tmpfiles: create subvolumes for "v", "q", and "Q" only if / is a subvolume

It's not a good idea to create subvolumes for parts of the OS tree (such
as /home, or /var) if the root directory is not a subvolume too. We
shouldn't assume control of "heavier" objects such as subvolumes, if the
originating object (the root directory) is a "light-weight" object, i.e.
a plain directory.

Effectively this means that chroot() environments that are run on a
plain directory do not have to deal with problems around systemd
creating subvolumes that cannot be removed with a simple "rm" anymore.
However, if the chroot manager creates a proper subvolume for such an
environment it will also get further subvolumes placed in there, under
the assumption that the manager understands the concept of subvolumes in
that case.

src/basic/btrfs-util.c

@@ -105,7 +105,7 @@ int btrfs_is_filesystem(int fd) {
         return F_TYPE_EQUAL(sfs.f_type, BTRFS_SUPER_MAGIC);
 }
 
-int btrfs_is_subvol(int fd) {
+int btrfs_is_subvol_fd(int fd) {
         struct stat st;
 
         assert(fd >= 0);
@@ -121,6 +121,18 @@ int btrfs_is_subvol(int fd) {
         return btrfs_is_filesystem(fd);
 }
 
+int btrfs_is_subvol(const char *path) {
+        _cleanup_close_ int fd = -1;
+
+        assert(path);
+
+        fd = open(path, O_RDONLY|O_NOCTTY|O_CLOEXEC|O_DIRECTORY);
+        if (fd < 0)
+                return -errno;
+
+        return btrfs_is_subvol_fd(fd);
+}
+
 int btrfs_subvol_make(const char *path) {
         struct btrfs_ioctl_vol_args args = {};
         _cleanup_close_ int fd = -1;
@@ -1682,7 +1694,7 @@ int btrfs_subvol_snapshot_fd(int old_fd, const char *new_path, BtrfsSnapshotFlag
         assert(old_fd >= 0);
         assert(new_path);
 
-        r = btrfs_is_subvol(old_fd);
+        r = btrfs_is_subvol_fd(old_fd);
         if (r < 0)
                 return r;
         if (r == 0) {
@@ -1868,7 +1880,7 @@ int btrfs_subvol_auto_qgroup_fd(int fd, uint64_t subvol_id, bool insert_intermed
          */
 
         if (subvol_id == 0) {
-                r = btrfs_is_subvol(fd);
+                r = btrfs_is_subvol_fd(fd);
                 if (r < 0)
                         return r;
                 if (!r)

src/basic/btrfs-util.h

@@ -56,7 +56,9 @@ typedef enum BtrfsRemoveFlags {
 } BtrfsRemoveFlags;
 
 int btrfs_is_filesystem(int fd);
-int btrfs_is_subvol(int fd);
+
+int btrfs_is_subvol_fd(int fd);
+int btrfs_is_subvol(const char *path);
 
 int btrfs_reflink(int infd, int outfd);
 int btrfs_clone_range(int infd, uint64_t in_offset, int ofd, uint64_t out_offset, uint64_t sz);