QProcess/Win: improve reading in waitForBytesWritten()

On Windows, tryReadFromChannel() just publishes the data that was
already buffered by the pipe reader. Reading from the pipe was still
possible only because the pipe writer enters an alertable wait state
which can trigger an I/O callback for the read operation as well. This
made the implementation unclear and tied to APC behavior.

To avoid this, we now call the "proper" pipe reader wait function
instead, which also unifies the code with the other waitFor...()
implementations.

Change-Id: I7376d7973477a722472ff8763f95f39e6ea2fce9
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>

src/corelib/io/qprocess_win.cpp

@@ -758,18 +758,12 @@ bool QProcessPrivate::waitForBytesWritten(int msecs)
             return true;
 
         // If we wouldn't write anything, check if we can read stdout.
-        if (stdoutChannel.pipe[0] != INVALID_Q_PIPE
-                && bytesAvailableInChannel(&stdoutChannel) != 0) {
-            tryReadFromChannel(&stdoutChannel);
+        if (stdoutChannel.reader && stdoutChannel.reader->waitForReadyRead(0))
             timer.resetIncrements();
-        }
 
         // Check if we can read stderr.
-        if (stderrChannel.pipe[0] != INVALID_Q_PIPE
-                && bytesAvailableInChannel(&stderrChannel) != 0) {
-            tryReadFromChannel(&stderrChannel);
+        if (stderrChannel.reader && stderrChannel.reader->waitForReadyRead(0))
             timer.resetIncrements();
-        }
 
         // Check if the process died while reading.
         if (!pid)