fix(mqtt): mask the broker password in native error lines

aimdb-mqtt-connector/src/native.rs

@@ -60,7 +60,7 @@ pub(crate) fn build<'a>(
         })?;
 
         let futures: Vec<BoxFuture> = vec![
-            Box::pin(run_event_loop(event_loop, inbound, broker_url.to_string())),
+            Box::pin(run_event_loop(event_loop, inbound, redacted(broker_url))),
             Box::pin(run_publish_loop(client, outbound, opts)),
         ];
         Ok(futures)
@@ -91,8 +91,10 @@ impl MqttConnectorImpl {
             url = format!("{}/dummy", url.trim_end_matches('/'));
         }
 
-        let connector_url =
-            ConnectorUrl::parse(&url).map_err(|e| format!("Invalid MQTT URL: {}", e))?;
+        let connector_url = ConnectorUrl::parse(&url).map_err(|e| {
+            // The parse error quotes its input, password included.
+            format!("Invalid MQTT URL: {}", e).replace(&url, &redacted(&url))
+        })?;
 
         let host = connector_url.host.clone();
         let port = connector_url.port.unwrap_or_else(|| {
@@ -254,6 +256,26 @@ async fn run_publish_loop(
     log_info!("MQTT publish loop: every outbound route has closed");
 }
 
+/// `url` with the password in its authority masked, as `ConnectorUrl`'s
+/// `Display` masks it: `mqtt://user:****@host:1883`. Safe for logs and errors.
+/// A URL missing its scheme (the one input `ConnectorUrl::parse` rejects, and
+/// quotes) is masked the same way.
+fn redacted(url: &str) -> String {
+    let (prefix, rest) = match url.split_once("://") {
+        Some((_, rest)) => (url.len() - rest.len(), rest),
+        None => (0, url),
+    };
+    let authority_end = rest.find('/').unwrap_or(rest.len());
+    let (authority, path) = rest.split_at(authority_end);
+    let Some((userinfo, host)) = authority.rsplit_once('@') else {
+        return url.to_string();
+    };
+    match userinfo.split_once(':') {
+        Some((user, _password)) => format!("{}{user}:****@{host}{path}", &url[..prefix]),
+        None => url.to_string(),
+    }
+}
+
 /// The TLS configuration for `mqtts://`, from whichever backend this build
 /// selected.
 #[cfg(feature = "tokio-native-tls")]
@@ -420,6 +442,42 @@ mod tests {
         }
     }
 
+    #[test]
+    fn redacted_masks_only_the_password() {
+        assert_eq!(
+            redacted("mqtt://aimdb:s3cret@localhost:1884"),
+            "mqtt://aimdb:****@localhost:1884"
+        );
+        assert_eq!(
+            redacted("mqtts://user:p@ss:w0rd@broker.example.com:8883/x"),
+            "mqtts://user:****@broker.example.com:8883/x"
+        );
+        // Without a scheme, as a mistyped URL arrives.
+        assert_eq!(
+            redacted("user:s3cret@localhost:1884"),
+            "user:****@localhost:1884"
+        );
+        // Nothing to mask: unchanged.
+        for url in [
+            "mqtt://localhost:1883",
+            "mqtt://user@localhost:1883",
+            "not-a-url",
+        ] {
+            assert_eq!(redacted(url), url);
+        }
+    }
+
+    #[tokio::test]
+    async fn an_invalid_url_error_does_not_repeat_the_password() {
+        let Err(err) =
+            MqttConnectorImpl::build_internal("user:s3cret@localhost:1884", None, None, 60, &[])
+                .await
+        else {
+            panic!("a URL without a scheme must not build");
+        };
+        assert!(!err.contains("s3cret"), "{err}");
+    }
+
     /// The plain scheme is unaffected by which backend, if any, is selected.
     #[tokio::test]
     async fn test_connector_mqtt_url_needs_no_tls_backend() {