From a6d66bfe247cdc73413d032fde3cbecf4e00ca26 Mon Sep 17 00:00:00 2001
From: "Anna (navi) Figueiredo Gomes" <navi@vlhl.dev>
Date: Sun, 5 Jan 2025 02:36:06 +0100
Subject: [PATCH] supervise-daemon: add s6-style readiness notification.

---
 sh/supervise-daemon.sh                  |  1 +
 src/supervise-daemon/supervise-daemon.c | 45 +++++++++++++++++++++++--
 2 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/sh/supervise-daemon.sh b/sh/supervise-daemon.sh
index a2881f3a9..8cdde18b5 100644
--- a/sh/supervise-daemon.sh
+++ b/sh/supervise-daemon.sh
@@ -43,6 +43,7 @@ supervise_start()
 		${no_new_privs:+--no-new-privs} \
 		${command_user+--user} $command_user \
 		${umask+--umask} $umask \
+		${ready+--ready} $ready \
 		${supervise_daemon_args-${start_stop_daemon_args}} \
 		$command \
 		-- $command_args $command_args_foreground
diff --git a/src/supervise-daemon/supervise-daemon.c b/src/supervise-daemon/supervise-daemon.c
index ed60f5e46..03d6a42c9 100644
--- a/src/supervise-daemon/supervise-daemon.c
+++ b/src/supervise-daemon/supervise-daemon.c
@@ -82,6 +82,7 @@ enum {
   LONGOPT_SECBITS,
   LONGOPT_STDERR_LOGGER,
   LONGOPT_STDOUT_LOGGER,
+  LONGOPT_READY,
 };
 
 const char *applet = NULL;
@@ -115,6 +116,7 @@ const struct option longopts[] = {
 	{ "stderr",       1, NULL, '2'},
 	{ "stdout-logger",1, NULL, LONGOPT_STDOUT_LOGGER},
 	{ "stderr-logger",1, NULL, LONGOPT_STDERR_LOGGER},
+	{ "ready",        1, NULL, LONGOPT_READY},
 	{ "reexec",       0, NULL, '3'},
 	longopts_COMMON
 };
@@ -165,6 +167,8 @@ static int devnull_fd = -1;
 static int stdin_fd;
 static int stdout_fd;
 static int stderr_fd;
+static int ready_fd = -1;
+static int ready_pipe[2];
 static char *redirect_stderr = NULL;
 static char *redirect_stdout = NULL;
 static char *stderr_process = NULL;
@@ -588,6 +592,9 @@ RC_NORETURN static void child_process(char *exec, char **argv)
 
 	cloexec_fds_from(3);
 
+	if (ready_fd != -1)
+		dup2(ready_pipe[1], ready_fd);
+
 	cmdline = make_cmdline(argv);
 	syslog(LOG_INFO, "Child command line: %s", cmdline);
 	free(cmdline);
@@ -786,6 +793,28 @@ RC_NORETURN static void supervisor(char *exec, char **argv)
 	exit(EXIT_SUCCESS);
 }
 
+static void wait_ready(void)
+{
+	if (ready_fd == -1)
+		exit(EXIT_SUCCESS);
+
+	close(ready_pipe[1]);
+	/* TODO: add timeout failure and stop child supervisor */
+	for (;;) {
+		char buf[BUFSIZ];
+		int bytes = read(ready_pipe[0], buf, BUFSIZ);
+		if (bytes == -1 && errno != EINTR) {
+			eerror("%s: read failed '%s'\n", applet, strerror(errno));
+			exit(EXIT_FAILURE);
+		}
+
+		if (memchr(buf, '\n', bytes))
+			break;
+	}
+
+	exit(EXIT_SUCCESS);
+}
+
 int main(int argc, char **argv)
 {
 	int opt;
@@ -1068,6 +1097,12 @@ int main(int argc, char **argv)
 			stderr_process = optarg;
 			break;
 
+		case LONGOPT_READY:
+			if (sscanf(optarg, "fd:%d", &ready_fd) != 1)
+				eerrorx("%s: invalid ready '%s'.", applet, optarg);
+			pipe(ready_pipe);
+			break;
+
 		case_RC_COMMON_GETOPT
 		}
 
@@ -1205,8 +1240,8 @@ int main(int argc, char **argv)
 		if (child_pid == -1)
 			eerrorx("%s: fork: %s", applet, strerror(errno));
 		if (child_pid != 0)
-			/* first parent process, do nothing. */
-			exit(EXIT_SUCCESS);
+			wait_ready();
+
 #ifdef TIOCNOTTY
 		tty_fd = open("/dev/tty", O_RDWR);
 #endif
@@ -1214,10 +1249,16 @@ int main(int argc, char **argv)
 		dup2(devnull_fd, STDIN_FILENO);
 		dup2(devnull_fd, STDOUT_FILENO);
 		dup2(devnull_fd, STDERR_FILENO);
+
+		if (ready_fd != -1)
+			close(ready_pipe[0]);
+
 		child_pid = fork();
 		if (child_pid == -1)
 			eerrorx("%s: fork: %s", applet, strerror(errno));
 		else if (child_pid != 0) {
+			if (ready_fd != -1)
+				close(ready_pipe[1]);
 			c = argv;
 			x = 0;
 			while (c && *c) {