[PATCH v3 2/2] btrfs: test ENOSPC handling on fsverity rollback

[Daniel Linjama <daniel@dev.linjama.com>]

Test that the filesystem stays writable when an fsverity enable and its
rollback run out of space on a small mixed block group filesystem, and
that it still mounts afterwards with no verity or orphan items left behind.

The -ENOSPC counterpart of btrfs/354.

This exercises a bug fixed by kernel commit 76bf149cd029 ("btrfs: handle
lack of space when cleaning up verity items").

Assisted-by: LLM
Signed-off-by: Daniel Linjama <daniel@dev.linjama.com>
---
Changes since v2:
- The fix is in mainline as 76bf149cd029, fill it in.

Changes since v1:
- Fill the filesystem with a single xfs_io write, like btrfs/354 now
  does. The room for the enable to start is made by deleting a small
  file written beforehand, since a truncate fails on a full filesystem.
- Look for leftover items of the target inode only, like btrfs/058 does.

 tests/btrfs/355     | 117 ++++++++++++++++++++++++++++++++++++++++++++
 tests/btrfs/355.out |   2 +
 2 files changed, 119 insertions(+)
 create mode 100755 tests/btrfs/355
 create mode 100644 tests/btrfs/355.out

diff --git a/tests/btrfs/355 b/tests/btrfs/355
new file mode 100755
index 0000000..f7918a4
--- /dev/null
+++ b/tests/btrfs/355
@@ -0,0 +1,117 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 Daniel Linjama.  All Rights Reserved.
+#
+# FS QA Test 355
+#
+# Test that a failed fsverity enable on a full filesystem leaves the
+# filesystem usable. The -ENOSPC counterpart of btrfs/354.
+#
+. ./common/preamble
+_begin_fstest auto quick verity enospc
+
+# Override the default cleanup function.
+_cleanup()
+{
+	cd /
+	_restore_fsverity_signatures
+	rm -f $tmp.*
+}
+
+# Import common functions.
+. ./common/filter
+. ./common/verity
+
+# real QA test starts here
+
+_require_scratch_verity
+_require_btrfs_command inspect-internal dump-tree
+_require_no_compress
+_require_scratch_size $((1024 * 1024))
+_disable_fsverity_signatures
+
+_fixed_by_kernel_commit 76bf149cd029 \
+	"btrfs: handle lack of space when cleaning up verity items"
+
+target=$SCRATCH_MNT/target
+
+prepare()
+{
+	# Mixed block groups, so filling with data starves metadata too.
+	_scratch_mkfs --mixed -b 1G &>> $seqres.full || _fail "mkfs failed"
+	_scratch_mount
+}
+
+create_target()
+{
+	$XFS_IO_PROG -f -c "pwrite -q -b 1m 0 512m" $target
+	ino=$(stat -c %i $target)
+	sync
+}
+
+fill_fs()
+{
+	# Deleting this afterwards leaves room for the enable to start, not
+	# to finish.
+	$XFS_IO_PROG -f -c "pwrite -q 0 1m" $SCRATCH_MNT/margin
+	# Writes until the filesystem is full.
+	$XFS_IO_PROG -f -c "pwrite -q -b 64k 0 1g" $SCRATCH_MNT/filler &>/dev/null
+	sync
+	rm -f $SCRATCH_MNT/margin
+	sync
+	$BTRFS_UTIL_PROG filesystem usage $SCRATCH_MNT >> $seqres.full 2>&1
+}
+
+enable_fsverity()
+{
+	if _fsv_enable $target >> $seqres.full 2>&1; then
+		_notrun "could not fill the filesystem, verity enable succeeded"
+	fi
+}
+
+check_rollback()
+{
+	# Deleting a filler needs no new space, it fails only if the fs went read-only.
+	rm -f $SCRATCH_MNT/filler 2>> $seqres.full || \
+		echo "filesystem was forced read-only by the failed verity enable"
+	if $FSVERITY_PROG measure $target >> $seqres.full 2>&1; then
+		echo "verity is enabled on the target after a failed enable"
+	fi
+}
+
+check_remount()
+{
+	_scratch_unmount
+	_try_scratch_mount >> $seqres.full 2>&1 || \
+		_fail "cannot mount the filesystem after the failed verity enable"
+	$XFS_IO_PROG -c "pread -q 0 1m" $target >> $seqres.full 2>&1 || \
+		echo "cannot read the target file after the failed verity enable"
+	_scratch_unmount
+}
+
+check_leftover_items()
+{
+	local dump=$($BTRFS_UTIL_PROG inspect-internal dump-tree -t 5 $SCRATCH_DEV)
+	local verity_items=$(echo "$dump" | grep -c "($ino VERITY_\(DESC\|MERKLE\)_ITEM")
+	local orphans=$(echo "$dump" | grep -c "(ORPHAN ORPHAN_ITEM $ino)")
+
+	echo "$dump" >> $seqres.full
+	[ "$verity_items" -eq 0 ] || \
+		echo "$verity_items verity items left behind by the failed enable"
+	[ "$orphans" -eq 0 ] || \
+		echo "$orphans orphan items left behind by the failed enable"
+}
+
+prepare
+create_target
+fill_fs
+enable_fsverity
+check_rollback
+check_remount
+check_leftover_items
+
+echo "Silence is golden"
+
+# success, all done
+status=0
+exit
diff --git a/tests/btrfs/355.out b/tests/btrfs/355.out
new file mode 100644
index 0000000..4e4c5e9
--- /dev/null
+++ b/tests/btrfs/355.out
@@ -0,0 +1,2 @@
+QA output created by 355
+Silence is golden
-- 
2.55.0