conf: Add -P, --pid, to specify a file where own PID is written to

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

conf.c

@@ -565,6 +565,7 @@ static void usage(const char *name)
 	else
 		info("      /tmp/passt_ISO8601-TIMESTAMP_INSTANCE-NUMBER.pcap");
 
+	info(   "  -P, --pid FILE	Write own PID to the given file");
 	info(   "  -m, --mtu MTU	Assign MTU via DHCP/NDP");
 	info(   "    a zero value disables assignment");
 	info(   "    default: 65520: maximum 802.3 MTU minus 802.3 header");
@@ -764,6 +765,7 @@ void conf(struct ctx *c, int argc, char **argv)
 		{"socket",	required_argument,	NULL,		's' },
 		{"ns-ifname",	required_argument,	NULL,		'I' },
 		{"pcap",	optional_argument,	NULL,		'p' },
+		{"pid",		required_argument,	NULL,		'P' },
 		{"mtu",		required_argument,	NULL,		'm' },
 		{"address",	required_argument,	NULL,		'a' },
 		{"netmask",	required_argument,	NULL,		'n' },
@@ -807,9 +809,9 @@ void conf(struct ctx *c, int argc, char **argv)
 		const char *optstring;
 
 		if (c->mode == MODE_PASST)
-			optstring = "dqfehs:p::m:a:n:M:g:i:D::S::46t:u:";
+			optstring = "dqfehs:p::P:m:a:n:M:g:i:D::S::46t:u:";
 		else
-			optstring = "dqfehI:p::m:a:n:M:g:i:D::S::46t:u:T:U:";
+			optstring = "dqfehI:p::P:m:a:n:M:g:i:D::S::46t:u:T:U:";
 
 		name = getopt_long(argc, argv, optstring, options, NULL);
 
@@ -949,6 +951,19 @@ void conf(struct ctx *c, int argc, char **argv)
 				usage(argv[0]);
 			}
 			break;
+		case 'P':
+			if (*c->pid_file) {
+				err("Multiple --pid options given");
+				usage(argv[0]);
+			}
+
+			ret = snprintf(c->pid_file, sizeof(c->pid_file), "%s",
+				       optarg);
+			if (ret <= 0 || ret >= (int)sizeof(c->pid_file)) {
+				err("Invalid PID file: %s", optarg);
+				usage(argv[0]);
+			}
+			break;
 		case 'm':
 			if (c->mtu) {
 				err("Multiple --mtu options given");

passt.1

@@ -106,6 +106,11 @@ in \fBpasst\fR mode and to
 in \fBpasta\fR mode, where \fIinstance-number\fR is a progressive count of
 other detected instances running on the same host.
 
+.TP
+.BR \-P ", " \-\-pid " " \fIfile
+Write own PID to \fIfile\fR once initialisation is done, before forking to
+background (if configured to do so).
+
 .TP
 .BR \-m ", " \-\-mtu " " \fImtu
 Assign \fImtu\fR via DHCP (option 26) and NDP (option type 5).

passt.c

@@ -227,6 +227,27 @@ static void drop_caps(void)
 	}
 }
 
+/**
+ * pid_file() - Write own PID to file, if configured
+ * @c:		Execution context
+ */
+static void pid_file(struct ctx *c) {
+	char pid_buf[12];
+	int pid_fd, n;
+
+	if (!c->pid_file)
+		return;
+
+	pid_fd = open(c->pid_file, O_CREAT | O_WRONLY);
+	if (pid_fd < 0)
+		return;
+
+	n = snprintf(pid_buf, sizeof(pid_buf), "%i\n", getpid());
+
+	write(pid_fd, pid_buf, n);
+	close(pid_fd);
+}
+
 /**
  * main() - Entry point and main loop
  * @argc:	Argument count
@@ -237,7 +258,7 @@ static void drop_caps(void)
  * #syscalls read write open close fork dup2 exit chdir ioctl writev syslog
  * #syscalls prlimit64 epoll_ctl epoll_create1 epoll_wait accept4 accept listen
  * #syscalls socket bind connect getsockopt setsockopt recvfrom sendto shutdown
- * #syscalls openat fstat fcntl lseek clone setsid exit_group
+ * #syscalls openat fstat fcntl lseek clone setsid exit_group getpid
  * #syscalls:pasta rt_sigreturn
  */
 int main(int argc, char **argv)
@@ -327,6 +348,8 @@ int main(int argc, char **argv)
 	if (isatty(fileno(stdout)) && !c.foreground)
 		daemon(0, 0);
 
+	pid_file(&c);
+
 	timer_init(&c, &now);
 loop:
 	nfds = epoll_wait(c.epollfd, events, EPOLL_EVENTS, TIMER_INTERVAL);

passt.h

@@ -91,6 +91,7 @@ enum passt_modes {
  * @stderr:		Force logging to stderr
  * @sock_path:		Path for UNIX domain socket
  * @pcap:		Path for packet capture file
+ * @pid_file:		Path to PID file, empty string if not configured
  * @pasta_netns_fd:	File descriptor for network namespace in pasta mode
  * @pasta_userns_fd:	File descriptor for user namespace in pasta mode
  * @netns_only:		In pasta mode, don't join or create a user namespace
@@ -142,6 +143,7 @@ struct ctx {
 	int stderr;
 	char sock_path[UNIX_PATH_MAX];
 	char pcap[PATH_MAX];
+	char pid_file[PATH_MAX];
 
 	int pasta_netns_fd;
 	int pasta_userns_fd;