Synchronize writing thread with existing writers

configuration/spotbugs-filters.xml

@@ -87,4 +87,10 @@
 		<!-- Allow writing to static field org.tinylog.policies.DynamicPolicy.reset from instance method org.tinylog.policies.DynamicPolicy.reset -->
 		<Bug pattern="ST_WRITE_TO_STATIC_FROM_INSTANCE_METHOD" />
 	</Match>
+	<Match>
+		<!-- Writing Thread -->
+		<Class name="org.tinylog.core.WritingThread" />
+		<!-- Allow unconditional waits - a rare spurious thread is a no-op other than synchronizing on the task mutex-->
+		<Bug pattern="UW_UNCOND_WAIT" />
+	</Match>
 </FindBugsFilter>

tinylog-impl/src/main/java/org/tinylog/core/WritingThread.java

@@ -28,9 +28,8 @@
 public final class WritingThread extends Thread {
 
 	private static final String THREAD_NAME = "tinylog-WritingThread";
-	private static final long MILLISECONDS_TO_SLEEP = 10L;
-
 	private final Object mutex;
+	private final Object threadMutex;
 	private final Collection<Writer> writers;
 	private List<Task> tasks;
 
@@ -40,6 +39,7 @@ public final class WritingThread extends Thread {
 	 */
 	WritingThread(final Collection<Writer> writers) {
 		this.mutex = new Object();
+		this.threadMutex = new Object();
 		this.writers = writers;
 		this.tasks = new ArrayList<Task>();
 
@@ -56,23 +56,30 @@ public void run() {
 		Collection<Writer> writers = new ArrayList<Writer>(1);
 
 		while (true) {
+			synchronized (threadMutex) {
+				try {
+					// Only wake up when there's something to write
+					threadMutex.wait();
+				} catch (InterruptedException ex) {
+					// Ignore and continue
+				}
+			}
+			boolean flushOnShutdown = false;
 			for (Task task : receiveTasks()) {
 				if (task == Task.POISON) {
+					if (flushOnShutdown) {
+						flush(writers);
+					}
 					close();
 					return;
 				} else {
+					flushOnShutdown = true;
 					write(writers, task);
 				}
 			}
 
 			flush(writers);
 			writers.clear();
-
-			try {
-				sleep(MILLISECONDS_TO_SLEEP);
-			} catch (InterruptedException ex) {
-				// Ignore and continue
-			}
 		}
 	}
 
@@ -89,6 +96,9 @@ public void add(final Writer writer, final LogEntry logEntry) {
 		synchronized (mutex) {
 			tasks.add(task);
 		}
+		synchronized (threadMutex) {
+			threadMutex.notifyAll();
+		}
 	}
 
 	/**
@@ -102,6 +112,10 @@ public void add(final Writer writer, final LogEntry logEntry) {
 	public void shutdown() {
 		synchronized (mutex) {
 			tasks.add(Task.POISON);
+			mutex.notify();
+		}
+		synchronized (threadMutex) {
+			threadMutex.notifyAll();
 		}
 
 		interrupt();