usbdmx: Move widget deletion to main thread

Currently when a usb device is remove the libusb event thread, though the
hotplug handler, will cause the widget's Device to be deleted in the main
thread and wait for this to complete in the libusb thread.

If any of the destructors triggered by device deletion causes libusb_close
to be called this causes a deadlock as libusb_close will try to take over
handling of events in the main thread while the libusb thread is waiting
for libusb_close, via the destructors, to finish.

Just move both widget and device deletion to the main thread to prevent
this scenario.

Author: DanielG

plugins/usbdmx/AsyncPluginImpl.cpp

@@ -264,16 +264,9 @@ void AsyncPluginImpl::DeviceEvent(HotplugAgent::EventType event,
     // Sunlite plugin, if we make the f/w load async we'll need to let the
     // factory cancel the load.
 
-    // Unregister & delete the device in the main thread.
-    if (state->ola_device) {
-      Future<void> f;
-      m_plugin_adaptor->Execute(
-          NewSingleCallback(this, &AsyncPluginImpl::ShutdownDevice,
-                            state->ola_device, &f));
-      f.Get();
-      state->ola_device = NULL;
-    }
-    state->DeleteWidget();
+    // Unregister & delete the device and widget in the main thread.
+    m_plugin_adaptor->Execute(
+        NewSingleCallback(this, &AsyncPluginImpl::ShutdownDeviceState, state));
   }
 }
 
@@ -353,17 +346,26 @@ bool AsyncPluginImpl::StartAndRegisterDevice(Widget *widget, Device *device) {
 
 /*
  * @brief Signal widget removal.
- * @param device_id The device id to remove.
+ * @param state The DeviceState owning the widget to be removed.
  *
- * This is run within the main thread.
+ * This must be run within the main thread. The Device destructor below may
+ * cause libusb_close() to be called, which would deadlock if the hotplug
+ * event thread were to wait for it, say in AsyncPluginImpl::DeviceEvent().
  */
-void AsyncPluginImpl::ShutdownDevice(Device *device, Future<void> *f) {
-  m_plugin_adaptor->UnregisterDevice(device);
-  device->Stop();
-  delete device;
-  if (f) {
-    f->Set();
+void AsyncPluginImpl::ShutdownDeviceState(DeviceState *state) {
+  if (state->ola_device) {
+    Device *device = state->ola_device;
+    m_plugin_adaptor->UnregisterDevice(device);
+    device->Stop();
+    delete device;
+    state->ola_device = NULL;
+  } else {
+          /* This case can be legitimate when the widget setup through the
+           * widget factory is delayed and an unplug event happens before
+           * that is completed. */
+          OLA_DEBUG << "ola_device was NULL at shutdown";
   }
+  state->DeleteWidget();
 }
 }  // namespace usbdmx
 }  // namespace plugin

plugins/usbdmx/AsyncPluginImpl.h

@@ -37,7 +37,6 @@
 #include "libs/usb/HotplugAgent.h"
 
 #include "ola/base/Macro.h"
-#include "ola/thread/Future.h"
 #include "olad/Preferences.h"
 #include "plugins/usbdmx/PluginImplInterface.h"
 #include "plugins/usbdmx/SynchronizedWidgetObserver.h"
@@ -111,7 +110,7 @@ class AsyncPluginImpl: public PluginImplInterface, public WidgetObserver {
   template <typename Widget>
   bool StartAndRegisterDevice(Widget *widget, Device *device);
 
-  void ShutdownDevice(Device *device, ola::thread::Future<void> *f);
+  void ShutdownDeviceState(DeviceState *state);
 
   DISALLOW_COPY_AND_ASSIGN(AsyncPluginImpl);
 };