[PATCH v2 3/5] btrfs-progs: rootdir: warn about hard links

[Qu Wenruo <wqu@suse.com>]

The recent rework changes how we detect hard links.

[OLD BEHAVIOR]
We trusted st_nlink and st_ino, reuse them without extra sanity
checks.

That behavior has problems handling cross mount-point or hard links out
of the rootdir cases.

[NEW BEHAVIOR]
The new refactored code will treat every inode, no matter if it's a
hardlink, as a new inode.

This means we will break the hard link detection, and every hard link
will be created as a different inode.

For the most common use case, like populating a rootfs, it's toally
fine.

[EXTRA WARNING]
But for cases where the user have extra hard links inside the rootdir,
output a warning just to inform the end user.

This will not cause any content difference, just breaking the hard links
into new inodes.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 mkfs/rootdir.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/mkfs/rootdir.c b/mkfs/rootdir.c
index 3db32982550b..05787dc3f46c 100644
--- a/mkfs/rootdir.c
+++ b/mkfs/rootdir.c
@@ -419,6 +419,21 @@ static int ftw_add_inode(const char *full_path, const struct stat *st,
 	u64 ino;
 	int ret;
 
+	/*
+	 * Hard link need extra detection code, not supported for now, but
+	 * it's not to break anything but splitting the hard links into
+	 * new inodes.
+	 * And we do not even know if the hard links are inside the rootdir.
+	 *
+	 * So here we only need to do extra warning.
+	 *
+	 * On most filesystems st_nlink of a directory is the number of
+	 * subdirs, including "." and "..", so skip directory inodes.
+	 */
+	if (unlikely(!S_ISDIR(st->st_mode) && st->st_nlink > 1))
+		warning("'%s' has extra hard links, they will be converted into new inodes.",
+			full_path);
+
 	/* The rootdir itself. */
 	if (unlikely(ftwbuf->level == 0)) {
 		u64 root_ino = btrfs_root_dirid(&root->root_item);
-- 
2.45.2