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();