`SwfMovie::from_data` is falliable and we should not hide those errors behind panics.

core/src/loader.rs

@@ -294,9 +294,9 @@ impl<'gc> Loader<'gc> {
                 },
             )?;
 
-            let data = fetch.await;
-            if let Ok(data) = data {
-                let movie = Arc::new(SwfMovie::from_data(&data));
+            let data = (fetch.await).and_then(|data| Ok((data.len(), SwfMovie::from_data(&data)?)));
+            if let Ok((length, movie)) = data {
+                let movie = Arc::new(movie);
 
                 player
                     .lock()
@@ -322,8 +322,8 @@ impl<'gc> Loader<'gc> {
                                 &[
                                     "onLoadProgress".into(),
                                     Value::Object(broadcaster),
-                                    data.len().into(),
-                                    data.len().into(),
+                                    length.into(),
+                                    length.into(),
                                 ],
                             );
                             avm.run_stack_till_empty(uc)?;
@@ -374,6 +374,8 @@ impl<'gc> Loader<'gc> {
                 //TODO: Inspect the fetch error.
                 //This requires cooperation from the backend to send abstract
                 //error types we can actually inspect.
+                //This also can get errors from decoding an invalid SWF file,
+                //too. We should distinguish those to player code.
                 player.lock().expect("Could not lock player!!").update(
                     |avm, uc| -> Result<(), Error> {
                         let (clip, broadcaster) = match uc.load_manager.get_loader(handle) {

core/src/player.rs

@@ -149,7 +149,7 @@ impl Player {
         input: Input,
         swf_data: Vec<u8>,
     ) -> Result<Arc<Mutex<Self>>, Error> {
-        let movie = Arc::new(SwfMovie::from_data(&swf_data));
+        let movie = Arc::new(SwfMovie::from_data(&swf_data)?);
 
         info!(
             "{}x{}",

core/src/tag_utils.rs

@@ -2,7 +2,8 @@ use gc_arena::Collect;
 use std::sync::Arc;
 use swf::{Header, TagCode};
 
-pub type DecodeResult = Result<(), Box<dyn std::error::Error>>;
+pub type Error = Box<dyn std::error::Error>;
+pub type DecodeResult = Result<(), Error>;
 pub type SwfStream<R> = swf::read::Reader<std::io::Cursor<R>>;
 
 /// An open, fully parsed SWF movie ready to play back, either in a Player or a
@@ -41,8 +42,8 @@ impl SwfMovie {
     }
 
     /// Construct a movie based on the contents of the SWF datastream.
-    pub fn from_data(swf_data: &[u8]) -> Self {
-        let swf_stream = swf::read::read_swf_header(&swf_data[..]).unwrap();
+    pub fn from_data(swf_data: &[u8]) -> Result<Self, Error> {
+        let swf_stream = swf::read::read_swf_header(&swf_data[..])?;
         let header = swf_stream.header;
         let mut reader = swf_stream.reader;
 
@@ -66,7 +67,7 @@ impl SwfMovie {
             data
         };
 
-        Self { header, data }
+        Ok(Self { header, data })
     }
 
     pub fn header(&self) -> &Header {