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

Fixes thread sanitizer warnings.

src/event/Loop.cxx

@@ -27,7 +27,7 @@
 #include <algorithm>
 
 EventLoop::EventLoop()
-	:SocketMonitor(*this)
+	:SocketMonitor(*this), quit(false)
 {
 	SocketMonitor::Open(wake_fd.Get());
 	SocketMonitor::Schedule(SocketMonitor::READ);
@@ -46,7 +46,9 @@ EventLoop::~EventLoop()
 void
 EventLoop::Break()
 {
-	quit = true;
+	if (quit.exchange(true))
+		return;
+
 	wake_fd.Write();
 }
 

src/event/Loop.hxx

@@ -30,6 +30,7 @@
 #include "SocketMonitor.hxx"
 
 #include <chrono>
+#include <atomic>
 #include <list>
 #include <set>
 
@@ -82,7 +83,7 @@ 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