Load saved LAN printers when the device manager first gets an agent

When the network plugin is not yet installed at startup, on_init_network builds
the DeviceManager without an agent, so its constructor skips loading the persisted
LAN printers. After the plugin is installed and the network stack hot-reloads, the
manager is reused via set_agent(), which never loaded them - so previously paired
printers stayed missing from the device list until an app restart. Load them once a
real agent first arrives.

src/slic3r/GUI/DeviceCore/DevManager.cpp

@@ -24,12 +24,23 @@ namespace Slic3r
 
         DevPrinterConfigUtil::InitFilePath(resources_dir());
 
-        // Load saved local machines
+        // Load saved local machines (needs an agent; when built without one the load is
+        // deferred to set_agent()).
         if (agent) {
+            load_local_machines_from_config();
+        }
+    }
+
+    void DeviceManager::load_local_machines_from_config()
+    {
         AppConfig* config = GUI::wxGetApp().app_config;
-            const auto local_machines = config->get_local_machines();
+        if (!config)
+            return;
+        const auto& local_machines = config->get_local_machines();
         for (auto& it : local_machines) {
             const auto& m = it.second;
+            if (localMachineList.count(m.dev_id))
+                continue;
             MachineObject* obj       = new MachineObject(this, m_agent, m.dev_name, m.dev_id, m.dev_ip);
             obj->printer_type        = m.printer_type;
             obj->dev_connection_type = "lan";
@@ -47,7 +58,6 @@ namespace Slic3r
             }
         }
     }
-    }
 
     void DeviceManager::update_local_machine(const MachineObject& m)
     {
@@ -98,6 +108,11 @@ namespace Slic3r
         BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ": updating agent for "
                                 << localMachineList.size() << " local and "
                                 << userMachineList.size() << " user machines";
+
+        // First real agent after an agent-less construction (network plugin wasn't ready at
+        // startup): run the persisted-LAN-printer load the constructor had to skip. See
+        // load_local_machines_from_config().
+        const bool first_real_agent = (m_agent == nullptr && agent != nullptr);
         m_agent = agent;
 
         std::lock_guard<std::mutex> lock(listMutex);
@@ -111,6 +126,10 @@ namespace Slic3r
                 it.second->set_agent(agent);
             }
         }
+
+        if (first_real_agent) {
+            load_local_machines_from_config();
+        }
     }
 
     void DeviceManager::EnableMultiMachine(bool enable)

src/slic3r/GUI/DeviceCore/DevManager.h

@@ -96,6 +96,13 @@ public:
     std::map<std::string, std::vector<std::string>> device_subseries;
 
 private:
+    // Load the LAN printers persisted in AppConfig into localMachineList. Runs from the
+    // constructor when an agent is available and, for the case where the DeviceManager was
+    // first built without one (network plugin not yet installed at startup), from set_agent()
+    // once a real agent finally arrives - so paired printers survive a plugin install/hot
+    // reload without an app restart.
+    void load_local_machines_from_config();
+
     void keep_alive();
     void check_pushing();