fix: do not validate utf8 for partial text frames

src/fragment.rs

@@ -222,6 +222,18 @@ impl Fragments {
           if self.fragments.is_some() {
             return Err(WebSocketError::InvalidFragment);
           }
+          // Validate UTF-8 for unfragmented text messages
+          if frame.opcode == OpCode::Text {
+            match utf8::decode(&frame.payload) {
+              Ok(_) => {}
+              Err(utf8::DecodeError::Incomplete { .. }) => {
+                return Err(WebSocketError::InvalidUTF8);
+              }
+              Err(utf8::DecodeError::Invalid { .. }) => {
+                return Err(WebSocketError::InvalidUTF8);
+              }
+            }
+          }
           return Ok(Some(Frame::new(true, frame.opcode, None, frame.payload)));
         } else {
           self.fragments = match frame.opcode {

src/lib.rs

@@ -645,13 +645,7 @@ impl ReadHalf {
       OpCode::Ping if self.auto_pong => {
         (Ok(None), Some(Frame::pong(frame.payload)))
       }
-      OpCode::Text => {
-        if frame.fin && !frame.is_utf8() {
-          (Err(WebSocketError::InvalidUTF8), None)
-        } else {
-          (Ok(Some(frame)), None)
-        }
-      }
+      OpCode::Text => (Ok(Some(frame)), None),
       _ => (Ok(Some(frame)), None),
     }
   }

