btrfs: test power failure while qgroups rescan is in progress

Test that if a power failure happens on a filesystem with quotas
(qgroups) enabled while the quota rescan kernel thread is running,
we will be able to mount the filesystem after the power failure.

This test is motivated by a recent regression introduced in the
linux kernel's 4.18 merge window and is fixed by a patch with the
title:

  "Btrfs: fix mount failure when qgroup rescan is in progress"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/btrfs/166

@@ -0,0 +1,57 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2018 SUSE Linux Products GmbH. All Rights Reserved.
+#
+# FSQA Test No. 166
+#
+# Test that if a power failure happens on a filesystem with quotas (qgroups)
+# enabled while the quota rescan kernel thread is running, we will be able
+# to mount the filesystem after the power failure.
+#
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+tmp=/tmp/$$
+status=1	# failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+_cleanup()
+{
+	_cleanup_flakey
+	cd /
+	rm -f $tmp.*
+}
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+. ./common/dmflakey
+
+# real QA test starts here
+_supported_fs btrfs
+_supported_os Linux
+_require_scratch
+_require_dm_target flakey
+
+rm -f $seqres.full
+
+_scratch_mkfs  >>$seqres.full 2>&1
+_require_metadata_journaling $SCRATCH_DEV
+_init_flakey
+_mount_flakey
+
+# Enable qgroups on the filesystem. This will start the qgroup rescan kernel
+# thread.
+_run_btrfs_util_prog quota enable $SCRATCH_MNT
+
+# Simulate a power failure, while the qgroup rescan kernel thread is running,
+# and then mount the filesystem to check that mounting the filesystem does not
+# fail.
+_flakey_drop_and_remount
+
+_unmount_flakey
+_cleanup_flakey
+
+echo "Silence is golden"
+status=0
+exit

tests/btrfs/166.out

@@ -0,0 +1,2 @@
+QA output created by 166
+Silence is golden

tests/btrfs/group

@@ -168,3 +168,4 @@
 163 auto quick volume
 164 auto quick volume
 165 auto quick subvol
+166 auto quick qgroup