feat(rumqttc): only expect "mqtt" in response header (#729)

Author: Devdutt Shenoi

rumqttc/src/websockets.rs

@@ -23,22 +23,17 @@ pub enum ValidationError {
 pub(crate) fn validate_response_headers(
     response: Response<Option<Vec<u8>>>,
 ) -> Result<(), ValidationError> {
-    let val = response
+    let subprotocol = response
         .headers()
         .get("Sec-WebSocket-Protocol")
-        .ok_or(ValidationError::SubprotocolHeaderMissing)?;
+        .ok_or(ValidationError::SubprotocolHeaderMissing)?
+        .to_str()?;
 
-    let subprotocols = val.to_str()?;
-
-    // In Sec-WebSocket-Protocol header
-    // multiple subprotocols can be listed in a comma-delimited format
-    // e.g. Sec-WebSocket-Protocol: mqtt, chat
-    if !subprotocols
-        .split(',')
-        .any(|protocol| protocol.trim() == "mqtt")
-    {
+    // Server must respond with Sec-WebSocket-Protocol header value of "mqtt"
+    // https://http.dev/ws#sec-websocket-protocol
+    if subprotocol.trim() != "mqtt" {
         return Err(ValidationError::SubprotocolMqttMissing(
-            subprotocols.to_owned(),
+            subprotocol.to_owned(),
         ));
     }