changeset 49802:8ac08fa69f00

8201315: (se) Allow SelectableChannel.register to be invoked while selection operation is in progress Reviewed-by: bpb
author alanb
date Wed, 18 Apr 2018 10:20:09 +0100
parents 466d4df248a6
children 14518ac8df87
files src/java.base/share/classes/java/nio/channels/SelectableChannel.java src/java.base/share/classes/java/nio/channels/SelectionKey.java src/java.base/share/classes/java/nio/channels/Selector.java src/java.base/share/classes/java/nio/channels/spi/AbstractSelectableChannel.java src/java.base/share/classes/sun/nio/ch/SelectorImpl.java test/jdk/java/nio/channels/Selector/RegisterDuringSelect.java
diffstat 6 files changed, 244 insertions(+), 142 deletions(-) [+]
line wrap: on
line diff
--- a/src/java.base/share/classes/java/nio/channels/SelectableChannel.java	Wed Apr 18 11:11:38 2018 +0200
+++ b/src/java.base/share/classes/java/nio/channels/SelectableChannel.java	Wed Apr 18 10:20:09 2018 +0100
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -117,12 +117,6 @@
      */
     public abstract int validOps();
 
-    // Internal state:
-    //   keySet, may be empty but is never null, typ. a tiny array
-    //   boolean isRegistered, protected by key set
-    //   regLock, lock object to prevent duplicate registrations
-    //   blocking mode, protected by regLock
-
     /**
      * Tells whether or not this channel is currently registered with any
      * selectors.  A newly-created channel is not registered.
@@ -135,8 +129,6 @@
      * @return {@code true} if, and only if, this channel is registered
      */
     public abstract boolean isRegistered();
-    //
-    // sync(keySet) { return isRegistered; }
 
     /**
      * Retrieves the key representing the channel's registration with the given
@@ -150,8 +142,6 @@
      *          currently registered with that selector
      */
     public abstract SelectionKey keyFor(Selector sel);
-    //
-    // sync(keySet) { return findKey(sel); }
 
     /**
      * Registers this channel with the given selector, returning a selection
@@ -171,12 +161,12 @@
      * will be {@code att}.
      *
      * <p> This method may be invoked at any time.  If this method is invoked
-     * while another invocation of this method or of the {@link
-     * #configureBlocking(boolean) configureBlocking} method is in progress
-     * then it will first block until the other operation is complete.  This
-     * method will then synchronize on the selector's key set and therefore may
-     * block if invoked concurrently with another registration or selection
-     * operation involving the same selector. </p>
+     * while a selection operation is in progress then it has no effect upon
+     * that operation; the new registration or change to the key's interest set
+     * will be seen by the next selection operation.  If this method is invoked
+     * while an invocation of {@link #configureBlocking(boolean) configureBlocking}
+     * is in progress then it will block until the channel's blocking mode has
+     * been adjusted.
      *
      * <p> If this channel is closed while this operation is in progress then
      * the key returned by this method will have been cancelled and will
@@ -218,16 +208,6 @@
      */
     public abstract SelectionKey register(Selector sel, int ops, Object att)
         throws ClosedChannelException;
-    //
-    // sync(regLock) {
-    //   sync(keySet) { look for selector }
-    //   if (channel found) { set interest ops -- may block in selector;
-    //                        return key; }
-    //   create new key -- may block somewhere in selector;
-    //   sync(keySet) { add key; }
-    //   attach(attachment);
-    //   return key;
-    // }
 
     /**
      * Registers this channel with the given selector, returning a selection
@@ -314,11 +294,6 @@
      */
     public abstract SelectableChannel configureBlocking(boolean block)
         throws IOException;
-    //
-    // sync(regLock) {
-    //   sync(keySet) { throw IBME if block && isRegistered; }
-    //   change mode;
-    // }
 
