stat-util: teach statx_mount_same() STATX_MNT_ID_UNIQUE

So far statx_mount_same() assumed STATX_MNT_ID_UNIQUE didn't exist.
However it does exist, hence do something useful if we see it set. Note
that this creates a certain ambiguity: if we compare one statx struct
with STATX_MNT_ID_UNIQUE and one without it (but with the regular mnt
id), then we cnanot really come to a clear conclusion, hence need to
introduce a third, unknown state.

Note that we don't request STATX_MNT_ID_UNIQUE yet wherever we call
statx_mount_same(). THis will be added in a later commit.

src/basic/chase.c

@@ -379,11 +379,20 @@ int chaseat(int dir_fd, const char *path, ChaseFlags flags, char **ret_path, int
                          * root tree we operate in is allowed, where an inode is moved up the tree while we
                          * look at it, and thus get the current path wrong and think we are deeper down than
                          * we actually are. */
-                        if (FLAGS_SET(flags, CHASE_AT_RESOLVE_IN_ROOT) &&
-                            (empty_or_root(done) || (statx_inode_same(&stx, &root_stx) && statx_mount_same(&stx, &root_stx)))) {
-                                if (FLAGS_SET(flags, CHASE_STEP))
-                                        goto chased_one;
-                                continue;
+                        if (FLAGS_SET(flags, CHASE_AT_RESOLVE_IN_ROOT)) {
+                                bool is_root = empty_or_root(done);
+                                if (!is_root && statx_inode_same(&stx, &root_stx)) {
+                                        r = statx_mount_same(&stx, &root_stx);
+                                        if (r < 0)
+                                                return r;
+
+                                        is_root = r > 0;
+                                }
+                                if (is_root) {
+                                        if (FLAGS_SET(flags, CHASE_STEP))
+                                                goto chased_one;
+                                        continue;
+                                }
                         }
 
                         fd_parent = openat(fd, "..", O_CLOEXEC|O_NOFOLLOW|O_PATH|O_DIRECTORY);

src/basic/fd-util.c

@@ -1094,7 +1094,11 @@ int fds_inode_and_mount_same(int fd1, int fd2) {
         if (r < 0)
                 return r;
 
-        return statx_inode_same(&sx1, &sx2) && statx_mount_same(&sx1, &sx2);
+        r = statx_mount_same(&sx1, &sx2);
+        if (r <= 0)
+                return r;
+
+        return statx_inode_same(&sx1, &sx2);
 }
 
 int resolve_xat_fdroot(int *fd, const char **path, char **ret_buffer) {

src/basic/stat-util.c

@@ -675,14 +675,15 @@ bool statx_inode_same(const struct statx *a, const struct statx *b) {
                 a->stx_ino == b->stx_ino;
 }
 
-bool statx_mount_same(const struct statx *a, const struct statx *b) {
+int statx_mount_same(const struct statx *a, const struct statx *b) {
         if (!statx_is_set(a) || !statx_is_set(b))
                 return false;
 
-        assert(FLAGS_SET(a->stx_mask, STATX_MNT_ID));
-        assert(FLAGS_SET(b->stx_mask, STATX_MNT_ID));
+        if ((FLAGS_SET(a->stx_mask, STATX_MNT_ID) && FLAGS_SET(b->stx_mask, STATX_MNT_ID)) ||
+            (FLAGS_SET(a->stx_mask, STATX_MNT_ID_UNIQUE) && FLAGS_SET(b->stx_mask, STATX_MNT_ID_UNIQUE)))
+                return a->stx_mnt_id == b->stx_mnt_id;
 
-        return a->stx_mnt_id == b->stx_mnt_id;
+        return -ENODATA;
 }
 
 usec_t statx_timestamp_load(const struct statx_timestamp *ts) {

src/basic/stat-util.h

@@ -110,7 +110,7 @@ bool stat_inode_same(const struct stat *a, const struct stat *b);
 bool stat_inode_unmodified(const struct stat *a, const struct stat *b);
 
 bool statx_inode_same(const struct statx *a, const struct statx *b);
-bool statx_mount_same(const struct statx *a, const struct statx *b);
+int statx_mount_same(const struct statx *a, const struct statx *b);
 
 int xstatfsat(int dir_fd, const char *path, struct statfs *ret);