Add HOOK_SVC_LOST to detect lost processes

This patch adds support for the runtime `HOOK_SVC_LOST` and the (very
noisy) *example* plugin `lost.so`.

Customer request.

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

README.md

@@ -480,6 +480,9 @@ For your convenience a set of *optional* plugins are available:
 
 * *initctl.so*: Extends finit with a traditional `initctl` functionality.
 
+* *lost.so*: Very simple `HOOK_SVC_LOST` example.  Logs process ID and
+  name to syslog.
+
 * *netlink.so*: Listens to Linux kernel Netlink events for gateway and
   interfaces.  These events are then sent to the Finit service monitor
   for services that may want to be SIGHUP'ed on new default route or
@@ -527,6 +530,14 @@ state, etc.  Available hook points are:
   all modified/removed services have been stopped.  When the hook has
   completed, Finit continues to start all modified and new services.
 
+* `HOOK_SVC_LOST`: Called when a process is lost.  When reconfiguring
+  services at runtime this hook may be called a lot.  However, it may be
+  a quite useful hook to monitor a system post bootstrap when no, or
+  few, services are expected to exit.  A default plugin `lost.so` is
+  available in the `plugins/` subdirectory as an example.
+
+  **NOTE:** This hook callback gets the lost PID as argument.
+
 * `HOOK_RUNLEVEL_CHANGE`: Called when the user has issued a runlevel
   change.  The hook is called when services not matching the new
   runlevel have been been stopped.  When the hook has completed, Finit

plugin.c

@@ -166,18 +166,26 @@ plugin_t *plugin_find(char *name)
 }
 
 /* Private daemon API *******************************************************/
-void plugin_run_hooks(hook_point_t no)
+
+/* Some hooks are called with a fixed argument, like HOOK_SVC_LOST */
+void plugin_run_hook(hook_point_t no, void *arg)
 {
 	plugin_t *p, *tmp;
 
 	PLUGIN_ITERATOR(p, tmp) {
 		if (p->hook[no].cb) {
-			_d("Calling %s hook n:o %d from runloop...", basename(p->name), no);
+			_d("Calling %s hook n:o %d (arg: %p) ...", basename(p->name), no, arg);
-			p->hook[no].cb(p->hook[no].arg);
+			p->hook[no].cb(arg ? arg : p->hook[no].arg);
 		}
 	}
 }
 
+/* Regular hooks are called with the registered plugin's argument */
+void plugin_run_hooks(hook_point_t no)
+{
+	plugin_run_hook(no, NULL);
+}
+
 /* Generic libev I/O callback, looks up correct plugin and calls its callback */
 static void generic_io_cb(uev_t *w, void *arg, int events)
 {

plugin.h

@@ -51,6 +51,7 @@ typedef enum {
 
 	/* Runtime hooks */
 	HOOK_SVC_RECONF,
+	HOOK_SVC_LOST,
 	HOOK_RUNLEVEL_CHANGE,
 
 	/* Shutdown hooks */

plugins/Makefile.am

@@ -5,12 +5,12 @@ AM_CPPFLAGS        += -DFINIT_FIFO=\"$(fifo)\"
 
 if STATIC
 noinst_LTLIBRARIES  = libplug.la
-libplug_la_SOURCES  = alsa-utils.c bootmisc.c dbus.c hwclock.c		\
+libplug_la_SOURCES  = alsa-utils.c bootmisc.c dbus.c hwclock.c			\
-		      initctl.c netlink.c pidfile.c resolvconf.c	\
+		      initctl.c lost.c netlink.c pidfile.c resolvconf.c		\
 		      time.c tty.c urandom.c x11-common.c
 else
-pkglib_LTLIBRARIES  = alsa-utils.la bootmisc.la dbus.la hwclock.la	\
+pkglib_LTLIBRARIES  = alsa-utils.la bootmisc.la dbus.la hwclock.la		\
-		      initctl.la netlink.la pidfile.la resolvconf.la	\
+		      initctl.la lost.la netlink.la pidfile.la resolvconf.la	\
 		      time.la tty.la urandom.la x11-common.la
 
 # Clean up .la files installed by libtool despite being in -module mode

plugins/lost.c

@@ -0,0 +1,60 @@
+/* Example HOOK_SVC_LOST plugin, very noisy do not use as-is!
+ *
+ * Copyright (c) 2016  Joachim Nilsson <troglobit@gmail.com>
+ *
+ * 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 "../finit.h"
+#include "../svc.h"
+#include "../plugin.h"
+
+static void lost_pid(void *arg)
+{
+	pid_t pid = (uintptr_t)arg;
+	svc_t *svc;
+
+	svc = svc_find_by_pid(pid);
+	if (!svc)
+		return;
+
+	FLOG_INFO("Lost PID %u, svc %s", pid, svc->cmd);
+}
+
+static plugin_t plugin = {
+	.name = __FILE__,
+	.hook[HOOK_SVC_LOST] = { .cb  = lost_pid },
+};
+
+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:
+ */

private.h

@@ -36,6 +36,7 @@ int       client           (int argc, char *argv[]);
 void      service_bootstrap(void);
 void      service_monitor  (pid_t lost);
 
+void      plugin_run_hook  (hook_point_t no, void *arg);
 void      plugin_run_hooks (hook_point_t no);
 int       plugin_load_all  (uev_ctx_t *ctx, char *path);
 

service.c

@@ -740,6 +740,8 @@ void service_monitor(pid_t lost)
 	if (tty_respawn(lost))
 		return;
 
+	plugin_run_hook(HOOK_SVC_LOST, (void *)(uintptr_t)lost);
+
 	svc = svc_find_by_pid(lost);
 	if (!svc) {
 		_d("collected unknown PID %d", lost);