diff: fix -N and nonexistent files. Closes 7454

function                                             old     new   delta
diffreg                                             1253    1310     +57
diff_main                                           1329    1355     +26
create_J                                            1819    1821      +2
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 85/0)               Total: 85 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 21 insertions, 5 deletions

diff --git a/editors/diff.c b/editors/diff.c
index 3304edb26..7687518f3 100644
--- a/editors/diff.c
+++ b/editors/diff.c
@@ -715,9 +715,19 @@ static int diffreg(char *file[2])
         fp[0] = stdin;
         fp[1] = stdin;
         for (i = 0; i < 2; i++) {
-                int fd = open_or_warn_stdin(file[i]);
-                if (fd == -1)
-                        goto out;
+                int fd = STDIN_FILENO;
+                if (!LONE_DASH(file[i])) {
+                        if (!(option_mask32 & FLAG(N))) {
+                                fd = open_or_warn(file[i], O_RDONLY);
+                                if (fd == -1)
+                                        goto out;
+                        } else {
+                                /* -N: if some file does not exist compare it like empty */
+                                fd = open(file[i], O_RDONLY);
+                                if (fd == -1)
+                                        fd = xopen("/dev/null", O_RDONLY);
+                        }
+                }
                 /* Our diff implementation is using seek.
                  * When we meet non-seekable file, we must make a temp copy.
                  */
@@ -978,17 +988,23 @@ int diff_main(int argc UNUSED_PARAM, char **argv)
         argv += optind;
         while (L_arg)
                 label[!!label[0]] = llist_pop(&L_arg);
+
+        /* Compat: "diff file name_which_doesnt_exist" exits with 2 */
         xfunc_error_retval = 2;
         for (i = 0; i < 2; i++) {
                 file[i] = argv[i];
-                /* Compat: "diff file name_which_doesnt_exist" exits with 2 */
                 if (LONE_DASH(file[i])) {
                         fstat(STDIN_FILENO, &stb[i]);
                         gotstdin++;
-                } else
+                } else if (option_mask32 & FLAG(N)) {
+                        if (stat(file[i], &stb[i]))
+                                xstat("/dev/null", &stb[i]);
+                } else {
                         xstat(file[i], &stb[i]);
+                }
         }
         xfunc_error_retval = 1;
+
         if (gotstdin && (S_ISDIR(stb[0].st_mode) || S_ISDIR(stb[1].st_mode)))
                 bb_error_msg_and_die("can't compare stdin to a directory");