CGDataProviderRefReleaseCallback not being called - objective-c

Question

I'm creating a CGBitmap using the code below, but the done_callback function is never called. What am I doing wrong? The bitmap is created correctly but I have no way to free the pixel buffer afterwards. Is it because the CGBitmap is never released?
void done_callback(void *info, const void *data, size_t size)
{
free((void *)data);
NSLog(#"Done!");
}
static CGImageRef SVG_to_CGImage(char const *svg, int width, int height)
{
using namespace lunasvg;
auto doc = Document::loadFromData(svg);
if(doc.get() == nullptr) {
return nullptr;
}
auto bmp = doc->renderToBitmap(width, height);
if(!bmp.valid()) {
return nullptr;
}
UInt32 *pixels = (UInt32 *)malloc(width * height * 4);
// rearrange the BGRA to RGBA
for(int y=0; y<height; ++y) {
UInt32 *s = (UInt32 *)(bmp.data() + y * width * 4);
UInt32 *d = pixels + y * width;
for(int x=0; x<width; ++x) {
UInt32 argb = *s++;
UInt32 a = argb & 0xff000000;
UInt32 r = (argb & 0xff0000) >> 16;
UInt32 g = argb & 0xff00;
UInt32 b = (argb & 0xff) << 16;
*d++ = a | r | g | b;
}
}
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, pixels, width * height * 4, done_callback);
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedLast;
CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
return CGImageCreate(width, height, 8, 32, 4 * width, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);
}