QProcess: discard unwanted output from the child process

Drop process output to nullDevice(), if an application does not request
forwarding, redirecting or reading from the device channel. This
prevents from accumulation of unnecessary data which can not be read.

Change-Id: Ia311a8c658a46cf580ffa9484c5369f3fc5f98a7
Reviewed-by: Thiago Macieira <[email protected]>
Reviewed-by: Oswald Buddenhagen <[email protected]>

Author: Alex Trotsenko

src/corelib/io/qprocess.cpp

@@ -2206,6 +2206,16 @@ void QProcessPrivate::start(QIODevice::OpenMode mode)
         mode &= ~QIODevice::ReadOnly;      // not open for reading
     if (mode == 0)
         mode = QIODevice::Unbuffered;
+#ifndef Q_OS_WINCE
+    if ((mode & QIODevice::ReadOnly) == 0) {
+        if (stdoutChannel.type == QProcessPrivate::Channel::Normal)
+            q->setStandardOutputFile(q->nullDevice());
+        if (stderrChannel.type == QProcessPrivate::Channel::Normal
+            && processChannelMode != QProcess::MergedChannels)
+            q->setStandardErrorFile(q->nullDevice());
+    }
+#endif
+
     q->QIODevice::open(mode);
 
     stdinChannel.closed = false;

tests/auto/corelib/io/qprocess/tst_qprocess.cpp

@@ -122,6 +122,7 @@ private slots:
     void removeFileWhileProcessIsRunning();
     void fileWriterProcess();
     void switchReadChannels();
+    void discardUnwantedOutput();
     void setWorkingDirectory();
     void setNonExistentWorkingDirectory();
 #endif // not Q_OS_WINCE
@@ -2228,6 +2229,25 @@ void tst_QProcess::switchReadChannels()
 }
 #endif
 
+#ifndef Q_OS_WINCE
+// Reading and writing to a process is not supported on Qt/CE
+void tst_QProcess::discardUnwantedOutput()
+{
+    QProcess process;
+
+    process.setProgram("testProcessEcho2/testProcessEcho2");
+    process.start(QIODevice::WriteOnly);
+    process.write("Hello, World");
+    process.closeWriteChannel();
+    QVERIFY(process.waitForFinished(5000));
+
+    process.setReadChannel(QProcess::StandardOutput);
+    QCOMPARE(process.bytesAvailable(), Q_INT64_C(0));
+    process.setReadChannel(QProcess::StandardError);
+    QCOMPARE(process.bytesAvailable(), Q_INT64_C(0));
+}
+#endif
+
 //-----------------------------------------------------------------------------
 #ifndef Q_OS_WINCE
 // Q_OS_WIN - setWorkingDirectory will chdir before starting the process on unices