Handle files with invalid filters (#410)

The bad filter case is reachable, and can happen when the PNG file is corrupted.

src/filters.rs

@@ -1,3 +1,5 @@
+use crate::error::PngError;
+
 pub fn filter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8], buf: &mut Vec<u8>) {
     assert!(data.len() >= bpp);
     assert!(last_line.is_empty() || data.len() == last_line.len());
@@ -65,7 +67,13 @@ pub fn filter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8], buf: &
     }
 }
 
-pub fn unfilter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8], buf: &mut Vec<u8>) {
+pub fn unfilter_line(
+    filter: u8,
+    bpp: usize,
+    data: &[u8],
+    last_line: &[u8],
+    buf: &mut Vec<u8>,
+) -> Result<(), PngError> {
     buf.clear();
     buf.reserve(data.len());
     assert!(data.len() >= bpp);
@@ -114,8 +122,9 @@ pub fn unfilter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8], buf:
                 );
             }
         }
-        _ => unreachable!(),
+        _ => return Err(PngError::InvalidData),
     }
+    Ok(())
 }
 
 fn paeth_predictor(a: u8, b: u8, c: u8) -> u8 {

src/png/mod.rs

@@ -136,7 +136,7 @@ impl PngData {
             transparency_pixel,
             aux_headers,
         };
-        raw.data = raw.unfilter_image();
+        raw.data = raw.unfilter_image()?;
         // Return the PngData
         Ok(Self {
             idat_data: idat_headers,
@@ -281,7 +281,7 @@ impl PngImage {
     }
 
     /// Reverse all filters applied on the image, returning an unfiltered IDAT bytestream
-    fn unfilter_image(&self) -> Vec<u8> {
+    fn unfilter_image(&self) -> Result<Vec<u8>, PngError> {
         let mut unfiltered = Vec::with_capacity(self.data.len());
         let bpp = ((self.ihdr.bit_depth.as_u8() * self.channels_per_pixel() + 7) / 8) as usize;
         let mut last_line: Vec<u8> = Vec::new();
@@ -293,13 +293,13 @@ impl PngImage {
                 last_pass = line.pass;
             }
             last_line.resize(line.data.len(), 0);
-            unfilter_line(line.filter, bpp, line.data, &last_line, &mut unfiltered_buf);
+            unfilter_line(line.filter, bpp, line.data, &last_line, &mut unfiltered_buf)?;
             unfiltered.push(0);
             unfiltered.extend_from_slice(&unfiltered_buf);
             std::mem::swap(&mut last_line, &mut unfiltered_buf);
             unfiltered_buf.clear();
         }
-        unfiltered
+        Ok(unfiltered)
     }
 
     /// Apply the specified filter type to all rows in the image