codec: LinesCodec should only return MaxLineLengthExceeded once per line (#3556)

tokio-util/src/codec/lines_codec.rs

@@ -133,7 +133,7 @@ impl Decoder for LinesCodec {
                     buf.advance(read_to);
                     self.next_index = 0;
                     if buf.is_empty() {
-                        return Err(LinesCodecError::MaxLineLengthExceeded);
+                        return Ok(None);
                     }
                 }
                 (false, Some(offset)) => {

tokio-util/tests/codecs.rs

@@ -201,7 +201,26 @@ fn lines_decoder_discard_repeat() {
     buf.put_slice(b"aa");
     assert!(codec.decode(buf).is_err());
     buf.put_slice(b"a");
+    assert_eq!(None, codec.decode(buf).unwrap());
+}
+
+// Regression test for [subsequent calls to LinesCodec decode does not return the desired results bug](https://github.com/tokio-rs/tokio/issues/3555)
+#[test]
+fn lines_decoder_max_length_underrun_twice() {
+    const MAX_LENGTH: usize = 11;
+
+    let mut codec = LinesCodec::new_with_max_length(MAX_LENGTH);
+    let buf = &mut BytesMut::new();
+
+    buf.reserve(200);
+    buf.put_slice(b"line ");
+    assert_eq!(None, codec.decode(buf).unwrap());
+    buf.put_slice(b"too very l");
     assert!(codec.decode(buf).is_err());
+    buf.put_slice(b"aaaaaaaaaaaaaaaaaaaaaaa");
+    assert_eq!(None, codec.decode(buf).unwrap());
+    buf.put_slice(b"ong\nshort\n");
+    assert_eq!("short", codec.decode(buf).unwrap().unwrap());
 }
 
 #[test]