Let's simplify things a bit: we so far called both functions every
single time, let's just merge one into the other, so that we have fewer
functions to call.

This reworks is_kernel_thread() a bit. Instead of checking whether
/proc/$pid/cmdline is entirely empty we now parse the 'flags' field from
/proc/$pid/stat and check the PF_KTHREAD flag, which directly encodes
whether something is a kernel thread.

Why all this? With current kernels userspace processes can set their
command line to empty too (through PR_SET_MM_ARG_START and friends), and
could potentially confuse us. Hence, let's use a more reliable way to
detect kernels like this.

I figure saying "systemd" here was a typo, and it should have been
"system". (Yes, it becomes very hard after a while typing "system"
correctly if you type "systemd" so often.) That said, "systemd" in some
ways is actually more correct, since BPF might be available for the
system instance but not in the user instance.

Either way, talking of "this systemd" is weird, let's reword this to be
"this manager", to emphasize that it's the local instance of systemd
where BPF is not available, but that it might be available otherwise.

No functional changes, but let's make this a bit more finegrained.

(The individual functions are exported, which is used in a later commit)

This removes the current bus_init() call, as it had multiple problems:
it munged  handling of the three bus connections we care about (private,
"api" and system) into one, even though the conditions when which was
ready are very different. It also added redundant logging, as the
individual calls it called all logged on their own anyway.

The three calls bus_init_api(), bus_init_private() and bus_init_system()
are now made public. A new call manager_dbus_is_running() is added that
works much like manager_journal_is_running() and is a lot more careful
when checking whether dbus is around. Optionally it checks the unit's
deserialized_state rather than state, in order to accomodate for cases
where we cant to connect to the bus before deserializing the
"subscribed" list, before coldplugging the units.

manager_recheck_dbus() is added, that works a lot like
manager_recheck_journal() and is invoked in unit_notify(), i.e. when
units change state.

All in all this should make handling a bit more alike to journal
handling, and it also fixes one major bug: when running in user mode
we'll now connect to the system bus early on, without conditionalizing
this in anyway.

In test mode, let's not consider the journal to be up ever: we want all
output to go to stderr.

Let's also use the journal if it is currently reloading. In that state
it should also be able to process our requests. Moreover, we might
otherwise end up disconnecting/reconnecting from the journal without
really any need to hence, relax the check accordingly.

No need for an if check if we just pass along a bool anyway.

This patch does four things:

1. Adds more comments that clarify the order in which things appear in
   the file

2. All entries are placed in the order in which their SD_BUS_METHOD()
   macros appear in the C vtables.

3. A couple of missing entries are added that should be open to all or
   do polkit

4. Corrects the interface name for the GetProcesses() calls. They belong
   to the per-unit interface, not to Unit

…s calls

This splits out the code that translates a unit name into a Unit* object
from method_get_unit(), and reuses it all other functions that operate
similar to it. This effectively means all those calls now optionally
take an empty unit string which now means the same as the client's unit.
This useful behaviour of the GetUnit() bus call is thus extended to all
other matching bus calls.

Similar, the same logic from method_load_unit() is also generalized and
reused wherever appropriate.

Let's make debugging easier, by synthesizing a name when we have some
indication what kind of bus this is.

Let's make things easier to debug

… event loop iteration

Previously, we'd synchronize bus names immediately when we succeeded
connecting to the bus, potentially even before coldplugging the units.
This was problematic, as synchronizing bus names meant invoking the
per-unit name change handler function which might change the unit's
state — which will result in consistency when done before we coldplug
things.

With this change we instead enqueue a job for the event loop to resync
the names in a later loop iteration, i.e. at a point where we know
coldplugging has finished.

…ssociated with it

Let's not keep the old bus object pinned this way, let's destroy all
relevant trackers for units, the way we already destroy them for jobs.

Let's better be safe than sorry, and validate that api_bus is not NULL
before we send messages to it. Of course, strictly speaking this
shouldn't actually be necessary, as the tracker object should not exist
without the bus, but let's be extra sure.

…we inherit the API bus

This corrects the control flow: when we reuse the API bus as system bus,
let's definitely invoke bus_init_system() too, so that it is called
regardless how we acquired the bus object.

(Note that this doesn't actually change anything, as we only inherit the
bus like this in system mode, and bus_init_system() doesn't do anything
in system bus, besides writing a log message)

…ervice units

This adds a new bus call to service and scope units called
AttachProcesses() that moves arbitrary processes into the cgroup of the
unit. The primary user for this new API is systemd itself: the systemd
--user instance uses this call of the systemd --system instance to
migrate processes if itself gets the request to migrate processes and
the kernel refuses this due to access restrictions.

The primary use-case of this is to make "systemd-run --scope --user …"
invoked from user session scopes work correctly on pure cgroupsv2
environments. There, the kernel refuses to migrate processes between two
unprivileged-owned cgroups unless the requestor as well as the ownership
of the closest parent cgroup all match. This however is not the case
between the session-XYZ.scope unit of a login session and the
[email protected] of the systemd --user instance.

The new logic always tries to move the processes on its own, but if
that doesn't work when being the user manager, then the system manager
is asked to do it instead.

The new operation is relatively restrictive: it will only allow to move
the processes like this if the caller is root, or the UID of the target
unit, caller and process all match. Note that this means that
unprivileged users cannot attach processes to scope units, as those do
not have "owning" users (i.e. they have now User= field).

Fixes: systemd#3388