In https://github.com/systemd/systemd/issues/28515, multiple people report that
services that have PrivateTmp=yes and DefaultDependencies=no fail to create the
temporary directories under /tmp, when /tmp is e.g. a bind mount or some other
kind of mount that takes more time.
Before PrivateTmp=disconnected was added, we didn't have a nice solution:
DefaultDependencies=no is used to start services very early, so we wouldn't
want to add a dependency on /tmp automatically. With PrivateTmp=disconnected we
have a fairly nice solution. Let's "upgrade" to this mode automatically.
Strictly speaking, it is a small compat break, but in practice it's unlikely to
matter for early-boot services whether their /tmp is private or disconnected.
The dependency on /tmp that is checked is After. I think this is enough,
since any tmp.mount would be pulled in by local-fs.target and the rest of
the transaction anyway, so we don't need to check more than After.
The asserts are relaxed, because now the two settings can now diverge
in either way.
Resolves https://github.com/systemd/systemd/issues/28515.
[yhndnzj: fix unit_add_exec_dependencies() to handle the new
combination; add a comment in exec_needs_sys_admin()]
Zbigniew Jędrzejewski-Szmek
fa33eef344