Support s6 service startup notification

This can be enabled via the s6-notify configure option; see
http://www.skarnet.org/software/s6/notifywhenup.html for details.

ndhc will signal that it is ready when the first valid lease is
obtained.  Programs dependent on a working network interface
can then simply use s6-wait on the associated ndhc service dir.

A typical command line option assuming the s6 service directory
notification-fd contains '3' would be '--s6-notify 3', and
a typical configure file option would be 's6-notify 3'.

cfg.rl

@@ -149,6 +149,10 @@ struct cfgparse {
         client_config.rfkillIdx = t;
         client_config.enable_rfkill = true;
     }
+    action s6_notify {
+        client_config.s6_notify_fd = atoi(ccfg.buf);
+        client_config.enable_s6_notify = true;
+    }
     action version { print_version(); exit(EXIT_SUCCESS); }
     action help { show_usage(); exit(EXIT_SUCCESS); }
 }%%
@@ -190,13 +194,14 @@ struct cfgparse {
     resolv_conf = 'resolv-conf' value @resolv_conf;
     dhcp_set_hostname = 'dhcp-set-hostname' boolval @dhcp_set_hostname;
     rfkill_idx = 'rfkill-idx' value @rfkill_idx;
+    s6_notify = 's6-notify' value @s6_notify;
 
     main := blankline |
         clientid | hostname | interface | now |
         request | vendorid | user | ifch_user | sockd_user | chroot |
         state_dir | seccomp_enforce | relentless_defense | arp_probe_wait |
         arp_probe_num | arp_probe_min | arp_probe_max | gw_metric |
-        resolv_conf | dhcp_set_hostname | rfkill_idx
+        resolv_conf | dhcp_set_hostname | rfkill_idx | s6_notify
     ;
 }%%
 
@@ -292,6 +297,7 @@ static void parse_cfgfile(const char *fname)
     resolv_conf = ('-R'|'--resolv-conf') argval @resolv_conf;
     dhcp_set_hostname = ('-H'|'--dhcp-set-hostname') tbv @dhcp_set_hostname;
     rfkill_idx = ('-K'|'--rfkill-idx') argval @rfkill_idx;
+    s6_notify = ('-N'|'--s6-notify') argval @s6_notify;
     version = ('-v'|'--version') 0 @version;
     help = ('-?'|'--help') 0 @help;
 
@@ -300,7 +306,7 @@ static void parse_cfgfile(const char *fname)
         now | request | vendorid | user | ifch_user | sockd_user |
         chroot | state_dir | seccomp_enforce | relentless_defense |
         arp_probe_wait | arp_probe_num | arp_probe_min | arp_probe_max |
-        gw_metric | resolv_conf | dhcp_set_hostname | rfkill_idx |
+        gw_metric | resolv_conf | dhcp_set_hostname | rfkill_idx | s6_notify |
         version | help
     )*;
 }%%

leasefile.c

@@ -42,6 +42,13 @@ void open_leasefile(void)
 
 void write_leasefile(struct in_addr ipnum)
 {
+    if (client_config.enable_s6_notify) {
+        static char buf[] = "\n";
+        safe_write(client_config.s6_notify_fd, buf, 1);
+        close(client_config.s6_notify_fd);
+        client_config.enable_s6_notify = false;
+    }
+
     char ip[INET_ADDRSTRLEN];
     char out[INET_ADDRSTRLEN*2];
     if (leasefilefd < 0) {

ndhc.8

@@ -125,6 +125,12 @@ an rfkill-idx parameter is specified, ndhc will print messages for any
 rfkill events that it sees, so it should not be too difficult to locate
 the proper rfkill device by checking the logs after hitting the switch.
 .TP
+.BI \-N\  NOTIFY_FDNUM ,\  \-\-s6\-notify= NOTIFY_FDNUM
+If set, specifies the file descriptor number that will have a '\n' written to
+and closed when the first DHCP lease is bound.  This option should be used when
+ndhc is run under a s6 supervisor that implements service startup
+notifications.
+.TP
 .BI \-v ,\  \-\-version
 Display the ndhc version number.
 .SH SIGNALS

ndhc.c

@@ -59,6 +59,7 @@ struct client_state_t cs = {
 struct client_config_t client_config = {
     .interface = "eth0",
     .arp = "\0\0\0\0\0\0",
+    .s6_notify_fd = 3,
     .clientid_len = 0,
     .metric = 0,
 };

ndhc.h

@@ -49,9 +49,11 @@ struct client_config_t {
     uint32_t rfkillIdx;          // Index of the corresponding rfkill device
     int metric;                  // Metric for the default route
     int ifindex;                 // Index number of the interface to use
+    int s6_notify_fd;            // File descriptor for s6 notify mechanism
     uint8_t clientid_len;        // Length of the clientid
     bool abort_if_no_lease;      // Abort if no lease
     bool enable_rfkill;          // Listen for rfkill events
+    bool enable_s6_notify;       // Perform s6 startup notification
 };
 
 enum {