further improve pullFromServerSide stability

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

@@ -340,25 +340,41 @@ class TcpConnectionSpec extends AkkaSpec("akka.io.tcp.register-timeout = 500ms")
       serverSideChannel.register(nioSelector, SelectionKey.OP_READ | SelectionKey.OP_WRITE)
 
     val buffer = ByteBuffer.allocate(TestSize)
-    @tailrec final def pullFromServerSide(remaining: Int): Unit =
-      if (remaining > 0) {
-        nioSelector.select(100)
-        if (clientSelectionKey.isValid && clientSelectionKey.isWritable)
-          selector.send(connectionActor, ChannelWritable)
 
-        if (serverSelectionKey.isValid && serverSelectionKey.isReadable) {
-          buffer.clear()
-          val read = serverSideChannel.read(buffer)
-          if (read == 0)
-            throw new IllegalStateException("Didn't make any progress")
-          else if (read == -1)
-            throw new IllegalStateException("Connection was closed unexpectedly with remaining bytes " + remaining)
+    /**
+     * Tries to simultaneously act on client and server side to read from the server
+     * all pending data from the client.
+     */
+    @tailrec final def pullFromServerSide(remaining: Int, waitingForWrite: Boolean = true): Unit =
+      if (remaining > 0)
+        pullFromServerSide(remaining - tryReading(), checkForWriteInterest(waitingForWrite))
 
-          pullFromServerSide(remaining - read)
-        } else
-          pullFromServerSide(remaining)
+    private def checkForWriteInterest(waitingForWrite: Boolean): Boolean = {
+      val waitingForWrite0 =
+        if (selector.msgAvailable) {
+          selector.expectMsg(WriteInterest)
+          true
+        } else waitingForWrite
 
-      }
+      nioSelector.select(1)
+
+      if (waitingForWrite0 && clientSelectionKey.isValid && clientSelectionKey.isWritable) {
+        selector.send(connectionActor, ChannelWritable)
+        false
+      } else waitingForWrite0
+    }
+    private def tryReading(): Int =
+      if (serverSelectionKey.isValid && serverSelectionKey.isReadable) {
+        buffer.clear()
+        val read = serverSideChannel.read(buffer)
+        if (read == 0)
+          throw new IllegalStateException("Didn't make any progress")
+        else if (read == -1)
+          throw new IllegalStateException("Connection was closed unexpectedly with remaining bytes " + remaining)
+
+        read
+      } else
+        0
 
     @tailrec final def expectReceivedString(data: String): Unit = {
       data.length must be > 0