win32: Fix lifetime tracking of create_similar_image()

As we return the child image to the user and so perform the reference
tracking on it and not the parent win32 display surface, we need to add
a call to destroy the parent from the image surface. This of course
complicates the normal scenario of destroying the parent first, and so
in that case we need to unhook the image->parent before freeing the
surface->image.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/cairo-image-surface-private.h

@@ -50,6 +50,19 @@ struct _cairo_image_surface {
 
     pixman_image_t *pixman_image;
     const cairo_compositor_t *compositor;
+
+    /* Parenting is tricky wrt lifetime tracking...
+     *
+     * One use for tracking the parent of an image surface is for
+     * create_similar_image() where we wish to create a device specific
+     * surface but return an image surface to the user. In such a case,
+     * the image may be owned by the device specific surface, its parent,
+     * but the user lifetime tracking is then performed on the image. So
+     * when the image is then finalized we call cairo_surface_destroy()
+     * on the parent. However, for normal usage where the lifetime tracking
+     * is done on the parent surface, we need to be careful to unhook
+     * the image->parent pointer before finalizing the image.
+     */
     cairo_surface_t *parent;
 
     pixman_format_code_t pixman_format;

src/cairo-image-surface.c

@@ -813,6 +813,11 @@ _cairo_image_surface_finish (void *abstract_surface)
 	surface->data = NULL;
     }
 
+    if (surface->parent) {
+	cairo_surface_destroy (surface->parent);
+	surface->parent = NULL;
+    }
+
     return CAIRO_STATUS_SUCCESS;
 }
 

src/win32/cairo-win32-display-surface.c

@@ -405,8 +405,12 @@ _cairo_win32_display_surface_finish (void *abstract_surface)
 {
     cairo_win32_display_surface_t *surface = abstract_surface;
 
-    if (surface->image)
+    if (surface->image) {
+	/* Unhook ourselves first to avoid the double-unref from the image */
+	surface->image->parent = NULL;
+	cairo_surface_finish (surface->image);
 	cairo_surface_destroy (surface->image);
+    }
 
     /* If we created the Bitmap and DC, destroy them */
     if (surface->bitmap) {