Fix #367: add support for mdev's netlink daemon mode

This commit replaces the `mdev -s` call to populate the device tree with
the "new" `mdev -df` daemon mode, introduced in BusyBox 1.31.0, 2019.

In daemon mode mdev listens to kernel uevents, replacing the hotplug use
of mdev.  After creating the netlink socket, 'mdev -df' performs the same
initial scan as 'mdev -s' did.

To perform device (re)discovery, module loading and setup, including any
firmware loading, a coldplug operation is typically required.  This is
done by the new /libexec/finit/coldplug script, by Alexander Zangerl.

Unlike 'mdevadm settle', mdev does not offer any mechanism to detect when
the discovery operation is done: on slower systems the triggering side,
the coldplug script, often completes quite a bit earlier than mdev's
uevent processing.  I.e., depending on <run/coldplug/success> is not an
indicator of all devices having been (re)discovered and fully set up.

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

configure.ac

@@ -15,7 +15,7 @@ AC_CONFIG_FILES([Makefile
 	man/Makefile
 	plugins/Makefile
 	src/Makefile
-	system/Makefile
+	system/Makefile system/10-hotplug.conf
 	tmpfiles.d/Makefile
 	test/test.env test/Makefile
 	test/lib/Makefile test/src/Makefile])

src/Makefile.am

@@ -19,6 +19,8 @@ if WATCHDOGD
 pkglibexec_PROGRAMS += watchdogd
 endif
 
+dist_pkglibexec_SCRIPTS = coldplug
+
 finit_pkglibdir      = $(pkglibdir)
 finit_pkglib_SCRIPTS = rescue.conf sample.conf
 

src/coldplug

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+# With the daemon mode ('mdev -df') we no longer need the hotplug helper
+echo "" > /proc/sys/kernel/hotplug
+
+# Wait for 'mdev -df' to set up its event queue.
+sleep 1
+
+# Trigger 'add' event for all devices and subsystems to load all modules
+# and any firmware needed by the hardware.
+find /sys/devices -name uevent -print | while IFS='' read X; do echo add > $X; done

system/10-hotplug.conf

@@ -1,17 +0,0 @@
-# System hotplug can be handled by udev or mdev
-
-# Either we have systemd-udevd or eudev, on the rare occasion we find
-# both, we opt for the latter.  Remember, you can override this by a
-# custom hotplug.conf in /etc/finit.d/ or /etc/finit.d/enabled/.
-service nowarn [S12345789] cgroup.system name:udevd pid:udevd log /lib/systemd/systemd-udevd -- Device event managing daemon (udev)
-service nowarn [S12345789] cgroup.system name:udevd pid:udevd log udevd -D -- Device event managing daemon (udev)
-
-# Wait for udevd to start, then wait a couple of seconds for it to become ready
-run nowarn if:udevd cgroup.init :1 [S] <pid/udevd> log udevadm settle -t 0                  --
-run nowarn if:udevd cgroup.init :2 [S] <pid/udevd> log udevadm control --reload             --
-run nowarn if:udevd cgroup.init :3 [S] <pid/udevd> log udevadm trigger -c add -t devices    -- Requesting device events
-run nowarn if:udevd cgroup.init :4 [S] <pid/udevd> log udevadm trigger -c add -t subsystems -- Requesting subsystem events
-run nowarn if:udevd cgroup.init :5 [S] <pid/udevd> log udevadm settle -t 30                 -- Waiting for udev to finish
-
-# If udevd is not available, try mdev
-run nowarn conflict:udevd [S] mdev -s -- Populating device tree

system/10-hotplug.conf.in

@@ -0,0 +1,42 @@
+# System hotplug handled by udev or mdev
+#
+# Your system usually has either udevd, mdev, or mdevd.  They probe the system
+# for devices, loading modules and any firmware needed.  Since the bootstrap
+# often needs to be synchronized carefully, we suggest using your system log
+# daemon as the barrier for all other services.  E.g.:
+#
+#    service if:udevd <run/udevadm:5/success> [S0123456789] syslogd -F -- System log daemon
+#    service if:mdev  <run/mdev/success>      [S0123456789] syslogd -F -- System log daemon
+#
+# This provdes a <pid/syslogd> condition that can act as a barrier for all
+# other services.  Notice the `if:` statement and the condition for starting
+# syslogd is only considered if either udevd or mdev (service) is loaded and
+# is guaranteed to run after each respective run stanza have completed.
+#
+# Override this file by copying it to /etc/finit.d/, using the same name, then
+# change the contents any way you like, it can even be empty.
+
+# Check for systemd-udevd and eudev, if we find both, we opt for the latter.
+service nowarn [S12345789] cgroup.system name:udevd pid:udevd log /lib/systemd/systemd-udevd -- Device event daemon (udev)
+service nowarn [S12345789] cgroup.system name:udevd pid:udevd log udevd -D -- Device event daemon (udev)
+
+# Wait for udevd to start, then trigger coldplug events and module loading.
+# The last 'settle' call waits for it to finalize processing all uevents.
+run nowarn if:udevd cgroup.init :1 [S] <pid/udevd> log udevadm settle -t 0                  --
+run nowarn if:udevd cgroup.init :2 [S] <pid/udevd> log udevadm control --reload             --
+run nowarn if:udevd cgroup.init :3 [S] <pid/udevd> log udevadm trigger -c add -t devices    -- Requesting device events
+run nowarn if:udevd cgroup.init :4 [S] <pid/udevd> log udevadm trigger -c add -t subsystems -- Requesting subsystem events
+run nowarn if:udevd cgroup.init :5 [S] <pid/udevd> log udevadm settle -t 30                 -- Waiting for udev to finish
+
+# If udevd is not available, try mdev.  The "new" daemon mode (-df) was added
+# in BusyBox 1.31.0, 2019.  It calls `mdev -s` internally but does not create
+# a PID file when that is done, so synchronization with coldplug is fragile.
+service nowarn conflict:udevd cgroup.init \
+	[S12345789] mdev -df -S -- Device event daemon (mdev)
+
+# Trigger a replay of all uevents from boot, before userspace was up and the
+# filesystem mounted.  Compared to udevd, we don't know when 'mdev -df' has
+# finished processing all events, i.e., depending on <run/coldplug/success> is
+# not an indicator of all devices having been (re)discovered and fully set up.
+run nowarn conflict:udevd cgroup.init name:coldplug if:<service/mdev/running> \
+	[S] @pkglibexecdir@/coldplug -- Cold plugging system

system/Makefile.am

@@ -1,8 +1,10 @@
+EXTRA_DIST        = 10-hotplug.conf.in
 systemdir         = $(system_path)
+system_DATA       =
 dist_system_DATA  =
 
 if BUILD_HOTPLUG_PLUGIN
-dist_system_DATA += 10-hotplug.conf
+system_DATA      += 10-hotplug.conf
 endif
 
 if BUILD_TESTSERV_PLUGIN