Fix #195: add mdevd plugin as complement to hotplug.sh

This patch adds the mdevd[1] plugin from Andy's fork of Finit, only
minor whitespace adjustment and group fix introduced, along with
updates to configure.ac and the documentation.

Currently the hotplug.sh calls mdevd-coldplug (instad of mdev -s), but
since mdevd is not guaranteed to have started and be ready, it is also
started with mdevd -C.  A better solution is to use the s6 notify code
to synchronize the start of mdevd-coldplug.  Support for this is coming
soon to Finit.

[1]: https://github.com/hongkongkiwi/finit/blob/master/plugins/mdevd.c

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

plugins/mdevd.c

@@ -0,0 +1,123 @@
+/* Setup and start system hotplug helper mdevd
+ *
+ * Portions Copyright (c) 2012-2022  Joachim Wiberg <troglobit@gmail.com>
+ * Portions Copyright (c) 2022 Andy Savage <andy@savage.hk>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include <sys/types.h>
+#ifdef _LIBITE_LITE
+# include <libite/lite.h>
+#else
+# include <lite/lite.h>
+#endif
+
+#include "finit.h"
+#include "config.h"
+#include "helpers.h"
+#include "plugin.h"
+#include "service.h"
+#include "conf.h"
+
+#define MDEVD_DAEMON "mdevd"
+/* Rebroadcast events onto nlgroup 4 */
+#define MDEVD_ARGS   "-C -O 4"
+#define MDEVD_DESC   "MDEVD Extended Hotplug Daemon"
+
+#ifndef MDEVD_DAEMONUSER
+#define MDEVD_DAEMONUSER "root"
+#endif
+
+#ifndef MDEVD_DAEMONGROUP
+#define MDEVD_DAEMONGROUP "root"
+#endif
+
+#ifndef MDEVD_DAEMONPIDFILE
+#define MDEVD_DAEMONPIDFILE "/var/run/mdevd.pid"
+#endif
+
+static void setup(void *arg)
+{
+	char line[256];
+	mode_t prev;
+	char *cmd;
+	FILE *fp;
+
+	if (rescue) {
+		_d("Skipping %s plugin in rescue mode.", __FILE__);
+		return;
+	}
+
+	cmd = which(MDEVD_DAEMON);
+	if (!cmd) {
+		_d("Skipping plugin, %s is not installed.", MDEVD_DAEMON);
+		return;
+	}
+
+	prev =umask(0);
+
+	/* Clean up from any previous pre-bootstrap run */
+	remove(MDEVD_DAEMONPIDFILE);
+
+	/* Set hotplug helper */
+	fp = fopen("/proc/sys/kernel/hotplug", "w");
+	if (fp) {
+		fputc('\n', fp);
+		fclose(fp);
+	}
+
+	/* Register service with Finit */
+	if (debug)
+		snprintf(line, sizeof(line), "[S12345789] cgroup.system pid:!%s @%s:%s %s -v3 %s -- %s",
+			 MDEVD_DAEMONPIDFILE, MDEVD_DAEMONUSER, MDEVD_DAEMONGROUP, cmd, MDEVD_ARGS, MDEVD_DESC);
+	else
+		snprintf(line, sizeof(line), "[S12345789] cgroup.system pid:!%s @%s:%s %s %s -- %s",
+			 MDEVD_DAEMONPIDFILE, MDEVD_DAEMONUSER, MDEVD_DAEMONGROUP, cmd, MDEVD_ARGS, MDEVD_DESC);
+	if (service_register(SVC_TYPE_SERVICE, line, global_rlimit, NULL))
+		_pe("Failed registering %s", MDEVD_DAEMON);
+	free(cmd);
+
+	umask(prev);
+}
+
+static plugin_t plugin = {
+	.name = __FILE__,
+	.hook[HOOK_BASEFS_UP] = {
+		.cb  = setup
+	},
+	.depends = { "bootmisc", },
+};
+
+PLUGIN_INIT(plugin_init)
+{
+	plugin_register(&plugin);
+}
+
+PLUGIN_EXIT(plugin_exit)
+{
+	plugin_unregister(&plugin);
+}
+
+/**
+ * Local Variables:
+ *  indent-tabs-mode: t
+ *  c-file-style: "linux"
+ * End:
+ */