write::BzDecoder: Fix infinite loop on drop when no data is read or written

src/write.rs

@@ -204,7 +204,13 @@ impl<W: Write> BzDecoder<W> {
     /// [`write`]: Self::write
     pub fn try_finish(&mut self) -> io::Result<()> {
         while !self.done {
-            let _ = self.write(&[])?;
+            // The write is effectively a `self.flush()`, but we want to know how many
+            // bytes were written. exit if no input was read and no output was written
+            if self.write(&[])? == 0 {
+                // finishing the output stream is effectively EOF of the input
+                let msg = "Input EOF reached before logical stream end";
+                return Err(io::Error::new(io::ErrorKind::UnexpectedEof, msg));
+            }
         }
         self.dump()
     }
@@ -303,14 +309,39 @@ mod tests {
     }
 
     #[test]
-    fn write_empty() {
+    fn roundtrip_empty() {
+        // this encodes and then decodes an empty input file
         let d = BzDecoder::new(Vec::new());
         let mut c = BzEncoder::new(d, Compression::default());
         let _ = c.write(b"").unwrap();
         let data = c.finish().unwrap().finish().unwrap();
         assert_eq!(&data[..], b"");
     }
 
+    #[test]
+    fn finish_empty_explicit() {
+        // The empty sequence is not a valid .bzip2 file!
+        // A valid file at least includes the magic bytes, the checksum, etc.
+        //
+        // This used to loop infinitely, see
+        //
+        // - https://github.com/trifectatechfoundation/bzip2-rs/issues/96
+        // - https://github.com/trifectatechfoundation/bzip2-rs/pull/97
+        let mut d = BzDecoder::new(Vec::new());
+        d.write(b"").unwrap();
+        let e = d.finish().unwrap_err();
+        assert_eq!(e.kind(), std::io::ErrorKind::UnexpectedEof);
+    }
+
+    #[test]
+    fn finish_empty_drop() {
+        // the drop implementation used to loop infinitely for empty input
+        //
+        // see https://github.com/trifectatechfoundation/bzip2-rs/pull/118
+        let d = BzDecoder::new(Vec::new());
+        drop(d);
+    }
+
     #[test]
     fn write_invalid() {
         // see https://github.com/trifectatechfoundation/bzip2-rs/issues/98