event/Loop: add compile-time option to disable multithreading

Not for MPD, but for other applications which might want to copy its
event loop, but do not need multi-threading.

src/event/Loop.cxx

@@ -37,18 +37,27 @@ EventLoop::TimerCompare::operator()(const TimerEvent &a,
 	return a.due < b.due;
 }
 
-EventLoop::EventLoop(ThreadId _thread)
+EventLoop::EventLoop(
-	:SocketMonitor(*this),
+#ifdef HAVE_THREADED_EVENT_LOOP
+		     ThreadId _thread
+#endif
+		     )
+	:
+#ifdef HAVE_THREADED_EVENT_LOOP
+	SocketMonitor(*this),
+	 thread(_thread),
 	 /* if this instance is hosted by an EventThread (no ThreadId
 	    known yet) then we're not yet alive until the thread is
 	    started; for the main EventLoop instance, we assume it's
 	    already alive, because nobody but EventThread will call
 	    SetAlive() */
 	 alive(!_thread.IsNull()),
-	 quit(false),
+#endif
-	 thread(_thread)
+	 quit(false)
 {
+#ifdef HAVE_THREADED_EVENT_LOOP
 	SocketMonitor::Open(SocketDescriptor(wake_fd.Get()));
+#endif
 }
 
 EventLoop::~EventLoop() noexcept
@@ -83,13 +92,17 @@ EventLoop::Break() noexcept
 	if (quit.exchange(true))
 		return;
 
+#ifdef HAVE_THREADED_EVENT_LOOP
 	wake_fd.Write();
+#endif
 }
 
 bool
 EventLoop::Abandon(int _fd)  noexcept
 {
+#ifdef HAVE_THREADED_EVENT_LOOP
 	assert(!IsAlive() || IsInside());
+#endif
 
 	return poll_group.Abandon(_fd);
 }
@@ -97,7 +110,9 @@ EventLoop::Abandon(int _fd)  noexcept
 bool
 EventLoop::RemoveFD(int _fd) noexcept
 {
+#ifdef HAVE_THREADED_EVENT_LOOP
 	assert(!IsAlive() || IsInside());
+#endif
 
 	return poll_group.Remove(_fd);
 }
@@ -169,15 +184,19 @@ ExportTimeoutMS(Event::Duration timeout)
 void
 EventLoop::Run() noexcept
 {
+#ifdef HAVE_THREADED_EVENT_LOOP
 	if (thread.IsNull())
 		thread = ThreadId::GetCurrent();
+#endif
 
 	assert(IsInside());
 	assert(!quit);
+#ifdef HAVE_THREADED_EVENT_LOOP
 	assert(alive);
 	assert(busy);
 
 	SocketMonitor::Schedule(SocketMonitor::READ);
+#endif
 	AtScopeExit(this) {
 #ifdef HAVE_URING
 		/* make sure that the Uring::Manager gets destructed
@@ -188,7 +207,9 @@ EventLoop::Run() noexcept
 		uring_initialized = false;
 #endif
 
+#ifdef HAVE_THREADED_EVENT_LOOP
 		SocketMonitor::Cancel();
+#endif
 	};
 
 	do {
@@ -212,6 +233,7 @@ EventLoop::Run() noexcept
 				return;
 		}
 
+#ifdef HAVE_THREADED_EVENT_LOOP
 		/* try to handle DeferEvents without WakeFD
 		   overhead */
 		{
@@ -225,6 +247,7 @@ EventLoop::Run() noexcept
 				   new timeout */
 				continue;
 		}
+#endif
 
 		/* wait for new event */
 
@@ -240,10 +263,12 @@ EventLoop::Run() noexcept
 
 		now = std::chrono::steady_clock::now();
 
+#ifdef HAVE_THREADED_EVENT_LOOP
 		{
 			const std::lock_guard<Mutex> lock(mutex);
 			busy = true;
 		}
+#endif
 
 		/* invoke sockets */
 		while (!ready_sockets.empty() && !quit) {
@@ -254,11 +279,15 @@ EventLoop::Run() noexcept
 		}
 	} while (!quit);
 
+#ifdef HAVE_THREADED_EVENT_LOOP
 #ifndef NDEBUG
 	assert(thread.IsInside());
 #endif
+#endif
 }
 
+#ifdef HAVE_THREADED_EVENT_LOOP
+
 void
 EventLoop::AddDeferred(DeferEvent &d) noexcept
 {
@@ -316,3 +345,5 @@ EventLoop::OnSocketReady([[maybe_unused]] unsigned flags) noexcept
 
 	return true;
 }
+
+#endif

src/event/Loop.hxx

@@ -22,11 +22,15 @@
 
 #include "Chrono.hxx"
 #include "PollGroup.hxx"
-#include "WakeFD.hxx"
 #include "SocketMonitor.hxx"
+#include "event/Features.h"
+#include "util/Compiler.h"
+
+#ifdef HAVE_THREADED_EVENT_LOOP
+#include "WakeFD.hxx"
 #include "thread/Id.hxx"
 #include "thread/Mutex.hxx"
-#include "util/Compiler.h"
+#endif
 
 #include <boost/intrusive/set.hpp>
 #include <boost/intrusive/list.hpp>
@@ -54,9 +58,14 @@ class DeferEvent;
  *
  * @see SocketMonitor, MultiSocketMonitor, TimerEvent, IdleEvent
  */
-class EventLoop final : SocketMonitor
+class EventLoop final
+#ifdef HAVE_THREADED_EVENT_LOOP
+	: SocketMonitor
+#endif
 {
+#ifdef HAVE_THREADED_EVENT_LOOP
 	WakeFD wake_fd;
+#endif
 
 	struct TimerCompare {
 		constexpr bool operator()(const TimerEvent &a,
@@ -76,6 +85,7 @@ class EventLoop final : SocketMonitor
 				       boost::intrusive::constant_time_size<false>>;
 	IdleList idle;
 
