control: close the connection on malformed input before auth (#1259)

control::serve documents that the first request on a connection must be
`auth` and that anything else closes the connection. A first line that
wasn't valid JSON got a parse error but left the connection open, unlike a
wrong token or a non-auth request. Close it in that case too; after a
successful auth a malformed line still just gets the parse error.

Adds control::transport_tests::malformed_input_before_auth_closes_the_connection,
which fails before this change.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/control.rs

@@ -788,7 +788,8 @@ pub fn serve(client: ControlClient) -> std::io::Result<Endpoint> {
                         continue;
                     }
                     let (reply, close) = match serde_json::from_str::<Value>(&line) {
-                        Err(e) => (rpc_error(Value::Null, -32700, &format!("parse error: {e}")), false),
+                        // Before `auth`, like any other first request, this closes the connection.
+                        Err(e) => (rpc_error(Value::Null, -32700, &format!("parse error: {e}")), !authed),
                         Ok(msg) => {
                             let id = msg.get("id").cloned().unwrap_or(Value::Null);
                             let method = msg.get("method").and_then(Value::as_str).unwrap_or("");
@@ -839,3 +840,31 @@ fn random_token() -> std::io::Result<String> {
     getrandom::fill(&mut bytes).map_err(|e| std::io::Error::other(e.to_string()))?;
     Ok(bytes.iter().map(|b| format!("{b:02x}")).collect())
 }
+
+#[cfg(test)]
+mod transport_tests {
+    /// Before authenticating, a line that isn't JSON gets a parse error and closes the
+    /// connection, as [`super::serve`] documents for any first request other than `auth`.
+    #[cfg(not(target_arch = "wasm32"))]
+    #[test]
+    fn malformed_input_before_auth_closes_the_connection() {
+        use std::io::{BufRead, BufReader, ErrorKind, Write};
+        let (_control, client) = super::attach(&egui::Context::default());
+        let ep = super::serve(client).unwrap();
+        let s = std::net::TcpStream::connect(("127.0.0.1", ep.port)).unwrap();
+        s.set_read_timeout(Some(std::time::Duration::from_secs(5))).unwrap();
+        writeln!(s.try_clone().unwrap(), "not json").unwrap();
+        let mut r = BufReader::new(s);
+        let mut first = String::new();
+        r.read_line(&mut first).unwrap();
+        let reply: serde_json::Value = serde_json::from_str(&first).unwrap();
+        assert_eq!(reply["error"]["code"], -32700, "{first}");
+        // Closed: end of stream (or a reset), not a connection left waiting for more input.
+        let mut next = String::new();
+        let closed = match r.read_line(&mut next) {
+            Ok(n) => n == 0,
+            Err(e) => matches!(e.kind(), ErrorKind::ConnectionReset | ErrorKind::ConnectionAborted),
+        };
+        assert!(closed, "the connection stayed open: {next:?}");
+    }
+}