initctl: restore 'cond [set|clr] foo' for user-defined conditions

In 7447192 we dropped support for 'cond set' and 'cond clear' commands
with the motivation they were unsafe and sent the wrong message to the
user.  That was true, but mostly because it was too generic and required
the user to call `initctl reload` to apply the changes.

This patch restores the behavior, albeit in a very reduced and simple
format.  All conditions set with this command are constrained to the
'usr/...' namespace.  No subdirectories are allowed.  The argument to
the 'cond set|clear' command is disallowed if it contains '/' or '.'
but anything else is supported, for example:

    initctl cond set foo:2

creates a static/oneshot condition in /run/finit/cond/usr/foo:2

These conditions are static and are fully handled by the user.  The
initctl command is the recommended, and only supported, way of setting
and clearing usr conditions.

This patch also includes a new plugin, usr.so, which is a very simple
inotify plugin for the /run/finit/cond/usr/ directory.  When files are
created or removed here the plugin tells the Finit condition engine to
update and trigger service changes.

For instance, the following service is not started by default at boot:

    service <usr/foo> myservice -- MyService

However, as soon as `initctl cond set foo` is called, myservice starts.
Consequently, it is stopped when `initctl cond clr foo` is called.

Another major difference from the original is that this implementation
doesn't send IPC commands to create/delete the conditions.  This makes
calling these new commands non-blocking so they can be used very early
in the bootstrap, by plugins, if needed.

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

src/initctl.c

@@ -270,6 +270,12 @@ static int dump_one_cond(const char *fpath, const struct stat *sb, int tflag, st
 			nm  = svc_ident(svc, NULL, 0);
 			pid = svc->pid;
 		}
+	} else if (strncmp("usr/", cond, 4) == 0) {
+		nm  = "static";
+		pid = 0;
+	} else if (strncmp("hook/", cond, 4) == 0) {
+		nm  = "static";
+		pid = 1;
 	}
 
 	printf("%-*d  %-*s  %-6s  %s\n", pw, pid, iw, nm, asserted, cond);
@@ -296,6 +302,38 @@ static int do_cond_dump(char *arg)
 	return 0;
 }
 
+static int do_cond_act(char *arg, int creat)
+{
+	char oneshot[256];
+	size_t off;
+
+	if (arg && strncmp(arg, COND_USR, strlen(COND_USR)) == 0)
+		arg += strlen(COND_USR);
+
+	if (!arg || !arg[0])
+		errx(1, "Invalid condition (empty)");
+	if (strchr(arg, '/'))
+		errx(1, "Invalid condition (slashes)");
+	if (strchr(arg, '.'))
+		errx(1, "Invalid condition (periods)");
+
+	snprintf(oneshot, sizeof(oneshot), _PATH_CONDUSR "%s", arg);
+	off = strlen(_PATH_COND);
+
+	if (creat) {
+		if (symlink(_PATH_RECONF, oneshot) && errno != EEXIST)
+			err(1, "Failed asserting condition <%s>", &oneshot[off]);
+	} else {
+		if (erase(oneshot))
+			err(1, "Failed deasserting condition <%s>", &oneshot[off]);
+	}
+
+	return 0;
+}
+
+static int do_cond_set(char *arg) { return do_cond_act(arg, 1); }
+static int do_cond_clr(char *arg) { return do_cond_act(arg, 0); }
+
 static void show_cond_one(const char *_conds)
 {
 	static char conds[MAX_COND_LEN];
@@ -765,7 +803,9 @@ static int usage(int rc)
 	fprintf(stderr,
 //		"  reload   <NAME>[:ID]      Reload (SIGHUP) service by name\n"  
 		"\n"
-		"  cond     show             Show condition status\n"
+		"  cond     set   <COND>     Set (assert) user-defined condition     +usr/COND\n"
+		"  cond     clear <COND>     Clear (deassert) user-defined condition -usr/COND\n"
+		"  cond     status           Show condition status, default cond command\n"
 		"  cond     dump             Dump all conditions and their status\n"
 		"\n"
 		"  log      [NAME]           Show ten last Finit, or NAME, messages from syslog\n"
@@ -850,6 +890,9 @@ int main(int argc, char *argv[])
 	struct cmd cond[] = {
 		{ "status",   NULL, do_cond_show }, /* default cmd */
 		{ "dump",     NULL, do_cond_dump },
+		{ "set",      NULL, do_cond_set  },
+		{ "clr",      NULL, do_cond_clr  },
+		{ "clear",    NULL, do_cond_clr  },
 		{ NULL, NULL, NULL }
 	};
 	struct cmd command[] = {