Fix MySQL error 'Transactions couldn't be nested' (closes #1271)

crates/store/src/backend/mysql/write.rs

@@ -318,7 +318,13 @@ impl MysqlStore {
             }
         }
 
-        trx.commit().await.map(|_| result).map_err(Into::into)
+        match trx.commit().await {
+            Ok(_) => Ok(result),
+            Err(err) => {
+                let _ = conn.query_drop("ROLLBACK;").await;
+                Err(err.into())
+            }
+        }
     }
 
     pub(crate) async fn purge_store(&self) -> trc::Result<()> {