+#ifdef HAVE_THREADED_EVENT_LOOP
 	Mutex mutex;
 
 	using DeferredList =
@@ -83,6 +93,7 @@ class EventLoop final : SocketMonitor
 				       boost::intrusive::base_hook<boost::intrusive::list_base_hook<>>,
 				       boost::intrusive::constant_time_size<false>>;
 	DeferredList deferred;
+#endif
 
 	using ReadySocketList =
 		boost::intrusive::list<SocketMonitor,
@@ -103,6 +114,12 @@ class EventLoop final : SocketMonitor
 
 	Event::Clock::time_point now = Event::Clock::now();
 
+#ifdef HAVE_THREADED_EVENT_LOOP
+	/**
+	 * A reference to the thread that is currently inside Run().
+	 */
+	ThreadId thread = ThreadId::Null();
+
 	/**
 	 * Is this #EventLoop alive, i.e. can events be scheduled?
 	 * This is used by BlockingCall() to determine whether
@@ -110,6 +127,7 @@ class EventLoop final : SocketMonitor
 	 * there's no #EventThread yet/anymore).
 	 */
 	bool alive;
+#endif
 
 	std::atomic_bool quit;
 
@@ -119,6 +137,7 @@ class EventLoop final : SocketMonitor
 	 */
 	bool again;
 
+#ifdef HAVE_THREADED_EVENT_LOOP
 	/**
 	 * True when handling callbacks, false when waiting for I/O or
 	 * timeout.
@@ -126,6 +145,7 @@ class EventLoop final : SocketMonitor
 	 * Protected with #mutex.
 	 */
 	bool busy = true;
+#endif
 
 #ifdef HAVE_URING
 	bool uring_initialized = false;
@@ -133,18 +153,17 @@ class EventLoop final : SocketMonitor
 
 	PollGroup poll_group;
 
-	/**
-	 * A reference to the thread that is currently inside Run().
-	 */
-	ThreadId thread = ThreadId::Null();
-
 public:
 	/**
 	 * Throws on error.
 	 */
+#ifdef HAVE_THREADED_EVENT_LOOP
 	explicit EventLoop(ThreadId _thread);
 
 	EventLoop():EventLoop(ThreadId::GetCurrent()) {}
+#else
+	EventLoop();
+#endif
 
 	~EventLoop() noexcept;
 
@@ -152,7 +171,9 @@ public:
 	 * A caching wrapper for Event::Clock::now().
 	 */
 	auto GetTime() const {
+#ifdef HAVE_THREADED_EVENT_LOOP
 		assert(IsInside());
+#endif
 
 		return now;
 	}
@@ -170,13 +191,17 @@ public:
 	void Break() noexcept;
 
 	bool AddFD(int _fd, unsigned flags, SocketMonitor &m) noexcept {
+#ifdef HAVE_THREADED_EVENT_LOOP
 		assert(!IsAlive() || IsInside());
+#endif
 
 		return poll_group.Add(_fd, flags, &m);
 	}
 
 	bool ModifyFD(int _fd, unsigned flags, SocketMonitor &m) noexcept {
+#ifdef HAVE_THREADED_EVENT_LOOP
 		assert(!IsAlive() || IsInside());
+#endif
 
 		return poll_group.Modify(_fd, flags, &m);
 	}
@@ -195,6 +220,7 @@ public:
 
 	void AddTimer(TimerEvent &t, Event::Duration d) noexcept;
 
+#ifdef HAVE_THREADED_EVENT_LOOP
 	/**
 	 * Schedule a call to DeferEvent::RunDeferred().
 	 *
@@ -209,6 +235,7 @@ public:
 	 * This method is thread-safe.
 	 */
 	void RemoveDeferred(DeferEvent &d) noexcept;
+#endif
 
 	/**
 	 * The main function of this class.  It will loop until
@@ -217,12 +244,14 @@ public:
 	void Run() noexcept;
 
 private:
+#ifdef HAVE_THREADED_EVENT_LOOP
 	/**
 	 * Invoke all pending DeferEvents.
 	 *
 	 * Caller must lock the mutex.
 	 */
 	void HandleDeferred() noexcept;
+#endif
 
 	/**
 	 * Invoke all expired #TimerEvent instances and return the
@@ -231,9 +260,12 @@ private:
 	 */
 	Event::Duration HandleTimers() noexcept;
 
+#ifdef HAVE_THREADED_EVENT_LOOP
 	bool OnSocketReady(unsigned flags) noexcept override;
+#endif
 
 public:
+#ifdef HAVE_THREADED_EVENT_LOOP
 	void SetAlive(bool _alive) noexcept {
 		alive = _alive;
 	}
@@ -241,13 +273,18 @@ public:
 	bool IsAlive() const noexcept {
 		return alive;
 	}
+#endif
 
 	/**
 	 * Are we currently running inside this EventLoop's thread?
 	 */
 	gcc_pure
 	bool IsInside() const noexcept {
+#ifdef HAVE_THREADED_EVENT_LOOP
 		return thread.IsInside();
+#else
+		return true;
+#endif
 	}
 };
 

src/event/meson.build

@@ -2,6 +2,7 @@ event_features = configuration_data()
 event_features.set('USE_EVENTFD', is_linux and get_option('eventfd'))
 event_features.set('USE_SIGNALFD', is_linux and get_option('signalfd'))
 event_features.set('USE_EPOLL', is_linux and get_option('epoll'))
+event_features.set('HAVE_THREADED_EVENT_LOOP', true)
 configure_file(output: 'Features.h', configuration: event_features)
 
 event_sources = []