Revert "Remove blocking inside SocketPool."

This reverts commit d2e792a.

src/main/kotlin/org/ice4j/socket/SocketPool.kt

@@ -15,11 +15,12 @@
  */
 package org.ice4j.socket
 
+import java.io.Closeable
 import java.net.DatagramSocket
 import java.net.DatagramSocketImpl
 import java.net.SocketAddress
 import java.nio.channels.DatagramChannel
-import java.util.concurrent.atomic.AtomicLong
+import java.util.concurrent.Semaphore
 
 /** A pool of datagram sockets all bound on the same port.
  *
@@ -55,6 +56,8 @@ class SocketPool(
             Runtime.getRuntime().availableProcessors()
         }
 
+    private val semaphore = Semaphore(numSockets)
+
     private val sockets = buildList {
         val multipleSockets = numSockets > 1
         var bindAddr = address
@@ -71,7 +74,7 @@ class SocketPool(
         }
     }
 
-    private val sockIndex = AtomicLong(0)
+    private val availableSockets = ArrayDeque(sockets)
 
     /** The socket on which packets will be received. */
     val receiveSocket: DatagramSocket
@@ -80,18 +83,48 @@ class SocketPool(
         //  sockets, spreading load?
         get() = sockets.last()
 
-    /** Gets a socket on which packets can be sent, chosen from among all the available send sockets. */
-    val sendSocket: DatagramSocket
-        get() {
-            if (numSockets == 1) {
-                return sockets.first()
+    interface SocketHolder : Closeable {
+        val socket: DatagramSocket
+    }
+
+    /** Socket holder with autocloseable semantics, to ensure the socket is returned. */
+    private inner class MySocketHolder : SocketHolder {
+        override val socket = acquireSendSocket()
+        private var closed = false
+        override fun close() {
+            if (!closed) {
+                returnSendSocket(socket)
+                closed = true
             }
-            return sockets[nextIndex()]
         }
+    }
 
-    private fun nextIndex(): Int {
-        val nextIdx = sockIndex.getAndIncrement()
-        return nextIdx.rem(numSockets).toInt()
+    /** Trivial socket holder that gives out a single unique socket. */
+    private inner class TrivialSocketHolder : SocketHolder {
+        override val socket = sockets.first()
+        override fun close() {}
+    }
+
+    /** Gets a send socket holder.  Should be used with Kotlin [use] or Java try-with-resources.  May block. */
+    fun getSendSocket(): SocketHolder {
+        if (numSockets == 1) {
+            return TrivialSocketHolder()
+        }
+        return MySocketHolder()
+    }
+
+    private fun acquireSendSocket(): DatagramSocket {
+        semaphore.acquire()
+        synchronized(availableSockets) {
+            return availableSockets.removeFirst()
+        }
+    }
+
+    private fun returnSendSocket(socket: DatagramSocket) {
+        synchronized(availableSockets) {
+            availableSockets.addLast(socket)
+        }
+        semaphore.release()
     }
 
     fun close() {

src/test/kotlin/org/ice4j/socket/SocketPoolTest.kt

@@ -35,30 +35,30 @@ class SocketPoolTest : ShouldSpec() {
         context("Getting multiple send sockets from a pool") {
             val numSockets = 4
             val pool = SocketPool(loopbackAny, numSockets)
-            val sockets = mutableListOf<DatagramSocket>()
+            val holders = mutableListOf<SocketPool.SocketHolder>()
             should("be possible") {
                 repeat(numSockets) {
-                    sockets.add(pool.sendSocket)
+                    holders.add(pool.getSendSocket())
                 }
             }
-            // All sockets should be distinct
-            sockets.toSet().size shouldBe sockets.size
+            holders.forEach { it.close() }
             pool.close()
         }
 
         context("Packets sent from each of the send sockets in the pool") {
             val numSockets = 4
             val pool = SocketPool(loopbackAny, numSockets)
             val local = pool.receiveSocket.localSocketAddress
-            val sockets = mutableListOf<DatagramSocket>()
+            val holders = mutableListOf<SocketPool.SocketHolder>()
             repeat(numSockets) {
-                sockets.add(pool.sendSocket)
+                holders.add(pool.getSendSocket())
             }
-            sockets.forEachIndexed { i, it ->
+            holders.forEachIndexed { i, it ->
                 val buf = i.toString().toByteArray()
                 val packet = DatagramPacket(buf, buf.size, local)
-                it.send(packet)
+                it.socket.send(packet)
             }
+            holders.forEach { it.close() }
 
             should("be received") {
                 for (i in 0 until numSockets) {
@@ -70,7 +70,6 @@ class SocketPoolTest : ShouldSpec() {
                     packet.socketAddress shouldBe local
                 }
             }
-            pool.close()
         }
 
         context("The number of send sockets") {
@@ -81,7 +80,9 @@ class SocketPoolTest : ShouldSpec() {
 
             repeat(2 * numSockets) {
                 // This should cycle through all the available send sockets
-                sockets.add(pool.sendSocket)
+                pool.getSendSocket().use { holder ->
+                    sockets.add(holder.socket)
+                }
             }
 
             should("be correct") {
@@ -124,8 +125,9 @@ class SocketPoolTest : ShouldSpec() {
 
         private fun sendToSocket(count: Int) {
             for (i in 0 until count) {
-                val socket = pool.sendSocket
-                socket.send(DatagramPacket(buf, BUFFER_SIZE, destAddr))
+                pool.getSendSocket().use {
+                    it.socket.send(DatagramPacket(buf, BUFFER_SIZE, destAddr))
+                }
             }
         }