Thanks to Rhonda for the report
86ffe96 introduced the call_end function as the callbacks were not being executed in some cases. However, it also added a call to condvar->end, which turns out was done without a corresponding ->begin. I.e. it was ending the event loop too soon.
I knew I had already done this elsewhere. This reverts commit b39c5d5.
This has essentially been more or less broken for some time.
This should really be fixed by allowing zdp to take options via the CLI.