Expose channel count for FixedChannelPool (#8059)

Motivation:

We use FixedChannelPool in production, and we believe we have a leak that doesn't return sockets to the pool (but they should be closed), thus blocking us from creating new connections when we need them. I haven't confirmed this yet, but right now I have to resort to reflection to access this field which makes me sad.

Modification:

Expose the acquiredChannelCount field through a getter method.

Result:

Allows introspection of the pool size in FixedChannelPool.

transport/src/main/java/io/netty/channel/pool/FixedChannelPool.java

@@ -28,6 +28,7 @@ import io.netty.util.internal.ThrowableUtil;
 import java.nio.channels.ClosedChannelException;
 import java.util.ArrayDeque;
 import java.util.Queue;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
@@ -70,7 +71,7 @@ public class FixedChannelPool extends SimpleChannelPool {
     private final Queue<AcquireTask> pendingAcquireQueue = new ArrayDeque<AcquireTask>();
     private final int maxConnections;
     private final int maxPendingAcquires;
-    private int acquiredChannelCount;
+    private final AtomicInteger acquiredChannelCount = new AtomicInteger();
     private int pendingAcquireCount;
     private boolean closed;
 
@@ -229,6 +230,11 @@ public class FixedChannelPool extends SimpleChannelPool {
         this.maxPendingAcquires = maxPendingAcquires;
     }
 
+    /** Returns the number of acquired channels that this pool thinks it has. */
+    public int acquiredChannelCount() {
+        return acquiredChannelCount.get();
+    }
+
     @Override
     public Future<Channel> acquire(final Promise<Channel> promise) {
         try {
@@ -255,8 +261,8 @@ public class FixedChannelPool extends SimpleChannelPool {
             promise.setFailure(POOL_CLOSED_ON_ACQUIRE_EXCEPTION);
             return;
         }
-        if (acquiredChannelCount < maxConnections) {
-            assert acquiredChannelCount >= 0;
+        if (acquiredChannelCount.get() < maxConnections) {
+            assert acquiredChannelCount.get() >= 0;
 
             // We need to create a new promise as we need to ensure the AcquireListener runs in the correct
             // EventLoop
@@ -319,10 +325,9 @@ public class FixedChannelPool extends SimpleChannelPool {
     }
 
     private void decrementAndRunTaskQueue() {
-        --acquiredChannelCount;
-
         // We should never have a negative value.
-        assert acquiredChannelCount >= 0;
+        int currentCount = acquiredChannelCount.decrementAndGet();
+        assert currentCount >= 0;
 
         // Run the pending acquire tasks before notify the original promise so if the user would
         // try to acquire again from the ChannelFutureListener and the pendingAcquireCount is >=
@@ -332,7 +337,7 @@ public class FixedChannelPool extends SimpleChannelPool {
     }
 
     private void runTaskQueue() {
-        while (acquiredChannelCount < maxConnections) {
+        while (acquiredChannelCount.get() < maxConnections) {
             AcquireTask task = pendingAcquireQueue.poll();
             if (task == null) {
                 break;
@@ -352,7 +357,7 @@ public class FixedChannelPool extends SimpleChannelPool {
 
         // We should never have a negative value.
         assert pendingAcquireCount >= 0;
-        assert acquiredChannelCount >= 0;
+        assert acquiredChannelCount.get() >= 0;
     }
 
     // AcquireTask extends AcquireListener to reduce object creations and so GC pressure
@@ -431,7 +436,7 @@ public class FixedChannelPool extends SimpleChannelPool {
             if (acquired) {
                 return;
             }
-            acquiredChannelCount++;
+            acquiredChannelCount.incrementAndGet();
             acquired = true;
         }
     }
@@ -464,7 +469,7 @@ public class FixedChannelPool extends SimpleChannelPool {
                 }
                 task.promise.setFailure(new ClosedChannelException());
             }
-            acquiredChannelCount = 0;
+            acquiredChannelCount.set(0);
             pendingAcquireCount = 0;
 
             // Ensure we dispatch this on another Thread as close0 will be called from the EventExecutor and we need