event/Loop: use std::atomic_bool for the "quit" variable

Fixes thread sanitizer warnings.

src/event/Loop.cxx

@@ -25,7 +25,7 @@
 #include "util/ScopeExit.hxx"
 
 EventLoop::EventLoop(ThreadId _thread)
-	:SocketMonitor(*this), thread(_thread)
+	:SocketMonitor(*this), quit(false), thread(_thread)
 {
 	SocketMonitor::Open(SocketDescriptor(wake_fd.Get()));
 }
@@ -39,10 +39,9 @@ EventLoop::~EventLoop()
 void
 EventLoop::Break()
 {
-	if (quit)
+	if (quit.exchange(true))
 		return;
 
-	quit = true;
 	wake_fd.Write();
 }
 

src/event/Loop.hxx

@@ -36,6 +36,7 @@
 #include <boost/intrusive/list.hpp>
 
 #include <chrono>
+#include <atomic>
 
 #include <assert.h>
 
@@ -85,7 +86,7 @@ class EventLoop final : SocketMonitor
 
 	std::chrono::steady_clock::time_point now = std::chrono::steady_clock::now();
 
-	bool quit = false;
+	std::atomic_bool quit;
 
 	/**
 	 * True when the object has been modified and another check is