This is a small wrapper around daemon(3).
The implementation does not violate any of the constraints documented on Command::pre_exec, and this code is expected to be safe.
This code may however cause a process hang if not used appropriately. Reading on the subtleties of CLOEXEC, CLOFORK and forking multi-threaded programs will provide ample background reading. For the sake of safe use, callers should work to ensure that uses of daemonize occur early in the program lifecycle, before many threads have been spawned, libraries have been used or files have been opened that may introduce CLOEXEC behaviors that could cause EXTBUSY outcomes in a Linux environment.