Use some magic devices to prevent users hurting themselves with --file-read=/dev/stdin as we re-use fd 0

adsbus/adsbus.c

@@ -1,8 +1,11 @@
 #include <assert.h>
+#include <fcntl.h>
 #include <getopt.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/types.h>
+#include <sys/stat.h>
 #include <unistd.h>
 
 #include "beast.h"
@@ -166,6 +169,12 @@ static bool parse_opts(int argc, char *argv[]) {
 	return true;
 }
 
+static void reopen(int fd, char *path, int flags) {
+	// Presumes that all fds < fd are open
+	assert(!close(fd));
+	assert(open(path, flags | O_CLOEXEC) == fd);
+}
+
 int main(int argc, char *argv[]) {
 	hex_init();
 	rand_init();
@@ -185,8 +194,8 @@ int main(int argc, char *argv[]) {
 		peer_shutdown(0);
 	}
 
-	assert(!close(0));
-	assert(!close(1));
+	reopen(STDIN_FILENO, "/dev/null", O_RDONLY);
+	reopen(STDOUT_FILENO, "/dev/full", O_WRONLY);
 
 	peer_loop();
 
@@ -208,7 +217,9 @@ int main(int argc, char *argv[]) {
 
 	peer_cleanup();
 
-	assert(!close(2));
+	assert(!close(STDIN_FILENO));
+	assert(!close(STDOUT_FILENO));
+	assert(!close(STDERR_FILENO));
 
 	return EXIT_SUCCESS;
 }

adsbus/opts.c

@@ -146,7 +146,7 @@ bool opts_add_exec_send_receive(char *arg) {
 }
 
 bool opts_add_stdin(char __attribute__((unused)) *arg) {
-	int fd = fcntl(0, F_DUPFD_CLOEXEC, 0);
+	int fd = fcntl(STDIN_FILENO, F_DUPFD_CLOEXEC, 0);
 	assert(fd >= 0);
 	return flow_new_send_hello(fd, receive_flow, NULL, NULL);
 }
@@ -156,7 +156,7 @@ bool opts_add_stdout(char *arg) {
 	if (!serializer) {
 		return false;
 	}
-	int fd = fcntl(1, F_DUPFD_CLOEXEC, 0);
+	int fd = fcntl(STDOUT_FILENO, F_DUPFD_CLOEXEC, 0);
 	assert(fd >= 0);
 	return flow_new_send_hello(fd, send_flow, serializer, NULL);
 }