nspawn: add fallback top normal copy/reflink when we cannot btrfs snapshot

Given that other file systems (notably: xfs) support reflinks these days, let's
extend the file system snapshotting logic to fall back to plan copies or
reflinks when full btrfs subvolume snapshots are not available.

This essentially makes "systemd-nspawn --ephemeral" and "systemd-nspawn
--template=" available on non-btrfs subvolumes. Of course, both operations will
still be slower on non-btrfs than on btrfs (simply because reflinking each file
individually in a directory tree is still slower than doing this in one step
for a whole subvolume), but it's probably good enough for many cases, and we
should provide the users with the tools, they have to figure out what's good
for them.

Note that "machinectl clone" already had a fallback like this in place, this
patch generalizes this, and adds similar support to our other cases.

1 files changed, 18 insertions, 7 deletions

diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index 29755a9e36..2770770cd0 100644
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -4070,7 +4070,7 @@ int main(int argc, char *argv[]) {
         _cleanup_fdset_free_ FDSet *fds = NULL;
         int r, n_fd_passed, loop_nr = -1, ret = EXIT_SUCCESS;
         char veth_name[IFNAMSIZ] = "";
-        bool secondary = false, remove_subvol = false, remove_image = false;
+        bool secondary = false, remove_directory = false, remove_image = false;
         pid_t pid = 0;
         union in_addr_union exposed = {};
         _cleanup_release_lock_file_ LockFile tree_global_lock = LOCK_FILE_INIT, tree_local_lock = LOCK_FILE_INIT;
@@ -4152,7 +4152,12 @@ int main(int argc, char *argv[]) {
                                 goto finish;
                         }
 
-                        r = btrfs_subvol_snapshot(arg_directory, np, (arg_read_only ? BTRFS_SNAPSHOT_READ_ONLY : 0) | BTRFS_SNAPSHOT_FALLBACK_COPY | BTRFS_SNAPSHOT_RECURSIVE | BTRFS_SNAPSHOT_QUOTA);
+                        r = btrfs_subvol_snapshot(arg_directory, np,
+                                                  (arg_read_only ? BTRFS_SNAPSHOT_READ_ONLY : 0) |
+                                                  BTRFS_SNAPSHOT_FALLBACK_COPY |
+                                                  BTRFS_SNAPSHOT_FALLBACK_DIRECTORY |
+                                                  BTRFS_SNAPSHOT_RECURSIVE |
+                                                  BTRFS_SNAPSHOT_QUOTA);
                         if (r < 0) {
                                 log_error_errno(r, "Failed to create snapshot %s from %s: %m", np, arg_directory);
                                 goto finish;
@@ -4162,7 +4167,7 @@ int main(int argc, char *argv[]) {
                         arg_directory = np;
                         np = NULL;
 
-                        remove_subvol = true;
+                        remove_directory = true;
 
                 } else {
                         r = image_path_lock(arg_directory, (arg_read_only ? LOCK_SH : LOCK_EX) | LOCK_NB, &tree_global_lock, &tree_local_lock);
@@ -4176,7 +4181,13 @@ int main(int argc, char *argv[]) {
                         }
 
                         if (arg_template) {
-                                r = btrfs_subvol_snapshot(arg_template, arg_directory, (arg_read_only ? BTRFS_SNAPSHOT_READ_ONLY : 0) | BTRFS_SNAPSHOT_FALLBACK_COPY | BTRFS_SNAPSHOT_RECURSIVE | BTRFS_SNAPSHOT_QUOTA);
+                                r = btrfs_subvol_snapshot(arg_template, arg_directory,
+                                                          (arg_read_only ? BTRFS_SNAPSHOT_READ_ONLY : 0) |
+                                                          BTRFS_SNAPSHOT_FALLBACK_COPY |
+                                                          BTRFS_SNAPSHOT_FALLBACK_DIRECTORY |
+                                                          BTRFS_SNAPSHOT_FALLBACK_IMMUTABLE |
+                                                          BTRFS_SNAPSHOT_RECURSIVE |
+                                                          BTRFS_SNAPSHOT_QUOTA);
                                 if (r == -EEXIST) {
                                         if (!arg_quiet)
                                                 log_info("Directory %s already exists, not populating from template %s.", arg_directory, arg_template);
@@ -4359,12 +4370,12 @@ finish:
 
         loop_remove(loop_nr, &image_fd);
 
-        if (remove_subvol && arg_directory) {
+        if (remove_directory && arg_directory) {
                 int k;
 
-                k = btrfs_subvol_remove(arg_directory, BTRFS_REMOVE_RECURSIVE|BTRFS_REMOVE_QUOTA);
+                k = rm_rf(arg_directory, REMOVE_ROOT|REMOVE_PHYSICAL|REMOVE_SUBVOLUME);
                 if (k < 0)
-                        log_warning_errno(k, "Cannot remove subvolume '%s', ignoring: %m", arg_directory);
+                        log_warning_errno(k, "Cannot remove '%s', ignoring: %m", arg_directory);
         }
 
         if (remove_image && arg_image) {