keventd: serve org.finit.Device1 on /run/keventd/bus

The udev parity gaps that were blocked on IPC -- settle, trigger,
info, queue introspection, runtime rule reload -- become bus methods.
keventd serves its own socket the way Finit serves /run/finit/bus:
brokerless, libink, one socket per daemon, no forwarding between the
two.

  Settle(u) -> b      parked until the queue drains or the timeout
                      passes; true when settled
  Trigger(s, s)       replay events, action + subsystem glob
  Info(s) -> a{ss}    /run/udev/data properties for a devpath
  RulesReload() -> u  re-read rules dirs, returns rule count
  QueueEmpty (b), SeqnumProcessed (t) properties
  DeviceProcessed (ss) signal after each fully handled event

The queue state is the highest kernel seqnum keventd has handled,
baselined at startup, against /sys/kernel/uevent_seqnum.  keventd -S
now asks the running daemon first and falls back to seqnum polling.
In passive mode Trigger and RulesReload refuse.  Adds
link_call_connection() for the park bookkeeping.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/keventd.md

@@ -40,6 +40,8 @@ kernel events and handles:
   system when device nodes appear or disappear
 - **Netlink rebroadcast**: rebroadcasts processed uevents to netlink
   group 0x4 for [libudev-zero][] consumers (enabled by default)
+- **D-Bus interface**: `org.finit.Device1` -- settle, trigger, device
+  info, rules reload, and queue state over its own bus socket
 
 
 Device Nodes
@@ -331,12 +333,15 @@ It is the `udevadm settle` equivalent for migration scenarios:
 
     keventd -S -t 10 && start-graphical-session
 
-It does not talk to the running keventd (or any device manager) -- it
-simply opens `/sys/kernel/uevent_seqnum` and polls until the kernel's
-sequence counter has been stable for 200ms, then exits zero.  After
-the `-t SECONDS` timeout (default 30) it exits non-zero instead.
-Because `uevent_seqnum` is maintained by the kernel itself, `-S` works
-regardless of which device manager is active, or even if none is.
+With D-Bus support it asks the running keventd over
+[`Device1.Settle`](#d-bus-interface-orgfinitdevice1), which tracks the
+event queue where the events actually flow.  When no bus answers --
+another device manager, or a build without D-Bus -- it falls back to
+polling `/sys/kernel/uevent_seqnum` until the kernel's sequence
+counter has been stable for 200ms, then exits zero.  After the
+`-t SECONDS` timeout (default 30) it exits non-zero instead.  The
+fallback works regardless of which device manager is active, or even
+if none is.
 
 Prefer the condition-based model (`<dev/X>`, `<class/...>`,
 `<driver/...>`) over settle when you control the service definition --
@@ -349,6 +354,50 @@ Debug logging can also be toggled at runtime by sending `SIGUSR1`:
     kill -USR1 $(pidof keventd)
 
 
+D-Bus Interface (`org.finit.Device1`)
+-------------------------------------
+
+With D-Bus support (default, `--disable-dbus` opts out) keventd serves
+`org.finit.Device1` at `/org/finit/device` on its own brokerless bus,
+`unix:path=/run/keventd/bus`, the same way Finit serves
+[`org.finit`](dbus.md) on `/run/finit/bus`.  There is no forwarding
+between the two -- clients that want both connect to both.
+
+| Method        | In sig | Out sig | Priv. | Notes                                        |
+|---------------|--------|---------|-------|----------------------------------------------|
+| `Settle`      | `u`    | `b`     | no    | Wait until the event queue drains, timeout in seconds; `false` on timeout |
+| `Trigger`     | `ss`   | —       | yes   | Replay events: action, subsystem glob (empty = all) |
+| `Info`        | `s`    | `a{ss}` | no    | `/run/udev/data` properties for a devpath    |
+| `RulesReload` | —      | `u`     | yes   | Re-read the rules directories, returns rule count |
+
+| Property          | Sig | Notes                                     |
+|-------------------|-----|-------------------------------------------|
+| `QueueEmpty`      | `b` | No device events in flight                |
+| `SeqnumProcessed` | `t` | Highest kernel seqnum keventd has handled |
+
+| Signal            | Body                    | Fires when                      |
+|-------------------|-------------------------|---------------------------------|
+| `DeviceProcessed` | `ss` — devpath, action  | An event has been fully handled: node, symlinks, database |
+
+Example, wait up to ten seconds for the queue to drain:
+
+```sh
+dbus-send --address=unix:path=/run/keventd/bus \
+          --type=method_call --print-reply --dest=org.finit \
+          /org/finit/device org.finit.Device1.Settle uint32:10
+```
+
+In passive mode (`-p`) `Trigger` and `RulesReload` refuse -- device
+events are not handled here -- while `Settle` and the queue-state
+properties remain meaningful.
+
+keventd's conditions are symlinks to Finit's `reconf` generation
+marker, so they read the current generation by construction and never
+enter `flux` -- like user-defined conditions, they need no reassert
+after `initctl reload`.  Device state does not change because Finit
+re-read its configuration.
+
+
 Integration with Finit
 ----------------------