     /**
      * Tells whether or not every I/O operation on this channel will block
--- a/src/java.base/share/classes/java/nio/channels/SelectionKey.java	Wed Apr 18 11:11:38 2018 +0200
+++ b/src/java.base/share/classes/java/nio/channels/SelectionKey.java	Wed Apr 18 10:20:09 2018 +0100
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -86,15 +86,9 @@
  * attached via the {@link #attach attach} method and then later retrieved via
  * the {@link #attachment() attachment} method.
  *
- * <p> Selection keys are safe for use by multiple concurrent threads.  The
- * operations of reading and writing the interest set will, in general, be
- * synchronized with certain operations of the selector.  Exactly how this
- * synchronization is performed is implementation-dependent: In a naive
- * implementation, reading or writing the interest set may block indefinitely
- * if a selection operation is already in progress; in a high-performance
- * implementation, reading or writing the interest set may block briefly, if at
- * all.  In any case, a selection operation will always use the interest-set
- * value that was current at the moment that the operation began.  </p>
+ * <p> Selection keys are safe for use by multiple concurrent threads.  A
+ * selection operation will always use the interest-set value that was current
+ * at the moment that the operation began.  </p>
  *
  *
  * @author Mark Reinhold
@@ -164,10 +158,7 @@
      * Retrieves this key's interest set.
      *
      * <p> It is guaranteed that the returned set will only contain operation
-     * bits that are valid for this key's channel.
-     *
-     * <p> This method may be invoked at any time.  Whether or not it blocks,
-     * and for how long, is implementation-dependent.  </p>
+     * bits that are valid for this key's channel. </p>
      *
      * @return  This key's interest set
      *
@@ -179,8 +170,10 @@
     /**
      * Sets this key's interest set to the given value.
      *
-     * <p> This method may be invoked at any time.  Whether or not it blocks,
-     * and for how long, is implementation-dependent.  </p>
+     * <p> This method may be invoked at any time.  If this method is invoked
+     * while a selection operation is in progress then it has no effect upon
+     * that operation; the change to the key's interest set will be seen by the
+     * next selection operation.
      *
      * @param  ops  The new interest set
      *
--- a/src/java.base/share/classes/java/nio/channels/Selector.java	Wed Apr 18 11:11:38 2018 +0200
+++ b/src/java.base/share/classes/java/nio/channels/Selector.java	Wed Apr 18 10:20:09 2018 +0100
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -84,11 +84,10 @@
  * operations.  A key may be removed directly from the selected-key set by
  * invoking the set's {@link java.util.Set#remove(java.lang.Object) remove}
  * method or by invoking the {@link java.util.Iterator#remove() remove} method
- * of an {@link java.util.Iterator iterator} obtained from the
- * set.  Keys are never removed from the selected-key set in any other way;
- * they are not, in particular, removed as a side effect of selection
- * operations.  Keys may not be added directly to the selected-key set. </p>
- *
+ * of an {@link java.util.Iterator iterator} obtained from the set.
+ * All keys may be removed from the selected-key set by invoking the set's
+ * {@link java.util.Set#clear() clear} method.  Keys may not be added directly
+ * to the selected-key set. </p>
  *
  * <a id="selop"></a>
  * <h2>Selection</h2>
@@ -144,12 +143,12 @@
  *
  * <h2>Concurrency</h2>
  *
- * <p> Selectors are themselves safe for use by multiple concurrent threads;
- * their key sets, however, are not.
+ * <p> A Selector and its key set are safe for use by multiple concurrent
+ * threads.  Its selected-key set and cancelled-key set, however, are not.
  *
- * <p> The selection operations synchronize on the selector itself, on the key
- * set, and on the selected-key set, in that order.  They also synchronize on
- * the cancelled-key set during steps (1) and (3) above.
+ * <p> The selection operations synchronize on the selector itself, on the
+ * selected-key set, in that order.  They also synchronize on the cancelled-key
+ * set during steps (1) and (3) above.
  *
  * <p> Changes made to the interest sets of a selector's keys while a
  * selection operation is in progress have no effect upon that operation; they
@@ -180,20 +179,27 @@
  *
  * </ul>
  *
- * <p> The {@link #close close} method synchronizes on the selector and all
- * three key sets in the same order as in a selection operation.
+ * <p> The {@link #close close} method synchronizes on the selector and its
+ * selected-key set in the same order as in a selection operation.
  *
  * <a id="ksc"></a>
+ * <p> A Selector's key set is safe for use by multiple concurrent threads.
+ * Retrieval operations from the key set do not generally block and so may
+ * overlap with new registrations that add to the set, or with the cancellation
+ * steps of selection operations that remove keys from the set.  Iterators and
+ * spliterators return elements reflecting the state of the set at some point at
+ * or since the creation of the iterator/spliterator.  They do not throw
+ * {@link java.util.ConcurrentModificationException ConcurrentModificationException}.
  *
- * <p> A selector's key and selected-key sets are not, in general, safe for use
- * by multiple concurrent threads.  If such a thread might modify one of these
- * sets directly then access should be controlled by synchronizing on the set
- * itself.  The iterators returned by these sets' {@link
- * java.util.Set#iterator() iterator} methods are <i>fail-fast:</i> If the set
- * is modified after the iterator is created, in any way except by invoking the
- * iterator's own {@link java.util.Iterator#remove() remove} method, then a
- * {@link java.util.ConcurrentModificationException} will be thrown. </p>
- *
+ * <a id="sksc"></a>
+ * <p> A selector's selected-key set is not, in general, safe for use by
+ * multiple concurrent threads.  If such a thread might modify the set directly
+ * then access should be controlled by synchronizing on the set itself.  The
+ * iterators returned by the set's {@link java.util.Set#iterator() iterator}
+ * methods are <i>fail-fast:</i> If the set is modified after the iterator is
+ * created, in any way except by invoking the iterator's own {@link
+ * java.util.Iterator#remove() remove} method, then a {@link
+ * java.util.ConcurrentModificationException} will be thrown. </p>
  *
  * @author Mark Reinhold
  * @author JSR-51 Expert Group
@@ -249,7 +255,8 @@
      * attempt to modify the key set will cause an {@link
      * UnsupportedOperationException} to be thrown.
      *
-     * <p> The key set is <a href="#ksc">not thread-safe</a>. </p>
+     * <p> The set is <a href="#ksc">safe</a> for use by multiple concurrent
+     * threads.  </p>
      *
      * @return  This selector's key set
      *
@@ -265,7 +272,7 @@
      * selected-key set.  Any attempt to add an object to the key set will
      * cause an {@link UnsupportedOperationException} to be thrown.
      *
-     * <p> The selected-key set is <a href="#ksc">not thread-safe</a>. </p>
+     * <p> The selected-key set is <a href="#sksc">not thread-safe</a>.  </p>
      *
      * @return  This selector's selected-key set
      *
@@ -326,8 +333,7 @@
      * @throws  IllegalArgumentException
      *          If the value of the timeout argument is negative
      */
-    public abstract int select(long timeout)
-        throws IOException;
+    public abstract int select(long timeout) throws IOException;
 
