Tcp: cosmetic changes and more checks

* whitespace
 * Write.Empty => Write.empty
 * check requirements
 * better 0-checks

akka-actor-tests/src/test/scala/akka/io/TcpConnectionSpec.scala

@@ -258,8 +258,8 @@ class TcpConnectionSpec extends AkkaSpec("akka.io.tcp.register-timeout = 500ms")
         writer.expectMsg(CommandFailed(secondWrite))
 
         // reject even empty writes
-        writer.send(connectionActor, Write.Empty)
+        writer.send(connectionActor, Write.empty)
-        writer.expectMsg(CommandFailed(Write.Empty))
+        writer.expectMsg(CommandFailed(Write.empty))
 
         // there will be immediately more space in the send buffer because
         // some data will have been sent by now, so we assume we can write

akka-actor/src/main/scala/akka/io/Tcp.scala

@@ -105,16 +105,30 @@ object Tcp extends ExtensionKey[TcpExt] {
    */
   case class Write(data: ByteString, ack: Any) extends WriteCommand
   object Write {
-    val Empty: Write = Write(ByteString.empty, NoAck)
+    /**
+     * The empty Write doesn't write anything and isn't acknowledged.
+     * It will, however, be denied and sent back with `CommandFailed` if the
+     * connection isn't currently ready to send any data (because another WriteCommand
+     * is still pending).
+     */
+    val empty: Write = Write(ByteString.empty, NoAck)
+
+    /**
+     * Create a new unacknowledged Write command with the given data.
+     */
     def apply(data: ByteString): Write =
-      if (data.isEmpty) Empty else Write(data, NoAck)
+      if (data.isEmpty) empty else Write(data, NoAck)
   }
 
   /**
    * Write `count` bytes starting at `position` from file at `filePath` to the connection.
-   * When write is finished acknowledge with `ack`. If no ack is needed use `NoAck`.
+   * When write is finished acknowledge with `ack`. If no ack is needed use `NoAck`. The
+   * count must be > 0.
    */
-  case class WriteFile(filePath: String, position: Long, count: Long, ack: Any) extends WriteCommand
+  case class WriteFile(filePath: String, position: Long, count: Long, ack: Any) extends WriteCommand {
+    require(position >= 0, "WriteFile.position must be >= 0")
+    require(count > 0, "WriteFile.count must be > 0")
+  }
 
   case object StopReading extends Command
   case object ResumeReading extends Command

akka-actor/src/main/scala/akka/io/TcpConnection.scala

@@ -305,20 +305,23 @@ private[io] abstract class TcpConnection(val channel: SocketChannel,
       try innerWrite(this)
       catch { case e: IOException ⇒ handleError(handler, e) }
     }
-    def hasData = buffer.remaining() > 0 || remainingData.size > 0
+    def hasData = buffer.hasRemaining || remainingData.nonEmpty
     def consume(writtenBytes: Int): PendingBufferWrite =
-      if (buffer.remaining() == 0) {
+      if (buffer.hasRemaining) this
+      else {
         buffer.clear()
         val copied = remainingData.copyToBuffer(buffer)
         buffer.flip()
         copy(remainingData = remainingData.drop(copied))
-      } else this
+      }
   }
+
   private[io] case class PendingWriteFile(
     commander: ActorRef,
     write: WriteFile,
     fileChannel: FileChannel,
     alreadyWritten: Long) extends PendingWrite {
+
     def doWrite(handler: ActorRef): PendingWrite = {
       tcp.fileIoDispatcher.execute(writeFileRunnable(this))
       this