feat: implement Framed::map_codec (#4427)

tokio-util/tests/framed.rs

@@ -12,7 +12,10 @@ use std::task::{Context, Poll};
 const INITIAL_CAPACITY: usize = 8 * 1024;
 
 /// Encode and decode u32 values.
-struct U32Codec;
+#[derive(Default)]
+struct U32Codec {
+    read_bytes: usize,
+}
 
 impl Decoder for U32Codec {
     type Item = u32;
@@ -24,6 +27,7 @@ impl Decoder for U32Codec {
         }
 
         let n = buf.split_to(4).get_u32();
+        self.read_bytes += 4;
         Ok(Some(n))
     }
 }
@@ -39,6 +43,38 @@ impl Encoder<u32> for U32Codec {
     }
 }
 
+/// Encode and decode u64 values.
+#[derive(Default)]
+struct U64Codec {
+    read_bytes: usize,
+}
+
+impl Decoder for U64Codec {
+    type Item = u64;
+    type Error = io::Error;
+
+    fn decode(&mut self, buf: &mut BytesMut) -> io::Result<Option<u64>> {
+        if buf.len() < 8 {
+            return Ok(None);
+        }
+
+        let n = buf.split_to(8).get_u64();
+        self.read_bytes += 8;
+        Ok(Some(n))
+    }
+}
+
+impl Encoder<u64> for U64Codec {
+    type Error = io::Error;
+
+    fn encode(&mut self, item: u64, dst: &mut BytesMut) -> io::Result<()> {
+        // Reserve space
+        dst.reserve(8);
+        dst.put_u64(item);
+        Ok(())
+    }
+}
+
 /// This value should never be used
 struct DontReadIntoThis;
 
@@ -63,18 +99,39 @@ impl tokio::io::AsyncRead for DontReadIntoThis {
 
 #[tokio::test]
 async fn can_read_from_existing_buf() {
-    let mut parts = FramedParts::new(DontReadIntoThis, U32Codec);
+    let mut parts = FramedParts::new(DontReadIntoThis, U32Codec::default());
     parts.read_buf = BytesMut::from(&[0, 0, 0, 42][..]);
 
     let mut framed = Framed::from_parts(parts);
     let num = assert_ok!(framed.next().await.unwrap());
 
     assert_eq!(num, 42);
+    assert_eq!(framed.codec().read_bytes, 4);
+}
+
+#[tokio::test]
+async fn can_read_from_existing_buf_after_codec_changed() {
+    let mut parts = FramedParts::new(DontReadIntoThis, U32Codec::default());
+    parts.read_buf = BytesMut::from(&[0, 0, 0, 42, 0, 0, 0, 0, 0, 0, 0, 84][..]);
+
+    let mut framed = Framed::from_parts(parts);
+    let num = assert_ok!(framed.next().await.unwrap());
+
+    assert_eq!(num, 42);
+    assert_eq!(framed.codec().read_bytes, 4);
+
+    let mut framed = framed.map_codec(|codec| U64Codec {
+        read_bytes: codec.read_bytes,
+    });
+    let num = assert_ok!(framed.next().await.unwrap());
+
+    assert_eq!(num, 84);
+    assert_eq!(framed.codec().read_bytes, 12);
 }
 
 #[test]
 fn external_buf_grows_to_init() {
-    let mut parts = FramedParts::new(DontReadIntoThis, U32Codec);
+    let mut parts = FramedParts::new(DontReadIntoThis, U32Codec::default());
     parts.read_buf = BytesMut::from(&[0, 0, 0, 42][..]);
 
     let framed = Framed::from_parts(parts);
@@ -85,7 +142,7 @@ fn external_buf_grows_to_init() {
 
 #[test]
 fn external_buf_does_not_shrink() {
-    let mut parts = FramedParts::new(DontReadIntoThis, U32Codec);
+    let mut parts = FramedParts::new(DontReadIntoThis, U32Codec::default());
     parts.read_buf = BytesMut::from(&vec![0; INITIAL_CAPACITY * 2][..]);
 
     let framed = Framed::from_parts(parts);