     /**
      * Selects a set of keys whose corresponding channels are ready for I/O
--- a/src/java.base/share/classes/java/nio/channels/spi/AbstractSelectableChannel.java	Wed Apr 18 11:11:38 2018 +0200
+++ b/src/java.base/share/classes/java/nio/channels/spi/AbstractSelectableChannel.java	Wed Apr 18 10:20:09 2018 +0100
@@ -108,7 +108,7 @@
                 if (keys[i] == null)
                     break;
         } else if (keys == null) {
-            keys =  new SelectionKey[3];
+            keys = new SelectionKey[2];
         } else {
             // Grow key array
             int n = keys.length * 2;
@@ -123,14 +123,14 @@
     }
 
     private SelectionKey findKey(Selector sel) {
-        synchronized (keyLock) {
-            if (keys == null)
-                return null;
-            for (int i = 0; i < keys.length; i++)
-                if ((keys[i] != null) && (keys[i].selector() == sel))
-                    return keys[i];
+        assert Thread.holdsLock(keyLock);
+        if (keys == null)
             return null;
-        }
+        for (int i = 0; i < keys.length; i++)
+            if ((keys[i] != null) && (keys[i].selector() == sel))
+                return keys[i];
+        return null;
+
     }
 
     void removeKey(SelectionKey k) {                    // package-private
@@ -166,7 +166,9 @@
     }
 
     public final SelectionKey keyFor(Selector sel) {
-        return findKey(sel);
+        synchronized (keyLock) {
+            return findKey(sel);
+        }
     }
 
     /**
@@ -195,32 +197,31 @@
      *
      * @throws  IllegalArgumentException {@inheritDoc}
      */
-    public final SelectionKey register(Selector sel, int ops,
-                                       Object att)
+    public final SelectionKey register(Selector sel, int ops, Object att)
         throws ClosedChannelException
     {
+        if ((ops & ~validOps()) != 0)
+            throw new IllegalArgumentException();
+        if (!isOpen())
+            throw new ClosedChannelException();
         synchronized (regLock) {
-            if (!isOpen())
-                throw new ClosedChannelException();
-            if ((ops & ~validOps()) != 0)
-                throw new IllegalArgumentException();
             if (isBlocking())
                 throw new IllegalBlockingModeException();
-            SelectionKey k = findKey(sel);
-            if (k != null) {
-                k.interestOps(ops);
-                k.attach(att);
-            }
-            if (k == null) {
-                // New registration
-                synchronized (keyLock) {
-                    if (!isOpen())
-                        throw new ClosedChannelException();
+            synchronized (keyLock) {
+                // re-check if channel has been closed
+                if (!isOpen())
+                    throw new ClosedChannelException();
+                SelectionKey k = findKey(sel);
+                if (k != null) {
+                    k.attach(att);
+                    k.interestOps(ops);
+                } else {
+                    // New registration
                     k = ((AbstractSelector)sel).register(this, ops, att);
                     addKey(k);
                 }
+                return k;
             }
-            return k;
         }
     }
 
