libbb: better coreutils compatibility for realpath

Add some tests which coreutils realpath pass but BusyBox realpath
fails (bar one).  Adjust xmalloc_realpath_coreutils() so the tests
pass:

- Expand symbolic links before testing whether the last path component
  exists.

- When the link target is a relative path canonicalize it by passing
  it through xmalloc_realpath_coreutils() as already happens for
  absolute paths.

- Ignore trailing slashes when finding the last path component and
  correctly handle the case where the only slash is at the start of
  the path.  This requires ignoring superfluous leading slashes.

- Undo all changes to the path so error messages from the caller show
  the original filename.

function                                             old     new   delta
xmalloc_realpath_coreutils                           214     313     +99

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/xreadlink.c

@@ -123,7 +123,7 @@ char* FAST_FUNC xmalloc_realpath(const char *path)
 #endif
 }
 
-char* FAST_FUNC xmalloc_realpath_coreutils(const char *path)
+char* FAST_FUNC xmalloc_realpath_coreutils(char *path)
 {
 	char *buf;
 
@@ -137,32 +137,19 @@ char* FAST_FUNC xmalloc_realpath_coreutils(const char *path)
 	 * (the directory must exist).
 	 */
 	if (!buf && errno == ENOENT) {
-		char *last_slash = strrchr(path, '/');
-		if (last_slash) {
-			*last_slash++ = '\0';
-			buf = xmalloc_realpath(path);
-			if (buf) {
-				unsigned len = strlen(buf);
-				buf = xrealloc(buf, len + strlen(last_slash) + 2);
-				buf[len++] = '/';
-				strcpy(buf + len, last_slash);
-			}
-		} else {
-			char *target = xmalloc_readlink(path);
-			if (target) {
-				char *cwd;
-				if (target[0] == '/') {
-					/*
-					 * $ ln -s /bin/qwe symlink  # note: /bin is a link to /usr/bin
-					 * $ readlink -f symlink
-					 * /usr/bin/qwe/target_does_not_exist
-					 * $ realpath symlink
-					 * /usr/bin/qwe/target_does_not_exist
-					 */
-					buf = xmalloc_realpath_coreutils(target);
-					free(target);
-					return buf;
-				}
+		char *target, c, *last_slash;
+		size_t i;
+
+		target = xmalloc_readlink(path);
+		if (target) {
+			/*
+			 * $ ln -s /bin/qwe symlink  # note: /bin is a link to /usr/bin
+			 * $ readlink -f symlink
+			 * /usr/bin/qwe
+			 * $ realpath symlink
+			 * /usr/bin/qwe
+			 */
+			if (target[0] != '/') {
 				/*
 				 * $ ln -s target_does_not_exist symlink
 				 * $ readlink -f symlink
@@ -170,13 +157,41 @@ char* FAST_FUNC xmalloc_realpath_coreutils(const char *path)
 				 * $ realpath symlink
 				 * /CURDIR/target_does_not_exist
 				 */
-				cwd = xrealloc_getcwd_or_warn(NULL);
-				buf = concat_path_file(cwd, target);
+				char *cwd = xrealloc_getcwd_or_warn(NULL);
+				char *tmp = concat_path_file(cwd, target);
 				free(cwd);
 				free(target);
-				return buf;
+				target = tmp;
+			}
+			buf = xmalloc_realpath_coreutils(target);
+			free(target);
+			return buf;
+		}
+
+		/* ignore leading and trailing slashes */
+		while (path[0] == '/' && path[1] == '/')
+			++path;
+		i = strlen(path) - 1;
+		while (i > 0 && path[i] == '/')
+			i--;
+		c = path[i + 1];
+		path[i + 1] = '\0';
+
+		last_slash = strrchr(path, '/');
+		if (last_slash == path)
+			buf = xstrdup(path);
+		else if (last_slash) {
+			*last_slash = '\0';
+			buf = xmalloc_realpath(path);
+			*last_slash++ = '/';
+			if (buf) {
+				unsigned len = strlen(buf);
+				buf = xrealloc(buf, len + strlen(last_slash) + 2);
+				buf[len++] = '/';
+				strcpy(buf + len, last_slash);
 			}
 		}
+		path[i + 1] = c;
 	}
 
 	return buf;