[PATCH] btrfs-progs: subvolume: Add warning on deleting default subvolume

[Sidong Yang <realwakka@gmail.com>]

This patch add warning messages when user try to delete default
subvolume. When deleting default subvolume, kernel will not allow and
make error message on syslog. but there is only message that permission
denied on userspace. User can be noticed the reason by this warning message.

This patch implements github issue.
https://github.com/kdave/btrfs-progs/issues/274

Signed-off-by: Sidong Yang <realwakka@gmail.com>
---
 cmds/subvolume.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/cmds/subvolume.c b/cmds/subvolume.c
index 2020e486..0cdf7a68 100644
--- a/cmds/subvolume.c
+++ b/cmds/subvolume.c
@@ -264,6 +264,7 @@ static int cmd_subvol_delete(const struct cmd_struct *cmd,
 	struct seen_fsid *seen_fsid_hash[SEEN_FSID_HASH_SIZE] = { NULL, };
 	enum { COMMIT_AFTER = 1, COMMIT_EACH = 2 };
 	enum btrfs_util_error err;
+	uint64_t default_subvol_id = 0, target_subvol_id = 0;
 
 	optind = 0;
 	while (1) {
@@ -360,6 +361,25 @@ again:
 		goto out;
 	}
 
+	err = btrfs_util_get_default_subvolume_fd(fd, &default_subvol_id);
+	if (fd < 0) {
+		ret = 1;
+		goto out;
+	}
+
+	if (subvolid > 0)
+		target_subvol_id = subvolid;
+	else {
+		err = btrfs_util_subvolume_id(path, &target_subvol_id);
+		if (fd < 0) {
+			ret = 1;
+			goto out;
+		}
+	}
+
+	if (target_subvol_id == default_subvol_id)
+		warning("trying to delete default subvolume.");
+
 	pr_verbose(MUST_LOG, "Delete subvolume (%s): ",
 		commit_mode == COMMIT_EACH ||
 		(commit_mode == COMMIT_AFTER && cnt + 1 == argc) ?
-- 
2.25.1