feature: add read_size async fn to get the size

src/decode.rs

@@ -743,6 +743,15 @@ mod tokio_io {
     }
 
     impl<T: AsyncRead + Unpin> SliceDecoderInner<T> {
+
+        async fn read_size(&mut self) -> io::Result<u64> {
+            if let NextRead::Header = self.shared.state.read_next() {
+                // call poll_input until we have a chunk
+                std::future::poll_fn(|cx| self.poll_input(cx)).await?;
+            }
+            Ok(self.shared.state.parser.content_len().unwrap())
+        }
+
         fn poll_input(&mut self, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
             // If we haven't done the initial seek yet, do the full seek loop
             // first. Note that this will never leave any buffered output. The only
@@ -819,6 +828,14 @@ mod tokio_io {
             Self(SliceDecoderState::Reading(Box::new(state)))
         }
 
+        pub async fn read_size(&mut self) -> io::Result<u64> {
+            match &mut self.0 {
+                SliceDecoderState::Reading(state) => state.read_size().await,
+                SliceDecoderState::Output(state) => state.read_size().await,
+                SliceDecoderState::Taken => unreachable!(),
+            }
+        }
+
         pub fn into_inner(self) -> T {
             match self.0 {
                 SliceDecoderState::Reading(state) => state.shared.input,

src/encode.rs

@@ -614,6 +614,10 @@ impl ParseState {
         self.content_position
     }
 
+    pub fn content_len(&self) -> Option<u64> {
+        self.content_len
+    }
+
     fn at_root(&self) -> bool {
         self.content_position < CHUNK_SIZE as u64 && self.stack_depth == 1
     }