@@ -239,12 +240,20 @@
      */
     protected final void implCloseChannel() throws IOException {
         implCloseSelectableChannel();
+
+        // clone keys to avoid calling cancel when holding keyLock
+        SelectionKey[] copyOfKeys = null;
         synchronized (keyLock) {
-            int count = (keys == null) ? 0 : keys.length;
-            for (int i = 0; i < count; i++) {
-                SelectionKey k = keys[i];
-                if (k != null)
-                    k.cancel();
+            if (keys != null) {
+                copyOfKeys = keys.clone();
+            }
+        }
+
+        if (copyOfKeys != null) {
+            for (SelectionKey k : copyOfKeys) {
+                if (k != null) {
+                    k.cancel();   // invalidate and adds key to cancelledKey set
+                }
             }
         }
     }
--- a/src/java.base/share/classes/sun/nio/ch/SelectorImpl.java	Wed Apr 18 11:11:38 2018 +0200
+++ b/src/java.base/share/classes/sun/nio/ch/SelectorImpl.java	Wed Apr 18 10:20:09 2018 +0100
@@ -37,17 +37,18 @@
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 
 
 /**
  * Base Selector implementation class.
  */
 
-public abstract class SelectorImpl
+abstract class SelectorImpl
     extends AbstractSelector
 {
     // The set of keys registered with this Selector
-    protected final Set<SelectionKey> keys;
+    private final Set<SelectionKey> keys;
 
     // The set of keys with data ready for an operation
     protected final Set<SelectionKey> selectedKeys;
@@ -58,7 +59,7 @@
 
     protected SelectorImpl(SelectorProvider sp) {
         super(sp);
-        keys = new HashSet<>();
+        keys = ConcurrentHashMap.newKeySet();
         selectedKeys = new HashSet<>();
         publicKeys = Collections.unmodifiableSet(keys);
         publicSelectedKeys = Util.ungrowableSet(selectedKeys);
@@ -82,11 +83,8 @@
     }
 
     /**
-     * Returns the public view of the key sets
+     * Returns the public view of the selected-key set
      */
-    protected final Set<SelectionKey> nioKeys() {
-        return publicKeys;
-    }
     protected final Set<SelectionKey> nioSelectedKeys() {
         return publicSelectedKeys;
     }
@@ -116,18 +114,14 @@
     private int lockAndDoSelect(long timeout) throws IOException {
         synchronized (this) {
             ensureOpen();
-            synchronized (publicKeys) {
-                synchronized (publicSelectedKeys) {
-                    return doSelect(timeout);
-                }
+            synchronized (publicSelectedKeys) {
+                return doSelect(timeout);
             }
         }
     }
 
     @Override
-    public final int select(long timeout)
-        throws IOException
-    {
+    public final int select(long timeout) throws IOException {
         if (timeout < 0)
             throw new IllegalArgumentException("Negative timeout");
         return lockAndDoSelect((timeout == 0) ? -1 : timeout);
@@ -135,7 +129,7 @@
 
     @Override
     public final int select() throws IOException {
-        return select(0);
+        return lockAndDoSelect(-1);
     }
 
     @Override
@@ -143,32 +137,33 @@
         return lockAndDoSelect(0);
     }
 
+    /**
+     * Invoked by implCloseSelector to close the selector.
+     */
+    protected abstract void implClose() throws IOException;
+
     @Override
     public final void implCloseSelector() throws IOException {
         wakeup();
         synchronized (this) {
             implClose();
-            synchronized (publicKeys) {
-                synchronized (publicSelectedKeys) {
-                    // Deregister channels
-                    Iterator<SelectionKey> i = keys.iterator();
-                    while (i.hasNext()) {
-                        SelectionKeyImpl ski = (SelectionKeyImpl)i.next();
-                        deregister(ski);
-                        SelectableChannel selch = ski.channel();
-                        if (!selch.isOpen() && !selch.isRegistered())
-                            ((SelChImpl)selch).kill();
-                        selectedKeys.remove(ski);
-                        i.remove();
-                    }
-                    assert selectedKeys.isEmpty() && keys.isEmpty();
+            synchronized (publicSelectedKeys) {
+                // Deregister channels
+                Iterator<SelectionKey> i = keys.iterator();
+                while (i.hasNext()) {
+                    SelectionKeyImpl ski = (SelectionKeyImpl)i.next();
+                    deregister(ski);
+                    SelectableChannel selch = ski.channel();
+                    if (!selch.isOpen() && !selch.isRegistered())
+                        ((SelChImpl)selch).kill();
+                    selectedKeys.remove(ski);
+                    i.remove();
                 }
+                assert selectedKeys.isEmpty() && keys.isEmpty();
             }
         }
     }
 
