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>

1 files changed, 10 insertions, 0 deletions

diff --git a/src/gui/image/qimagewriter.cpp b/src/gui/image/qimagewriter.cpp
index ab15d8ee29..a39d204677 100644
--- a/src/gui/image/qimagewriter.cpp
+++ b/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;