core: first step towards a pidfd focused future #29142
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Let's start with the conversion of PID 1 to pidfds. Let's add a simple structure with just two fields that can be used to maintain a reference to arbitrary processes via both pid_t and pidfd. This is an embeddable struct, to keep it in line with where we previously used a pid_t directly to track a process. Of course, since this might contain an fd on systems where we have pidfd this structure has a proper lifecycle. (Note that this is quite different from sd_event_add_child() event source objects as that one is only for child processes and collects process results, while this infra is much simpler and more generic and can be used to reference any process, anywhere in the tree.)
The first conversion to PidRef. It's mostly an excercise of search/replace, but with some special care taken for life-cycle (i.e. we need to destroy the structure properly once done, to release the pidfd). It also uses pidfd based killing for some of the killing but leaves most as it is to make the conversion minimal.
Just like the previous commit, but for socket rather than service units.
Let's stop manually destroying the timers when we fail to spawn a child. We don't do this in any of the similar codepaths in any of the unit types, only in two specific ones in socket/swap. Destroying the timer is unnecessary, since this is done anyway in the _set_state() call of each unit type if not appropriate, and every failure path here runs through that anyway. This brings all these similar codepaths into sync.
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This was referenced Sep 11, 2023
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This is the first step towards putting pidfds front and center for service process tracking. It doesn't move all process operations done by PID 1 over to it, but at least the basic tracking is now via pidfd. Further infra should be moved over too, but can happen in separate steps, already to make this PR not ridiculously large.