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.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
 	}
 };