fs-util: add new CHASE_NOFOLLOW flag to chase_symlinks()

This flag mimics what "O_NOFOLLOW|O_PATH" does for open(2) that is
chase_symlinks() will not resolve the final pathname component if it's a
symlink and instead will return a file descriptor referring to the symlink
itself.

Note: if CHASE_SAFE is also passed, no safety checking is performed on the
transition done if the symlink would have been followed.

src/basic/fs-util.c

@@ -695,7 +695,7 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags,
         if (!original_root && !ret && (flags & (CHASE_NONEXISTENT|CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_OPEN|CHASE_STEP)) == CHASE_OPEN) {
                 /* Shortcut the CHASE_OPEN case if the caller isn't interested in the actual path and has no root set
                  * and doesn't care about any of the other special features we provide either. */
-                r = open(path, O_PATH|O_CLOEXEC);
+                r = open(path, O_PATH|O_CLOEXEC|((flags & CHASE_NOFOLLOW) ? O_NOFOLLOW : 0));
                 if (r < 0)
                         return -errno;
 
@@ -850,7 +850,7 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags,
                     fd_is_fs_type(child, AUTOFS_SUPER_MAGIC) > 0)
                         return -EREMOTE;
 
-                if (S_ISLNK(st.st_mode)) {
+                if (S_ISLNK(st.st_mode) && !((flags & CHASE_NOFOLLOW) && isempty(todo))) {
                         char *joined;
 
                         _cleanup_free_ char *destination = NULL;

src/basic/fs-util.h

@@ -73,6 +73,7 @@ enum {
         CHASE_OPEN        = 1 << 4, /* If set, return an O_PATH object to the final component */
         CHASE_TRAIL_SLASH = 1 << 5, /* If set, any trailing slash will be preserved */
         CHASE_STEP        = 1 << 6, /* If set, just execute a single step of the normalization */
+        CHASE_NOFOLLOW    = 1 << 7, /* Only valid with CHASE_OPEN: when the path's right-most component refers to symlink return O_PATH fd of the symlink, rather than following it. */
 };
 
 /* How many iterations to execute before returning -ELOOP */