Introduce HashedWheelTimer.pendingTimeouts()

Motivation:

Fixes #6681.

Modification:

For the sake of better timer observability, expose the number of pending timeouts through the new HashedWheelTimer.pendingTimeouts method .

Result:

It's now ridiculously easy to observe Netty timer's very basic and yet important metric, the number of pending tasks/timeouts.

common/src/main/java/io/netty/util/HashedWheelTimer.java

@@ -16,7 +16,6 @@
 package io.netty.util;
 
 import io.netty.util.internal.PlatformDependent;
-import io.netty.util.internal.StringUtil;
 import io.netty.util.internal.logging.InternalLogger;
 import io.netty.util.internal.logging.InternalLoggerFactory;
 
@@ -405,15 +404,15 @@ public class HashedWheelTimer implements Timer {
         if (unit == null) {
             throw new NullPointerException("unit");
         }
-        if (shouldLimitTimeouts()) {
+
         long pendingTimeoutsCount = pendingTimeouts.incrementAndGet();
-            if (pendingTimeoutsCount > maxPendingTimeouts) {
+
+        if (maxPendingTimeouts > 0 && pendingTimeoutsCount > maxPendingTimeouts) {
             pendingTimeouts.decrementAndGet();
             throw new RejectedExecutionException("Number of pending timeouts ("
                 + pendingTimeoutsCount + ") is greater than or equal to maximum allowed pending "
                 + "timeouts (" + maxPendingTimeouts + ")");
         }
-        }
 
         start();
 
@@ -425,8 +424,11 @@ public class HashedWheelTimer implements Timer {
         return timeout;
     }
 
-    private boolean shouldLimitTimeouts() {
+    /**
-        return maxPendingTimeouts > 0;
+     * Returns the number of pending timeouts of this {@link Timer}.
+     */
+    public long pendingTimeouts() {
+        return pendingTimeouts.get();
     }
 
     private static void reportTooManyInstances() {
@@ -629,7 +631,7 @@ public class HashedWheelTimer implements Timer {
             HashedWheelBucket bucket = this.bucket;
             if (bucket != null) {
                 bucket.remove(this);
-            } else if (timer.shouldLimitTimeouts()) {
+            } else {
                 timer.pendingTimeouts.decrementAndGet();
             }
         }
@@ -774,9 +776,7 @@ public class HashedWheelTimer implements Timer {
             timeout.prev = null;
             timeout.next = null;
             timeout.bucket = null;
-            if (timeout.timer.shouldLimitTimeouts()) {
             timeout.timer.pendingTimeouts.decrementAndGet();
-            }
             return next;
         }
 

common/src/test/java/io/netty/util/HashedWheelTimerTest.java

@@ -213,6 +213,23 @@ public class HashedWheelTimerTest {
         timer.stop();
     }
 
+    @Test()
+    public void reportPendingTimeouts() throws InterruptedException {
+        final CountDownLatch latch = new CountDownLatch(1);
+        final HashedWheelTimer timer = new HashedWheelTimer();
+        final Timeout t1 = timer.newTimeout(createNoOpTimerTask(), 100, TimeUnit.MINUTES);
+        final Timeout t2 = timer.newTimeout(createNoOpTimerTask(), 100, TimeUnit.MINUTES);
+        timer.newTimeout(createCountDownLatchTimerTask(latch), 90, TimeUnit.MILLISECONDS);
+
+        assertEquals(3, timer.pendingTimeouts());
+        t1.cancel();
+        t2.cancel();
+        latch.await();
+
+        assertEquals(0, timer.pendingTimeouts());
+        timer.stop();
+    }
+
     private static TimerTask createNoOpTimerTask() {
         return new TimerTask() {
             @Override