This undoes 531ac2b. I acked that patch without looking at the code
carefully enough. There are two problems:
- we want to process the fds anyway
- in principle empty notification messages are valid, and we should
  process them as usual, including logging using log_unit_debug().

It's probably easier to diagnose a bad notification message if the
contents are printed. But still, do anything only if debugging is on.

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Hmm I don't get the "we want to process the fds anyway" part.

To process an fds, isn't the service supposed to pass "FDSTORE=1" ?

If you want them stored, then yes. But from systemd side, we want to process and do something with them: if we are not going to handle the message we should close the fds.

Hmm you mean that a service may want to send a fds with an empty message and can expect that systemd will close them ? If so I don't see how this can be useful and therefore why we would want to support this.

BTW how are the remaining fds which are not handled by the message closed exactly ?

Remaining fds are cleanup up by the auto-cleanup function.

It's not about being useful, but about proper cleanup.

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