This simply checks if the specified PidRef refers to the process we are
running in.
(In case you wonder why this is not a static inline: to avoid cyclic
header inclusion problems between pidref.h + process-util.h)
This new helper can be used after reading process info from procfs, to
verify that the data that was just read actually matches the pidfd, and
does not belong to some new process that just reused the numeric PID of
the process we originally pinned.
Usually we want to embed PidRef in other structures, but sometimes it
makes sense to allocate it on the heap in case it should be used
standalone. Add helpers for that.
Primary usecase: use as key in Hashmap objects, that for example map
process to unit objects in PID 1.
This adds pidref_free()/pidref_freep() for freeing such an allocated
struct, as well as pidref_dup() (for duplicating an existing PidRef
on the heap 1:1), and pidref_new_pid() (for allocating a new PidRef from a
PID).
This helper truns a pid_t into a PidRef. It's different from
pidref_set_pid() in being "passive", i.e. it does not attempt to acquire
a pidfd for the pid.
This is useful when using the PidRef as a lookup key that shall also
work after a process is already dead, and hence no conversion to a pidfd
is possible anymore.
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.)
