Add mutex to serialize concurrent Update calls

src/dbus/query.rs

 use std::sync::Arc;
-use tokio::sync::RwLock;
+use tokio::sync::{Mutex, RwLock};
 use zbus::{interface, Connection};
 
 use crate::config::Config;
@@ -14,11 +14,12 @@ fn to_json<T: serde::Serialize>(data: &T) -> zbus::fdo::Result<String> {
 pub struct QueryInterface {
     config: Arc<RwLock<Config>>,
     connection: Connection,
+    update_lock: Mutex<()>,
 }
 
 impl QueryInterface {
     pub fn new(config: Arc<RwLock<Config>>, connection: Connection) -> Self {
-        Self { config, connection }
+        Self { config, connection, update_lock: Mutex::new(()) }
     }
 }
 
@@ -105,6 +106,8 @@ impl QueryInterface {
     }
 
     async fn update(&self, transaction: String) -> zbus::fdo::Result<String> {
+        let _guard = self.update_lock.lock().await;
+
         let config = self.config.read().await;
         let signal_emitter = SignalEmitter::new(self.connection.clone());
         let tx = if transaction.is_empty() {