
+		public static var defaultNull:String = null;
+		public static var defaultUndefined:* = undefined;
+	}
+}