[PATCH] common: add proper btrfs log tree detection to _has_metadata_journaling

[Qu Wenruo <wqu@suse.com>]

[BUG]
With the incoming btrfs shutdown ioctl/remove_bdev callback support,
btrfs can be tested with the shutdown group.

But test case generic/050 still fails on btrfs with shutdown support:

generic/050 1s ... - output mismatch (see /home/adam/xfstests/results//generic/050.out.bad)
    --- tests/generic/050.out	2022-05-11 11:25:30.763333331 +0930
    +++ /home/adam/xfstests/results//generic/050.out.bad	2025-06-30 10:22:21.752068622 +0930
    @@ -13,9 +13,7 @@
     setting device read-only
     mounting filesystem that needs recovery on a read-only device:
     mount: device write-protected, mounting read-only
    -mount: cannot mount device read-only
     unmounting read-only filesystem
    -umount: SCRATCH_DEV: not mounted
     mounting filesystem with -o norecovery on a read-only device:
    ...
    (Run 'diff -u /home/adam/xfstests/tests/generic/050.out /home/adam/xfstests/results//generic/050.out.bad'  to see the entire diff)
Ran: generic/050

[CAUSE]
The test case generic/050 has several different golden output depending
on the fs features.

For fses which requires data write (e.g. replay the journal) during
mount, mounting a read-only block device should fail.
And that is the default golden output.

However there are cases that the fs doesn't need to write anything, e.g.
the fs has no metadata journal support at all, or the fs has no dirty
journal to replay.

In that case, there is the generic/050.nojournal golden output.

The test case is using the helper _has_metadata_journaling() to
determine the feature.

Unfortunately that helper doesn't support btrfs, thus the default
behavior is to assume the fs has journal to replay, thus for btrfs it
always assume there is a journal to replay and results the wrong golden
output to be used.

[FIX]
Add btrfs specific log tree check into _has_metadata_journaling(), so
that if there is no log tree to replay, expose the "nojournal" feature
correctly to pass generic/050.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 common/rc | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/common/rc b/common/rc
index 2d8e7167..a78b779a 100644
--- a/common/rc
+++ b/common/rc
@@ -4283,6 +4283,14 @@ _has_metadata_journaling()
 			return 1
 		fi
 		;;
+	btrfs)
+		_require_btrfs_command inspect-internal dump-super
+		if "$BTRFS_UTIL_PROG" inspect-internal dump-super $dev | \
+		   grep -q "log_root\s\+0" ; then
+			echo "$FSTYPE on $dev has empty log tree"
+			return 1
+		fi
+		;;
 	*)
 		# by default we pass; if you need to, add your fs above!
 		;;
-- 
2.50.0