readlink: support --, -n always

POSIX will be standardizing readlink (just the -n option) and realpath
(just -E and -e options):
https://www.austingroupbugs.net/view.php?id=1457

Change things for readlink so that the POSIX-mandated -n and -- work
even when disabling the non-standard (and partially non-working) -f
when FEATURE_READLINK_FOLLOW is clear.

POSIX also wants readlink to be verbose by default (if the argument is
not a symlink, readlink must output a diagnostic); I did NOT address
that one, because I'm waiting to see what the GNU Coreutils folks do:
https://lists.gnu.org/archive/html/bug-coreutils/2023-03/msg00035.html

Partial fix for https://bugs.busybox.net/show_bug.cgi?id=15466

function                                             old     new   delta
packed_usage                                       34538   34557     +19

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 12 insertions, 17 deletions

diff --git a/coreutils/readlink.c b/coreutils/readlink.c
index b2e867883..0a9aa957e 100644
--- a/coreutils/readlink.c
+++ b/coreutils/readlink.c
@@ -25,12 +25,14 @@
 //kbuild:lib-$(CONFIG_READLINK) += readlink.o
 
 //usage:#define readlink_trivial_usage
-//usage:        IF_FEATURE_READLINK_FOLLOW("[-fnv] ") "FILE"
+//usage:        IF_FEATURE_READLINK_FOLLOW("[-fnv] ")
+//usage:        IF_NOT_FEATURE_READLINK_FOLLOW("[-n] ")
+//usage:        "FILE"
 //usage:#define readlink_full_usage "\n\n"
-//usage:       "Display the value of a symlink"
-//usage:        IF_FEATURE_READLINK_FOLLOW( "\n"
-//usage:     "\n        -f      Canonicalize by following all symlinks"
+//usage:       "Display the value of a symlink" "\n"
 //usage:     "\n        -n      Don't add newline"
+//usage:        IF_FEATURE_READLINK_FOLLOW(
+//usage:     "\n        -f      Canonicalize by following all symlinks"
 //usage:     "\n        -v      Verbose"
 //usage:        )
 
@@ -67,25 +69,18 @@ int readlink_main(int argc UNUSED_PARAM, char **argv)
 {
         char *buf;
         char *fname;
+        unsigned opt;
 
-        IF_FEATURE_READLINK_FOLLOW(
-                unsigned opt;
-                /* We need exactly one non-option argument.  */
-                opt = getopt32(argv, "^" "fnvsq" "\0" "=1");
-                fname = argv[optind];
-        )
-        IF_NOT_FEATURE_READLINK_FOLLOW(
-                const unsigned opt = 0;
-                if (argc != 2) bb_show_usage();
-                fname = argv[1];
-        )
+        opt = getopt32(argv, "^" "n" IF_FEATURE_READLINK_FOLLOW("fvsq")
+                       "\0" "=1");
+        fname = argv[optind];
 
         /* compat: coreutils readlink reports errors silently via exit code */
         if (!(opt & 4)) /* not -v */
                 logmode = LOGMODE_NONE;
 
         /* NOFORK: only one alloc is allowed; must free */
-        if (opt & 1) { /* -f */
+        if (opt & 2) { /* -f */
                 buf = xmalloc_realpath_coreutils(fname);
         } else {
                 buf = xmalloc_readlink_or_warn(fname);
@@ -93,7 +88,7 @@ int readlink_main(int argc UNUSED_PARAM, char **argv)
 
         if (!buf)
                 return EXIT_FAILURE;
-        printf((opt & 2) ? "%s" : "%s\n", buf);
+        printf((opt & 1) ? "%s" : "%s\n", buf);
         free(buf);
 
         fflush_stdout_and_exit_SUCCESS();