keventd: add class/<subsystem>/<name> and driver/<name> conditions

dev/<X> only fires when a device gets a /dev node, which leaves a lot of
embedded-relevant hardware uncoverable: DSA switch cores, IIO sensors,
LEDs, backlight, PHYs, regulators -- all live purely under sysfs.

Two new condition namespaces:

  class/<subsystem>/<sysname>  asserted on every sysfs class device add
                                (e.g. <class/leds/blue>)
  driver/<name>                 asserted while the driver is bound to at
                                least one device (e.g. <driver/mt7530>)

A driver can bind to several devices, so driver/ conditions are
refcounted: asserted on first bind, cleared when the last device is
unbound.

dev_cond() is generalized into a static cond_emit(prefix, rel, set) so
class_cond() and driver_cond() share the same mkpath + symlink/erase
code.  The name avoids colliding with src/cond.h's public cond_path()
helper (unrelated function that returns a condition's filesystem path).

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

doc/conditions.md

@@ -177,7 +177,8 @@ Built-in Conditions
 
 Finit comes with a set of plugins for conditions:
 
- - `keventd`: provides `<dev/...>` and `<sys/pwr/...>`
+ - `keventd`: provides `<dev/...>`, `<class/...>`, `<driver/...>`, and
+   `<sys/pwr/...>`
  - `devmon` (built-in fallback for `<dev/...>` without keventd)
  - `netlink`: provides `<net/...>`
  - `pidfile`: provides `<pid/...>`
@@ -191,6 +192,11 @@ condition is cleared.  If keventd is not in use (an external device
 manager like udevd is used instead), the `devmon` built-in provides the
 same conditions by monitoring `/dev` and `/dev/dir` with inotify.
 
+keventd also asserts `class/<subsystem>/<name>` for devices that have
+no `/dev` node, e.g. LEDs and DSA switch ports, and `driver/<name>`
+while a driver is bound to at least one device.  See
+[keventd](keventd.md) for details.
+
 The `pidfile` plugin recursively watches `/run/` for PID files created
 by the monitored services, and sets a corresponding condition in the
 `pid/` namespace.
@@ -234,6 +240,8 @@ Built-in conditions:
 - `usr/foo`
 - `boot/arg`
 - `dev/node` and `dev/dir/node`
+- `class/<subsystem>/<name>`
+- `driver/<name>`
 
 > [!NOTE]
 > Here, `up` means administratively up, the interface flag `IFF_UP`.

doc/keventd.md

@@ -227,9 +227,60 @@ devices:
         command     = "/usr/sbin/gps-daemon"
     }
 
+Network interfaces are not device nodes and do not live in the `dev/`
+namespace -- a `/dev/wan` node created by a user must not be confused
+with a `wan` interface.  To wait for an interface, use the `netlink`
+plugin's `net/<iface>/exist` condition, plus `net/<iface>/up` (admin
+up) and `net/<iface>/running` (carrier present) to gate on link state:
+
+    service dhcpcd {
+        description = "DHCP client"
+        runlevel    = "2345"
+        conditions  = { "net/wan/exist" }
+        command     = "/usr/sbin/dhcpcd"
+    }
+
+keventd provides the parallel `class/net/<iface>` condition, like for
+any other sysfs class device.
+
 When the device is removed, the condition is cleared and Finit stops
 the dependent services.
 
+### Class Conditions (`class/`)
+
+Many devices live in sysfs without a `/dev/` node -- DSA switch ports,
+IIO sensors, LEDs, backlight, PHYs, regulators.  For those, keventd
+asserts `class/<subsystem>/<sysname>` on every `add` event so services
+can still wait for them:
+
+    service blink-blue {
+        description = "LED driver"
+        runlevel    = "2345"
+        conditions  = { "class/leds/blue" }
+        command     = "/usr/sbin/blink-blue"
+    }
+
+The condition is cleared on `remove`.
+
+### Driver Conditions (`driver/`)
+
+`driver/<name>` is asserted while the driver `<name>` is bound to at
+least one device, from the kernel's `bind`/`unbind` uevents.  Use this
+to gate on slow-probing hardware whose readiness isn't marked by a
+class device or `/dev` node, such as a switch core or complex PHY:
+
+    service dsa-probe {
+        description = "DSA topology probe"
+        runlevel    = "2345"
+        conditions  = { "driver/mt7530" }
+        command     = "/usr/sbin/dsa-probe"
+    }
+
+A driver bound to several devices keeps the condition asserted until
+the last device is unbound.  To wait for one specific device instance,
+gate on what its probe creates instead: the `class/` condition or
+`/dev` node of the child device.
+
 ### Power Supply Conditions (`sys/pwr/`)
 
 keventd monitors the `power_supply` subsystem and provides: