fix panic with corrupt webp file

There were a couple of layers of issue here:

* In the ImageDataType::decode method, we didn't detect and do something
  reasonable when the decoded image had 0 frames, later leading to
  a panic in glyphcache when trying to index frame 0 of an empty vec.
* We shouldn't have been using ImageDataType::decode for window
  background images
* Make the fallback/placeholder black rather than fully transparent
  in the more modern decoder thread routine that we use for image
  decoding at the gui layer.

refs: #3614

docs/changelog.md

@@ -77,6 +77,7 @@ As features stabilize some brief notes about them will accumulate here.
   The about dialog has been replaced with a menu item that you can click to
   copy the version number. #3507 #3585
 * Synthesized italics were double-skewed. Thanks to @rozbb! #3613 #3555
+* Panic when using corrupt/invalid webp images as window background #3614
 
 #### Updated
 * Bundled harfbuzz to 7.1.0

termwiz/src/image.rs

@@ -284,6 +284,16 @@ impl ImageDataType {
         }
     }
 
+    /// Black pixels
+    pub fn placeholder() -> Self {
+        let mut data = vec![];
+        let size = 8;
+        for _ in 0..size * size {
+            data.extend_from_slice(&[0, 0, 0, 0xff]);
+        }
+        ImageDataType::new_single_frame(size, size, data)
+    }
+
     pub fn hash_bytes(bytes: &[u8]) -> [u8; 32] {
         use sha2::Digest;
         let mut hasher = sha2::Sha256::new();
@@ -380,7 +390,14 @@ impl ImageDataType {
                 match format {
                     ImageFormat::Gif => image::codecs::gif::GifDecoder::new(&*data)
                         .and_then(|decoder| decoder.into_frames().collect_frames())
-                        .and_then(|frames| Ok(Self::decode_frames(frames)))
+                        .and_then(|frames| {
+                            if frames.is_empty() {
+                                log::error!("decoded image has 0 frames, using placeholder");
+                                Ok(Self::placeholder())
+                            } else {
+                                Ok(Self::decode_frames(frames))
+                            }
+                        })
                         .unwrap_or_else(|err| {
                             log::error!(
                                 "Unable to parse animated gif: {:#}, trying as single frame",
@@ -395,6 +412,10 @@ impl ImageDataType {
                         };
                         if decoder.is_apng() {
                             match decoder.apng().into_frames().collect_frames() {
+                                Ok(frames) if frames.is_empty() => {
+                                    log::error!("decoded image has 0 frames, using placeholder");
+                                    Self::placeholder()
+                                }
                                 Ok(frames) => Self::decode_frames(frames),
                                 _ => Self::EncodedFile(data),
                             }
@@ -408,6 +429,10 @@ impl ImageDataType {
                             _ => return Self::EncodedFile(data),
                         };
                         match decoder.into_frames().collect_frames() {
+                            Ok(frames) if frames.is_empty() => {
+                                log::error!("decoded image has 0 frames, using placeholder");
+                                Self::placeholder()
+                            }
                             Ok(frames) => Self::decode_frames(frames),
                             _ => Self::EncodedFile(data),
                         }

wezterm-gui/src/glyphcache.rs

@@ -317,7 +317,8 @@ impl FrameDecoder {
             .next()
             .ok_or_else(|| {
                 anyhow::anyhow!(
-                    "Image format is not fully supported by \
+                    "Unable to decode image data. Either it is corrupt, or \
+                    the Image format is not fully supported by \
                     https://github.com/image-rs/image/blob/master/README.md#supported-image-formats")
             })?;
         let frame = frame.context("first frame result")?;
@@ -386,15 +387,22 @@ struct FrameState {
 
 impl FrameState {
     fn new(rx: Receiver<DecodedFrame>) -> Self {
-        static EMPTY: Lazy<BlobLease> = Lazy::new(|| BlobManager::store(&[0u8; 4]).unwrap());
+        const BLACK_SIZE: usize = 8;
+        static BLACK: Lazy<BlobLease> = Lazy::new(|| {
+            let mut data = vec![];
+            for _ in 0..BLACK_SIZE * BLACK_SIZE {
+                data.extend_from_slice(&[0, 0, 0, 0xff]);
+            }
+            BlobManager::store(&data).unwrap()
+        });
 
         Self {
             source: FrameSource::Decoder(rx),
             frames: vec![],
             current_frame: DecodedFrame {
-                lease: EMPTY.clone(),
-                width: 1,
-                height: 1,
+                lease: BLACK.clone(),
+                width: BLACK_SIZE,
+                height: BLACK_SIZE,
                 duration: Duration::from_millis(0),
             },
         }
@@ -412,7 +420,7 @@ impl FrameState {
                 Err(TryRecvError::Disconnected) => {
                     self.source = FrameSource::FrameIndex(0);
                     if self.frames.is_empty() {
-                        log::warn!("decoder thread terminated");
+                        log::warn!("image decoder thread terminated");
                         self.current_frame.duration = Duration::from_secs(86400);
                         self.frames.push(self.current_frame.clone());
                         false
@@ -463,8 +471,7 @@ pub struct DecodedImage {
 
 impl DecodedImage {
     fn placeholder() -> Self {
-        // A single black pixel
-        let image = ImageData::with_data(ImageDataType::new_single_frame(1, 1, vec![0, 0, 0, 0]));
+        let image = ImageData::with_data(ImageDataType::placeholder());
         Self {
             frame_start: RefCell::new(Instant::now()),
             current_frame: RefCell::new(0),

wezterm-gui/src/termwindow/background.rs

@@ -206,7 +206,7 @@ impl CachedImage {
         let data = std::fs::read(path)
             .with_context(|| format!("Failed to load window_background_image {}", path))?;
         log::trace!("loaded {}", path);
-        let mut data = ImageDataType::EncodedFile(data).decode();
+        let mut data = ImageDataType::EncodedFile(data);
         data.adjust_speed(speed);
         let image = Arc::new(ImageData::with_data(data));