-    protected abstract void implClose() throws IOException;
-
     @Override
     protected final SelectionKey register(AbstractSelectableChannel ch,
                                           int ops,
@@ -179,12 +174,21 @@
         SelectionKeyImpl k = new SelectionKeyImpl((SelChImpl)ch, this);
         k.attach(attachment);
 
-        // register with selector (if needed) before adding to key set
+        // register (if needed) before adding to key set
         implRegister(k);
-        synchronized (publicKeys) {
-            keys.add(k);
+
+        // add to the selector's key set, removing it immediately if the selector
+        // is closed. The key is not in the channel's key set at this point but
+        // it may be observed by a thread iterating over the selector's key set.
+        keys.add(k);
+        try {
+            k.interestOps(ops);
+        } catch (ClosedSelectorException e) {
+            assert ch.keyFor(this) == null;
+            keys.remove(k);
+            k.cancel();
+            throw e;
         }
-        k.interestOps(ops);
         return k;
     }
 
@@ -198,11 +202,16 @@
         ensureOpen();
     }
 
+    /**
+     * Removes the key from the selector
+     */
     protected abstract void implDereg(SelectionKeyImpl ski) throws IOException;
 
+    /**
+     * Invoked by selection operations to process the cancelled-key set
+     */
     protected final void processDeregisterQueue() throws IOException {
         assert Thread.holdsLock(this);
-        assert Thread.holdsLock(publicKeys);
         assert Thread.holdsLock(publicSelectedKeys);
 
         Set<SelectionKey> cks = cancelledKeys();
@@ -231,7 +240,8 @@
     }
 
     /**
-     * Change the event set in the selector
+     * Invoked by interestOps to ensure the interest ops are updated at the
+     * next selection operation.
      */
     protected abstract void setEventOps(SelectionKeyImpl ski);
 }
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/jdk/java/nio/channels/Selector/RegisterDuringSelect.java	Wed Apr 18 10:20:09 2018 +0100
@@ -0,0 +1,109 @@
+/*
+ * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/* @test
+ * @bug 8201315
+ * @summary Test that channels can be registered, interest ops can changed,
+ *          and keys cancelled while a selection operation is in progress.
+ */
+
+import java.io.IOException;
+import java.nio.channels.Pipe;
+import java.nio.channels.SelectionKey;
+import java.nio.channels.Selector;
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.Phaser;
+
+public class RegisterDuringSelect {
+
+    static Callable<Void> selectLoop(Selector sel, Phaser barrier) {
+        return new Callable<Void>() {
+            @Override
+            public Void call() throws IOException {
+                for (;;) {
+                    sel.select();
+                    if (sel.isOpen()) {
+                        barrier.arriveAndAwaitAdvance();
+                    } else {
+                        // closed
+                        return null;
+                    }
+                }
+            }
+        };
+    }
+    /**
+     * Invoke register, interestOps, and cancel concurrently with a thread
+     * doing a selection operation
+     */
+    public static void main(String args[]) throws Exception {
+        Future<Void> result;
+
+        ExecutorService pool = Executors.newFixedThreadPool(1);
+        try (Selector sel = Selector.open()) {
+            Phaser barrier = new Phaser(2);
+
+            // submit task to do the select loop
+            result = pool.submit(selectLoop(sel, barrier));
+
+            Pipe p = Pipe.open();
+            try {
+                Pipe.SourceChannel sc = p.source();
+                sc.configureBlocking(false);
+
+                System.out.println("register ...");
+                SelectionKey key = sc.register(sel, SelectionKey.OP_READ);
+                if (!sel.keys().contains(key))
+                    throw new RuntimeException("key not in key set");
+                sel.wakeup();
+                barrier.arriveAndAwaitAdvance();
+
+                System.out.println("interestOps ...");
+                key.interestOps(0);
+                sel.wakeup();
+                barrier.arriveAndAwaitAdvance();
+
+                System.out.println("cancel ...");
+                key.cancel();
+                sel.wakeup();
+                barrier.arriveAndAwaitAdvance();
+                if (sel.keys().contains(key))
+                    throw new RuntimeException("key not removed from key set");
+
+            } finally {
+                p.source().close();
+                p.sink().close();
+            }
+
+        } finally {
+            pool.shutdown();
+        }
+
+        // ensure selectLoop completes without exception
+        result.get();
+
+    }
+}