[cairo] reference counting vs garbage collection

[Keith Packard]

In my nickle binding, I'm creating garbage collected objects that wrap 
cairo surfaces and contexts.  This allows me to automatically destroy 
surfaces which are no longer in use.

Cairo doesn't make this easy for some kinds of surfaces though; for png 
and ps surfaces, there is a 'FILE' object which is referenced by these 
surfaces and which must be fclose'd when the surface is finally destroyed.

Of course, each cairo_t object associated with the surface (either as 
target or pattern) holds a reference to it, so the surface isn't really 
destroyed until the cairo_t objects are also destroyed.

The problem here is that I have no idea which call to 
cairo_surface_destroy actually cleans up the cairo internal state, 
flushing the remaining output to the file and dropping the reference to 
the FILE object.

So, if the surface is finalized before the cairo_t and I fclose the file 
at that point, when the cairo_t gets finalized and the png library 
attempts to write the file, I get a nice segfault.

I don't really know what would work best in this situation; either some 
kind of callback (yuck) when the surface is actually destroyed, or some 
way of knowing whether a call to cairo_surface_destroy has actually 
destroyed the surface might work.  That would require some care on my part 
as things like cairo_destroy also call cairo_surface_destroy, but I think 
that could work.

What have other garbage collected bindings done in this situation?

-keith


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