[PATCH v8 05/13] 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 | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/common/rc b/common/rc
index 21b516accdf7..f73251365a60 100644
--- a/common/rc
+++ b/common/rc
@@ -6356,6 +6356,33 @@ _require_fanotify_ioerrors()
 	_notrun "$FSTYP does not support fanotify ioerrors"
 }
 
+# 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.
+#
+# 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()
+{
+	case "$FSTYP" in
+	ext*)
+		# Ext4 derives f_fsid from the superblock UUID, meaning
+		# clones share the same f_fsid until their UUIDs
+		# diverge.
+		_notrun "Target filesystem ($FSTYP) does not guarantee unique f_fsid on clones."
+		;;
+	*)
+		# Conversely, XFS, Btrfs, and F2FS ensure f_fsid remains
+		# unique per filesystem instance (often by deriving it
+		# from the UUID and underlying block device.)
+		;;
+	esac
+}
+
+
 # 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