feat(ServerApplication): termination callback #4643 (#4733)

* feat(ServerApplication): termination callback #4643

* chore(ServerApplication): spelling fix #4643

* chore(ServerApplication): correct documentation #4643

Util/include/Poco/Util/ServerApplication.h

@@ -24,6 +24,7 @@
 #if defined(POCO_OS_FAMILY_WINDOWS)
 #include "Poco/NamedEvent.h"
 #endif
+#include <functional>
 
 
 namespace Poco {
@@ -122,8 +123,20 @@ class Util_API ServerApplication: public Application
 	/// --pidfile=/var/run/sample.pid) may be useful to record the process ID of
 	/// the daemon in a file. The PID file will be removed when the daemon process
 	/// terminates (but not, if it crashes).
+	/// 
+	/// An application can register a callback to be called at termination time.
+	/// An example of the termination callback registration at some point
+	/// during the ServerApplication initialization time:
+	///
+	///     auto tCB = [](const std::string& message)
+	///     {
+	///         std::cout << message << std::endl;
+	///     };
+	///     ServerApplication::registerTerminateCallback(tCB, "custom termination message"s);
 {
 public:
+	using TerminateCallback = std::function<void(const std::string&)>;
+
 	ServerApplication();
 		/// Creates the ServerApplication.
 
@@ -158,6 +171,12 @@ public:
 		/// waitForTerminationRequest(), this method will return
 		/// and the application can shut down.
 
+	static void registerTerminateCallback(TerminateCallback tCB,
+		const std::string& message = _terminateMessage);
+		/// Registers a termination callback.
+		/// Used to register a function to be executed when the system
+		/// shutdown starts.
+
 protected:
 	int run();
 	void waitForTerminationRequest();
@@ -207,6 +226,11 @@ private:
 	static SERVICE_STATUS_HANDLE _serviceStatusHandle;
 	static Poco::NamedEvent      _terminate;
 #endif
+
+	static void terminateCallback();
+	inline static std::atomic<bool> _terminationGuard = false;
+	inline static TerminateCallback _terminateCallback = nullptr;
+	inline static std::string       _terminateMessage = "System terminating now!";
 };
 
 

Util/src/ServerApplication.cpp

@@ -91,8 +91,22 @@ int ServerApplication::run()
 }
 
 
+void ServerApplication::terminateCallback()
+{
+	if (!_terminationGuard.exchange(true))
+	{
+		if (_terminateCallback)
+		{
+			_terminateCallback(_terminateMessage);
+			_terminateCallback = nullptr;
+		}
+	}
+}
+
+
 void ServerApplication::terminate()
 {
+	terminateCallback();
 #if defined(POCO_OS_FAMILY_WINDOWS)
 	_terminate.set();
 #elif defined(POCO_VXWORKS) || POCO_OS == POCO_OS_ANDROID
@@ -103,6 +117,13 @@ void ServerApplication::terminate()
 }
 
 
+void ServerApplication::registerTerminateCallback(TerminateCallback tCB, const std::string& message)
+{
+	_terminateCallback = tCB;
+	_terminateMessage = message;
+}
+
+
 #if defined(POCO_OS_FAMILY_WINDOWS)
 
 
@@ -196,6 +217,7 @@ void ServerApplication::waitForTerminationRequest()
 {
 	SetConsoleCtrlHandler(ConsoleCtrlHandler, TRUE);
 	_terminate.wait();
+	terminateCallback();
 	_terminated.set();
 }
 
@@ -444,6 +466,7 @@ void ServerApplication::handleStartup(const std::string& name, const std::string
 void ServerApplication::waitForTerminationRequest()
 {
 	_terminate.wait();
+	terminateCallback();
 }
 
 
@@ -503,8 +526,10 @@ void ServerApplication::waitForTerminationRequest()
 	sigprocmask(SIG_BLOCK, &sset, NULL);
 	int sig;
 	sigwait(&sset, &sig);
+	terminateCallback();
 #else // POCO_OS != POCO_OS_ANDROID
 	_terminate.wait();
+	terminateCallback();
 #endif
 }