QImageWriter: Detect failure due to trying to write a QImage()

This is not a bug in QImageWriter, but caller code. We should be
explicit about what the problem was so it can be fixed.

[ChangeLog][QtGui][QImageWriter] Add QImageWriter::InvalidImageError to
communicate invalid attempts to write a bad QImage (for instance, a null
QImage).

Change-Id: I0333b8263f1da1c672bed17dab48bfd6cafe41a2
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/image/qimagewriter.cpp

@@ -87,6 +87,9 @@
     \value UnsupportedFormatError Qt does not support the requested
     image format.
 
+    \value InvalidImageError An attempt was made to write an invalid QImage. An
+    example of an invalid image would be a null QImage.
+
     \value UnknownError An unknown error occurred. If you get this
     value after calling write(), it is most likely caused by a bug in
     QImageWriter.
@@ -736,6 +739,13 @@ extern void qt_imageTransform(QImage &src, QImageIOHandler::Transformations orie
 */
 bool QImageWriter::write(const QImage &image)
 {
+    // Do this before canWrite, so it doesn't create a file if this fails.
+    if (Q_UNLIKELY(image.isNull())) {
+        d->imageWriterError = QImageWriter::InvalidImageError;
+        d->errorString = QImageWriter::tr("Image is empty");
+        return false;
+    }
+
     if (!canWrite())
         return false;
 

src/gui/image/qimagewriter.h

@@ -60,7 +60,8 @@ public:
     enum ImageWriterError {
         UnknownError,
         DeviceError,
-        UnsupportedFormatError
+        UnsupportedFormatError,
+        InvalidImageError
     };
 
     QImageWriter();

tests/auto/gui/image/qimagewriter/tst_qimagewriter.cpp

@@ -77,6 +77,9 @@ private slots:
     void saveWithNoFormat();
 
     void saveToTemporaryFile();
+
+    void writeEmpty();
+
 private:
     QTemporaryDir m_temporaryDir;
     QString prefix;
@@ -529,5 +532,18 @@ void tst_QImageWriter::saveToTemporaryFile()
     }
 }
 
+void tst_QImageWriter::writeEmpty()
+{
+    // check writing a null QImage errors gracefully
+    QTemporaryDir dir;
+    QVERIFY2(dir.isValid(), qPrintable(dir.errorString()));
+    QString fileName(dir.path() + QLatin1String("/testimage.bmp"));
+    QVERIFY(!QFileInfo(fileName).exists());
+    QImageWriter writer(fileName);
+    QVERIFY(!writer.write(QImage()));
+    QCOMPARE(writer.error(), QImageWriter::InvalidImageError);
+    QVERIFY(!QFileInfo(fileName).exists());
+}
+
 QTEST_MAIN(tst_QImageWriter)
 #include "tst_qimagewriter.moc"