test/smtp-dummy: add --background option and functionality

When shell executes background process using '&' the scheduling of
that new process is arbitrary. It could be that smtp-dummy doesn't
get execution time to listen() it's server socket until some other
process attempts to connect() to it. The --background option in
smtp-dummy makes it to go background *after* it started to listen
its server socket.

When --background option is used, the line "smtp_dummy_pid='<pid>'"
is printed to stdout from where shell can eval it.

test/smtp-dummy.c

@@ -119,6 +119,7 @@ do_smtp_to_file (FILE *peer, FILE *output)
 int
 main (int argc, char *argv[])
 {
+	const char * progname;
 	char *output_filename;
 	FILE *peer_file, *output;
 	int sock, peer, err;
@@ -126,9 +127,31 @@ main (int argc, char *argv[])
 	struct hostent *hostinfo;
 	socklen_t peer_addr_len;
 	int reuse;
+	int background;
+
+	progname = argv[0];
+
+	background = 0;
+	for (; argc >= 2; argc--, argv++) {
+		if (argv[1][0] != '-')
+			break;
+		if (strcmp (argv[1], "--") == 0) {
+			argc--;
+			argv++;
+			break;
+		}
+		if (strcmp (argv[1], "--background") == 0) {
+			background = 1;
+			continue;
+		}
+		fprintf(stderr, "%s: unregognized option '%s'\n",
+			progname, argv[1]);
+		return 1;
+	}
 
 	if (argc != 2) {
-		fprintf (stderr, "Usage: %s <output-file>\n", argv[0]);
+		fprintf (stderr,
+			 "Usage: %s [--background] <output-file>\n", progname);
 		return 1;
 	}
 
@@ -181,6 +204,36 @@ main (int argc, char *argv[])
 		return 1;
 	}
 
+	if (background) {
+		int pid = fork ();
+		if (pid > 0) {
+			printf ("smtp_dummy_pid='%d'\n", pid);
+			fflush (stdout);
+			close (sock);
+			return 0;
+		}
+		if (pid < 0) {
+			fprintf (stderr, "Error: fork() failed: %s\n",
+				 strerror (errno));
+			close (sock);
+			return 1;
+		}
+		/* Reached if pid == 0 (the child process). */
+		/* Close stdout so that the one interested in pid value will
+		   also get EOF. */
+		close (STDOUT_FILENO);
+		/* dup2() will re-reserve fd of stdout (1) (opportunistically),
+		   in case fd of stderr (2) is open. If that was not open we
+		   don't care fd of stdout (1) either. */
+		dup2 (STDERR_FILENO, STDOUT_FILENO);
+
+		/* This process is now out of reach of shell's job control.
+		   To resolve the rare but possible condition where this
+		   "daemon" is started but never connected this process will
+		   (only) have 30 seconds to exist. */
+		alarm (30);
+	}
+
 	peer_addr_len = sizeof (peer_addr);
 	peer = accept (sock, (struct sockaddr *) &peer_addr, &peer_addr_len);
 	if (peer == -1) {