[cairo-commit] src/cairo-xcb-surface.c

[Uli Schlachter]

 src/cairo-xcb-surface.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

New commits:
commit bfc243ca52e85e909e200363a687611d256ccc66
Author: Uli Schlachter <psychon at znc.in>
Date:   Tue May 30 18:04:38 2017 +0200

    xcb: Fix error reporting if fallback fails
    
    If we cannot let the X11 server do some operation (for example: the
    RENDER extension is not available), then we fall back to an image
    surface and do the operation locally instead. This fallback requires the
    current content of the surface to be downloaded from the X11 server.
    This fallback logic had an error.
    
    The fallback is implemented with _get_image() in the function
    _cairo_xcb_surface_fallback(). _get_image() is only called if we do not
    yet have a fallback available, so when we call _get_image we have
    surface->fallback == NULL. Then, if _get_image() fails, it returns a
    surface in an error state.
    
    Before this patch, the code would then just ignore this error surface
    and return &surface->fallback->base, a NULL pointer. This would then
    quickly cause a crash when e.g. the surface's ->status member is
    accessed.
    
    Fix this by returning the error surface instead as the fallback.
    
    The end result of this patch will be that the XCB surface that is
    currently drawn to ends up in an error state which is a lot better than
    a NULL pointer dereference and actually correct in this case. The error
    state is reached because the current drawing operation will fail and
    this error is reported up the call stack and eventually "taints" the
    surface.
    
    (However, the error code could be better: _get_image() too often fails
    with a generic CAIRO_STATUS_NO_MEMORY error, but that's left as future
    work)
    
    Signed-off-by: Uli Schlachter <psychon at znc.in>

diff --git a/src/cairo-xcb-surface.c b/src/cairo-xcb-surface.c
index b319b7f2..39127c14 100644
--- a/src/cairo-xcb-surface.c
+++ b/src/cairo-xcb-surface.c
@@ -833,12 +833,13 @@ _cairo_xcb_surface_fallback (cairo_xcb_surface_t *surface,
     image = (cairo_image_surface_t *)
 	    _get_image (surface, TRUE, 0, 0, surface->width, surface->height);
 
+    if (image->base.status != CAIRO_STATUS_SUCCESS)
+	return &image->base;
+
     /* If there was a deferred clear, _get_image applied it */
-    if (image->base.status == CAIRO_STATUS_SUCCESS) {
-	surface->deferred_clear = FALSE;
+    surface->deferred_clear = FALSE;
 
-	surface->fallback = image;
-    }
+    surface->fallback = image;
 
     return &surface->fallback->base;
 }