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Nils Goroll authored
Before this patch, layered directors needed to be destroyed top to bottom, and whenever that order was missed, we would panic, because a to-be-destroyed director still had references to it. One special case where this issue would always trigger are looped directors. Those should not be used and will cause havoc, which is a separate issue #3899. But we should still be able to unconfigure such a configuration. We solve the destruction order issue by making it a two step process: When a director is destroyed through VRT_DelDirector, a new release function is called, which has to disassociate any backends. The director then loses a reference, and when all references are gone, the destroy function is called. The new callback would not be necessary for the cases in varnish-cache today, directors could simply disassociate any backends before calling VRT_DelDirector. But this would complicate or even make impossible transfer of director ownership, where the code responsible for creating a director is not the same as the one calling VRT_DelDirector(). As a side effect, it also helps clarity. Fixes #3895
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