tests/fragment_utf8.rs

@@ -0,0 +1,159 @@
+// Test for fragmented text frames with partial UTF-8 characters
+// https://github.com/denoland/fastwebsockets/issues/122
+
+use tokio::io::AsyncWriteExt;
+use tokio::io::DuplexStream;
+
+use fastwebsockets::FragmentCollector;
+use fastwebsockets::Frame;
+use fastwebsockets::OpCode;
+use fastwebsockets::Role;
+use fastwebsockets::WebSocket;
+
+#[tokio::test]
+async fn test_fragmented_text_with_partial_utf8() {
+  let (client, server) = tokio::io::duplex(1024);
+
+  let server_task = tokio::spawn(async move {
+    handle_server(server).await.unwrap();
+  });
+  let client_task = tokio::spawn(async move {
+    handle_client(client).await.unwrap();
+  });
+
+  server_task.await.unwrap();
+  client_task.await.unwrap();
+}
+
+async fn handle_server(
+  stream: DuplexStream,
+) -> Result<(), Box<dyn std::error::Error>> {
+  let ws = WebSocket::after_handshake(stream, Role::Server);
+  let mut ws = FragmentCollector::new(ws);
+
+  let frame = ws.read_frame().await?;
+  assert_eq!(frame.opcode, OpCode::Text);
+  assert_eq!(frame.fin, true);
+  let text = std::str::from_utf8(&frame.payload)?;
+  assert_eq!(text, "Hello 😀!");
+
+  Ok(())
+}
+
+async fn handle_client(
+  mut stream: DuplexStream,
+) -> Result<(), Box<dyn std::error::Error>> {
+  // "Hello 😀!" where 😀 is U+1F600 (4 bytes in UTF-8: F0 9F 98 80)
+
+  // Frame 1: "Hello " + first 2 bytes of emoji (fin=false)
+  let mut frame1_payload = b"Hello ".to_vec();
+  frame1_payload.extend_from_slice(&[0xF0, 0x9F]); // First 2 bytes of 😀
+  let frame1 = create_raw_frame(false, OpCode::Text, &frame1_payload);
+  stream.write_all(&frame1).await?;
+
+  // Frame 2: last 2 bytes of emoji + "!" (fin=true, continuation)
+  let mut frame2_payload = vec![0x98, 0x80]; // Last 2 bytes of 😀
+  frame2_payload.extend_from_slice(b"!");
+  let frame2 = create_raw_frame(true, OpCode::Continuation, &frame2_payload);
+  stream.write_all(&frame2).await?;
+
+  Ok(())
+}
+
+fn create_raw_frame(fin: bool, opcode: OpCode, payload: &[u8]) -> Vec<u8> {
+  let mut frame = Vec::new();
+
+  // First byte: FIN + opcode
+  let first_byte = if fin { 0x80 } else { 0x00 } | (opcode as u8);
+  frame.push(first_byte);
+  // Second byte: MASK bit (0 for server->client) + payload length
+  let len = payload.len();
+  if len < 126 {
+    frame.push(len as u8);
+  } else if len < 65536 {
+    frame.push(126);
+    frame.extend_from_slice(&(len as u16).to_be_bytes());
+  } else {
+    frame.push(127);
+    frame.extend_from_slice(&(len as u64).to_be_bytes());
+  }
+  frame.extend_from_slice(payload);
+
+  frame
+}
+
+#[tokio::test]
+async fn test_low_level_fragmented_text_with_partial_utf8() {
+  // Test that the low-level WebSocket API doesn't validate UTF-8 on individual frames
+  let (client, server) = tokio::io::duplex(1024);
+
+  let server_task = tokio::spawn(async move {
+    handle_server_low_level(server).await.unwrap();
+  });
+
+  let client_task = tokio::spawn(async move {
+    handle_client(client).await.unwrap();
+  });
+
+  server_task.await.unwrap();
+  client_task.await.unwrap();
+}
+
+async fn handle_server_low_level(
+  stream: DuplexStream,
+) -> Result<(), Box<dyn std::error::Error>> {
+  let mut ws = WebSocket::after_handshake(stream, Role::Server);
+
+  // should succeed even though it contains partial UTF-8
+  let frame1 = ws.read_frame().await?;
+  assert_eq!(frame1.opcode, OpCode::Text);
+  assert_eq!(frame1.fin, false);
+
+  // should succeed even though it starts with partial UTF-8
+  let frame2 = ws.read_frame().await?;
+  assert_eq!(frame2.opcode, OpCode::Continuation);
+  assert_eq!(frame2.fin, true);
+
+  // When combined, they should form valid UTF-8
+  let mut combined = frame1.payload.to_vec();
+  combined.extend_from_slice(&frame2.payload);
+  let text = std::str::from_utf8(&combined)?;
+  assert_eq!(text, "Hello 😀!");
+
+  Ok(())
+}
+
+#[tokio::test]
+async fn test_invalid_unfragmented_utf8() {
+  // Test that FragmentCollector rejects unfragmented text with invalid UTF-8
+  // This corresponds to Autobahn test case 6.3.1
+  let (client, server) = tokio::io::duplex(1024);
+
+  let server_task = tokio::spawn(async move {
+    let ws = WebSocket::after_handshake(server, Role::Server);
+    let mut ws = FragmentCollector::new(ws);
+
+    // Should fail with InvalidUTF8 error
+    let result = ws.read_frame().await;
+    assert!(result.is_err());
+    match result {
+      Err(fastwebsockets::WebSocketError::InvalidUTF8) => {}
+      _ => panic!("Expected InvalidUTF8 error"),
+    }
+  });
+
+  let client_task = tokio::spawn(async move {
+    let mut stream = client;
+    // Send invalid UTF-8: κόσμε���edited (from Autobahn test 6.3.1)
+    // Hex: cebae1bdb9cf83cebcceb5eda080656469746564
+    let invalid_utf8 = vec![
+      0xce, 0xba, 0xe1, 0xbd, 0xb9, 0xcf, 0x83, 0xce, 0xbc, 0xce, 0xb5, 0xed,
+      0xa0, 0x80, 0x65, 0x64, 0x69, 0x74, 0x65, 0x64,
+    ];
+    let frame = create_raw_frame(true, OpCode::Text, &invalid_utf8);
+    stream.write_all(&frame).await.unwrap();
+  });
+
+  server_task.await.unwrap();
+  client_task.await.unwrap();
+}