[PATCH v7 04/11] fstests: add _require_unique_f_fsid() helper

Add a helper to check if the target filesystem supports unique f_fsid
tracking across cloned or snapshot instances.

Certain filesystems like XFS, Btrfs, and F2FS ensure unique f_fsid
identifiers per filesystem instance. However, Ext4 derives its f_fsid
directly from its superblock UUID, which leads to identical f_fsid
values on cloned images until the UUID is manually modified by userspace.

Introduce _require_unique_f_fsid() to allow test cases requiring strict
f_fsid uniqueness to skip gracefully on unsupported filesystems.

Signed-off-by: Anand Jain <[email protected]>
---
 common/rc | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/common/rc b/common/rc
index 968ba33686f3..d95eec94f7b7 100644
--- a/common/rc
+++ b/common/rc
@@ -6310,6 +6310,27 @@ _require_fanotify_ioerrors()
 	_notrun "$FSTYP does not support fanotify ioerrors"
 }
 
+# Ext4 derives f_fsid from the superblock UUID, meaning clones share the
+# same f_fsid until their UUIDs diverge. Conversely, XFS, Btrfs,
+# and F2FS ensure f_fsid remains unique per filesystem instance (often by
+# deriving it from the UUID and underlying block device.)
+#
+# Across all filesystems, a UUID collision causes libblkid tools to return
+# non-deterministic device mappings. It is ultimately the responsibility
+# of the userspace utility or use-case to enforce uniqueness when a clone
+# diverges. For details, see mailing list thread discussions:
+#   Link: https://lore.kernel.org/linux-ext4/[email protected]/
+_require_unique_f_fsid()
+{
+	# Skip the test if the filesystem does not enforce unique f_fsids
+	# natively. Checking this dynamically requires recreating a clone
+	# layout, so we use a static lookup based on FSTYP.
+	if [ "$FSTYP" == "ext4" ]; then
+		_notrun "Target filesystem ($FSTYP) does not guarantee unique f_fsid on clones."
+	fi
+}
+
+
 # Computes a percentage of the available space in a filesystem and
 # returns that quantity in MB. The percentage must not contain a percent
 # sign ("%").
-- 
2.43.0