Fix possible segfaults with side-by-side 3D movies

It was possible that the left & right PicturePtr& were reset
concurrently by _triggerFrameUpdate() while a getTileImage() was still
in progress, causing a segfault in left->getData(texture).

Another crash could occur when the 3D movie was looping back and not
all processes got the image, causing an invalid access in
_splitSideBySide(*image).

The first crash could be reproduced within seconds by opening a large
3D movie and moving the window around. The second was harder to
reproduce and its causes are not fully understood.

Due to the complexity of the movie decoding code there may still be a
unresolved underlying problem here; but at least this change
suppresses the crashes.

doc/Changelog.md

@@ -3,6 +3,8 @@ Changelog {#changelog}
 
 # Release 1.5 (git master)
 
+* [264](https://github.com/BlueBrain/Tide/pull/264):
+  Fix a crash that could occur when playing large 3D movies.
 * [261](https://github.com/BlueBrain/Tide/pull/261):
   Tweak UI design and controls style.
 * [260](https://github.com/BlueBrain/Tide/pull/260):

tide/wall/datasources/MovieUpdater.cpp

@@ -50,15 +50,14 @@
 
 namespace
 {
-void _splitSideBySide(const FFMPEGPicture& image, PicturePtr& left,
-                      PicturePtr& right)
+auto _splitSideBySide(const FFMPEGPicture& image)
 {
     const auto width = image.getWidth() / 2;
     const auto height = image.getHeight();
     const auto format = image.getFormat();
 
-    left = std::make_shared<FFMPEGPicture>(width, height, format);
-    right = std::make_shared<FFMPEGPicture>(width, height, format);
+    auto left = std::make_shared<FFMPEGPicture>(width, height, format);
+    auto right = std::make_shared<FFMPEGPicture>(width, height, format);
 
     const uint numTextures = (format == TextureFormat::rgba) ? 1 : 3 /*YUV*/;
     for (uint texture = 0; texture < numTextures; ++texture)
@@ -75,6 +74,14 @@ void _splitSideBySide(const FFMPEGPicture& image, PicturePtr& left,
             std::copy(input + targetWidth, input + lineWidth, outRight);
         }
     }
+    return std::make_pair(left, right);
+}
+
+auto _splitSideBySide(std::shared_ptr<FFMPEGPicture> image)
+{
+    if (!image)
+        return std::make_pair(image, image);
+    return _splitSideBySide(*image);
 }
 }
 
@@ -162,6 +169,8 @@ ImagePtr MovieUpdater::getTileImage(const uint tileIndex,
     if (loopBack)
         image = _ffmpegMovie->getFrame(0.0);
 
+    // Warning: in rare cases image may still be null at this point
+
     {
         const QMutexLocker lock(&_mutex);
         _currentPosition = _ffmpegMovie->getPosition();
@@ -172,7 +181,7 @@ ImagePtr MovieUpdater::getTileImage(const uint tileIndex,
     }
     if (_ffmpegMovie->isStereo())
     {
-        _splitSideBySide(*image, _pictureLeftOrMono, _pictureRight);
+        std::tie(_pictureLeftOrMono, _pictureRight) = _splitSideBySide(image);
         return view == deflect::View::right_eye ? _pictureRight
                                                 : _pictureLeftOrMono;
     }