udevd: add support for S6/dinit-compatible readiness notification

Add support for signalling readiness (complete initialization) by sending a
message via a file descriptor, provided by the parent process (such as a
service manager).

The mechanism is described here:
https://skarnet.org/software/s6/notifywhenup.html

Author: davmac314

man/udevd.8

@@ -25,7 +25,7 @@ udevd \- Device event managing daemon
 .PP
 udevd
 .HP \w'\fB/sbin/udevd\fR\ 'u
-\fB/sbin/udevd\fR [\fB\-\-daemon\fR] [\fB\-\-debug\fR] [\fB\-\-children\-max=\fR] [\fB\-\-exec\-delay=\fR] [\fB\-\-event\-timeout=\fR] [\fB\-\-resolve\-names=early|late|never\fR] [\fB\-\-version\fR] [\fB\-\-help\fR]
+\fB/sbin/udevd\fR [\fB\-\-daemon\fR] [\fB\-\-debug\fR] [\fB\-\-ready\-notify=\fR] [\fB\-\-children\-max=\fR] [\fB\-\-exec\-delay=\fR] [\fB\-\-event\-timeout=\fR] [\fB\-\-resolve\-names=early|late|never\fR] [\fB\-\-version\fR] [\fB\-\-help\fR]
 .SH "DESCRIPTION"
 .PP
 \fBudevd\fR
@@ -47,6 +47,11 @@ Detach and run in the background\&.
 Print debug messages to standard error\&.
 .RE
 .PP
+\fB\-r\fR, \fB\-\-ready\-notify=\fR
+.RS 4
+Issue (S6/dinit compatible) readiness notification via a file descriptor\&. A short message followed by a newline character will be sent via the specified file descriptor (inherited from the parent process) when initialization is complete\&.
+.RE
+.PP
 \fB\-c\fR, \fB\-\-children\-max=\fR
 .RS 4
 Limit the number of events executed in parallel\&.

man/udevd.xml

@@ -34,6 +34,7 @@
       <command>/sbin/udevd</command>
       <arg><option>--daemon</option></arg>
       <arg><option>--debug</option></arg>
+      <arg><option>--ready-notify=</option></arg>
       <arg><option>--children-max=</option></arg>
       <arg><option>--exec-delay=</option></arg>
       <arg><option>--event-timeout=</option></arg>
@@ -75,6 +76,16 @@
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><option>-r</option>, <option>--ready-notify=</option></term>
+        <listitem>
+          <para>Issue (S6/dinit compatible) readiness notification via a
+          file descriptor. A short message followed by a newline character
+          will be sent via the specified file descriptor (inherited from
+          the parent process) when initialization is complete.</para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><option>-c</option>, <option>--children-max=</option></term>
         <listitem>

src/udev/udevd.c

@@ -62,6 +62,7 @@ static int worker_watch[2] = { -1, -1 };
 static int fd_signal = -1;
 static int fd_ep = -1;
 static int fd_inotify = -1;
+static int fd_ready = -1;
 static bool stop_exec_queue;
 static bool reload;
 static bool arg_debug = false;
@@ -1036,6 +1037,7 @@ static void help(void) {
                "  -V --version                Print version of the program\n"
                "  -d --daemon                 Detach and run in the background\n"
                "  -D --debug                  Enable debug output\n"
+               "  -r --ready-notify=FD        Notify readiness via file descriptor\n"
                "  -c --children-max=INT       Set maximum number of workers\n"
                "  -e --exec-delay=SECONDS     Seconds to wait before executing RUN=\n"
                "  -t --event-timeout=SECONDS  Seconds to wait before terminating an event\n"
@@ -1048,6 +1050,7 @@ static int parse_argv(int argc, char *argv[]) {
         static const struct option options[] = {
                 { "daemon",             no_argument,            NULL, 'd' },
                 { "debug",              no_argument,            NULL, 'D' },
+                { "ready-notify",       required_argument,      NULL, 'r' },
                 { "children-max",       required_argument,      NULL, 'c' },
                 { "exec-delay",         required_argument,      NULL, 'e' },
                 { "event-timeout",      required_argument,      NULL, 't' },
@@ -1104,6 +1107,11 @@ static int parse_argv(int argc, char *argv[]) {
                                 return 0;
                         }
                         break;
+                case 'r':
+                        r = safe_atou(optarg, &fd_ready);
+                        if (r < 0)
+                                log_warning("Invalid --ready-notify ignored: %s", optarg);
+                        break;
                 case 'h':
                         help();
                         return 0;
@@ -1372,6 +1380,19 @@ int main(int argc, char *argv[]) {
                 goto exit;
         }
 
+        if (fd_ready) {
+                char ready_string[] = "udevd ready\n";
+                size_t to_write = sizeof(ready_string) - 1;
+                do {
+                        int r = write(fd_ready, ready_string, to_write);
+                        if (r < 0) {
+                                log_error_errno(errno, "failed to write to readiness fd: %m");
+                                break;
+                        }
+                        to_write -= r;
+                } while (to_write > 0);
+        }
+
         for (;;) {
                 static usec_t last_usec;
                 struct epoll_event ev[8];