A backdoor in xz

> System V init never needed that

Yes, and it sucked.

> This a well-defined POSIX-defined behavior

The fact that it is well-defined or POSIX-defined does not automatically mean that it's _good_. I hate to break it to you, but POSIX is not a pinnacle of system design.

> It shall be up to sshd process - and to all other system daemons - to die if it failed to parse configuration and/or bind necessary sockets

Setting up a proper readiness notification by double-forking is approximately tenfold more complicated and requires exponentially more moving parts than the sd_notify mechanism.

In fact, many daemons (including openssh) do not complete their initialization until after the fork, so the only correct implementation of the interface you describe entails the immediate child _waiting_ for the grandchild to finish its setup, and only then exiting. Which means that there has to be a temporary pipe or socket between the child and the grandchild.

So now we are choosing between a socket notification mechanism implemented _once_ in a well-audited, well-maintained project (systemd) and **the same socket notification mechanism** plus a bunch of historical nonsense implemented _all over again_ in each daemon.

I trust the choice is obvious.