QIODevice: fix interleaving read() and write() in text mode under Windows

Skip the correct number of bytes in the read buffer when expanding '\n'
into "\r\n" upon writing.

Change-Id: I5b01fc47c330dee5c83001abf0acd7d63d790b96
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

src/corelib/io/qiodevice.cpp

@@ -1289,6 +1289,7 @@ qint64 QIODevice::write(const char *data, qint64 maxSize)
         const char *startOfBlock = data;
 
         qint64 writtenSoFar = 0;
+        const qint64 savedPos = d->pos;
 
         forever {
             const char *endOfBlock = startOfBlock;
@@ -1300,7 +1301,7 @@ qint64 QIODevice::write(const char *data, qint64 maxSize)
                 qint64 ret = writeData(startOfBlock, blockSize);
                 if (ret <= 0) {
                     if (writtenSoFar && !sequential)
-                        d->buffer.skip(writtenSoFar);
+                        d->buffer.skip(d->pos - savedPos);
                     return writtenSoFar ? writtenSoFar : ret;
                 }
                 if (!sequential) {
@@ -1316,7 +1317,7 @@ qint64 QIODevice::write(const char *data, qint64 maxSize)
             qint64 ret = writeData("\r\n", 2);
             if (ret <= 0) {
                 if (writtenSoFar && !sequential)
-                    d->buffer.skip(writtenSoFar);
+                    d->buffer.skip(d->pos - savedPos);
                 return writtenSoFar ? writtenSoFar : ret;
             }
             if (!sequential) {
@@ -1329,7 +1330,7 @@ qint64 QIODevice::write(const char *data, qint64 maxSize)
         }
 
         if (writtenSoFar && !sequential)
-            d->buffer.skip(writtenSoFar);
+            d->buffer.skip(d->pos - savedPos);
         return writtenSoFar;
     }
 #endif

tests/auto/corelib/io/qiodevice/tst_qiodevice.cpp

@@ -59,6 +59,7 @@ private slots:
 
     void peekBug();
     void readAllKeepPosition();
+    void writeInTextMode();
 };
 
 void tst_QIODevice::initTestCase()
@@ -628,5 +629,45 @@ void tst_QIODevice::readAllKeepPosition()
     QCOMPARE(resultArray, buffer.buffer());
 }
 
+class RandomAccessBuffer : public QIODevice
+{
+public:
+    RandomAccessBuffer(const char *data) : QIODevice(), buf(data) { }
+
+protected:
+    qint64 readData(char *data, qint64 maxSize) Q_DECL_OVERRIDE
+    {
+        maxSize = qMin(maxSize, qint64(buf.size() - pos()));
+        memcpy(data, buf.constData() + pos(), maxSize);
+        return maxSize;
+    }
+    qint64 writeData(const char *data, qint64 maxSize) Q_DECL_OVERRIDE
+    {
+        maxSize = qMin(maxSize, qint64(buf.size() - pos()));
+        memcpy(buf.data() + pos(), data, maxSize);
+        return maxSize;
+    }
+
+private:
+    QByteArray buf;
+};
+
+// Test write() on skipping correct number of bytes in read buffer
+void tst_QIODevice::writeInTextMode()
+{
+    // Unlike other platforms, Windows implementation expands '\n' into
+    // "\r\n" sequence in write(). Ensure that write() properly works with
+    // a read buffer on random-access devices.
+#ifndef Q_OS_WIN
+    QSKIP("This is a Windows-only test");
+#else
+    RandomAccessBuffer buffer("one\r\ntwo\r\nthree\r\n");
+    buffer.open(QBuffer::ReadWrite | QBuffer::Text);
+    QCOMPARE(buffer.readLine(), QByteArray("one\n"));
+    QCOMPARE(buffer.write("two\n"), 4);
+    QCOMPARE(buffer.readLine(), QByteArray("three\n"));
+#endif
+}
+
 QTEST_MAIN(tst_QIODevice)
 #include "tst_qiodevice.moc"