CVRhsFn_wrapper unsafe use of @cfunction($f, ...)

[sjdaines]

See eg CVRhsFn_wrapper:

Sundials.jl/src/types_and_consts_additions.jl

Lines 9 to 10 in 64c98c8

	CVRhsFn_wrapper(fp::CVRhsFn) = fp
	CVRhsFn_wrapper(f) = @cfunction($f, Cint, (realtype, N_Vector, N_Vector, Ptr{Cvoid})).ptr

which is used in

Sundials.jl/lib/libsundials_api.jl

Lines 1668 to 1671 in 64c98c8

	function CVodeInit(cvode_mem, f, t0, y0)
	__y0 = convert(NVector, y0)
	CVodeInit(cvode_mem, CVRhsFn_wrapper(f), t0, convert(N_Vector, __y0))
	end

The problem is that a temporary Base.CFunction closure is returned by @cfunction($f, ...), which may be garbage collected so it is not safe to return the enclosed pointer (somewhat similar to the problem with NVector addressed by #380, but more fundamental as the RHS function needs to persist for the whole lifetime of the solver use).

Suggested fix is to remove CVRhsFn_wrapper (which is only used by a few tests of the low-level interface), and require that the caller either uses the @cfunction(f, ...) form to generate a C pointer (ie f known at compile time and the runtime Julia RHS function passed in to Sundials C code as part of the opaque user data, this is what the high-level interface does), or manages the lifetime of the @cfunction explicitly.

Addressed by PR #384

[ChrisRackauckas]

Fixed thanks. I wonder if this will end up fixing the ability to use Sundials' sensitivity analysis stuff too. It would be interesting to try and get https://github.com/SciML/Sundials.jl/blob/master/test/cvodes_dns.jl up and running