[cairo] Memory leak with image surfaces

[Chris Wilson]

On Fri, Jun 14, 2013 at 08:15:44PM +0200, Victor Goya wrote:
> I ran into the same problem with larger PDF files, the decompressed
> images are never freed.
> 
> The issue seems to be related to the "COW snapshots" feature :
> "_cairo_image_surface_snapshot" create a clone of the surface but set
> the "owns_data" flag of both the surface and its clone to FALSE. This
> can lead in a situation where neither the original surface nor its
> clone own the data, this results in a memory leak because none of them
> take the responsability to free it.
> 
> My guess is that the clone should copy the "owns_data" flag of the
> original surface before setting the "owns_data" flag of the original
> surface to FALSE. This way the clone is responsible to free the data
> only if the original surface owned it. I'm not quite sure about all
> the implications of the attached patch, but it fixes the leak for my
> use case.

Your analysis is correct, and the fix is indeed that I should have
marked the clone as owning the data along that fast path. Thanks!

commit e079e4e2297810ceb10798cf310a7fafe39b18f0
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Thu Jun 20 14:02:14 2013 +0100

    image: Mark the data as owned after stealing the snapshot's image
    
    Victor Goya found that we ended up leaking memory after reading a PNG
    into an image surface and drawing that onto a PDF surface. In
    particular, he discovered that
    
    commit 0bfd2acd35547fc2bd0de99cc67d153f0170697d
    Author: Chris Wilson <chris at chris-wilson.co.uk>
    Date:   Mon Aug 13 01:34:12 2012 +0100
    
        xlib: Implement SHM fallbacks and fast upload paths
    
    introduced a path to steal the image data for a snapshot (and thereby
    avoid a redundant copy), but that path then lead to the leak of the
    "owned" data.
    
    Reported-by: Victor Goya <victor.goya at af83.com>
    Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre