fix(plugins): Avoid double device control releases

This was not really broken, but it generated a debug log message
containing the keyword 'error' consistently, which is convoluting any
debugging session.

This commit also adds some trace log message on plugin device control
API calls.

fairmq/Plugin.h

@@ -79,7 +79,12 @@ class Plugin
     auto GetCurrentDeviceState() const -> DeviceState { return fPluginServices->GetCurrentDeviceState(); }
     auto TakeDeviceControl() -> void { fPluginServices->TakeDeviceControl(fkName); };
     auto StealDeviceControl() -> void { fPluginServices->StealDeviceControl(fkName); };
-    auto ReleaseDeviceControl() -> void { fPluginServices->ReleaseDeviceControl(fkName); };
+    auto ReleaseDeviceControl() -> void
+    {
+        if (fPluginServices->GetDeviceController() == fkName) {
+            fPluginServices->ReleaseDeviceControl(fkName);
+        }
+    };
     auto ChangeDeviceState(const DeviceStateTransition next) -> bool { return fPluginServices->ChangeDeviceState(fkName, next); }
     auto SubscribeToDeviceStateChange(std::function<void(DeviceState)> callback) -> void { fPluginServices->SubscribeToDeviceStateChange(fkName, callback); }
     auto UnsubscribeFromDeviceStateChange() -> void { fPluginServices->UnsubscribeFromDeviceStateChange(fkName); }

fairmq/PluginServices.cxx

@@ -6,6 +6,7 @@
  *                  copied verbatim in the file "LICENSE"                       *
  ********************************************************************************/
 
+#include <fairlogger/Logger.h>
 #include <fairmq/PluginServices.h>
 #include <fairmq/tools/Strings.h>
 
@@ -34,8 +35,10 @@ auto PluginServices::TakeDeviceControl(const string& controller) -> void
 
     if (!fDeviceController) {
         fDeviceController = controller;
+        LOG(trace) << "Plugin '" << controller << "' took over control.";
     } else if (fDeviceController == controller) {
         // nothing to do
+        LOG(trace) << "Plugin '" << controller << "' is already in control.";
     } else {
         throw DeviceControlError{tools::ToString(
             "Plugin '", controller, "' is not allowed to take over control. ",
@@ -49,6 +52,7 @@ auto PluginServices::StealDeviceControl(const string& controller) -> void
     lock_guard<mutex> lock{fDeviceControllerMutex};
 
     fDeviceController = controller;
+    LOG(trace) << "Plugin '" << controller << "' steals control!";
 }
 
 auto PluginServices::ReleaseDeviceControl(const string& controller) -> void
@@ -58,6 +62,7 @@ auto PluginServices::ReleaseDeviceControl(const string& controller) -> void
 
         if (fDeviceController == controller) {
             fDeviceController = boost::none;
+            LOG(trace) << "Plugin '" << controller << "' releases control.";
         } else {
             LOG(debug) << "Plugin '" << controller << "' cannot release control "
                        << "because it has no control.";