[PATCH v2] fstests: generic: Test if we can still do operations which don't take extra data space on a fs without data space

[Qu Wenruo <wqu@suse.com>]

This test will test if we can still do the following operations when a
fs has no extra data space:
- buffered write into unpopulated preallocated extent
- clone the untouched preallocated extent
- fsync
Above operations in a row should not fail, as they takes no extra data space.

Xfs passes the test, while btrfs fails at fsync.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
changelog:
v2:
- Change the comment and commit message to make it describe the test
  itself, not the btrfs specific part.
- Use $XFS_IO_PROG to replace xfs_io.
---
 tests/generic/545     | 70 +++++++++++++++++++++++++++++++++++++++++++
 tests/generic/545.out |  2 ++
 tests/generic/group   |  1 +
 3 files changed, 73 insertions(+)
 create mode 100755 tests/generic/545
 create mode 100644 tests/generic/545.out

diff --git a/tests/generic/545 b/tests/generic/545
new file mode 100755
index 00000000..9efaf58c
--- /dev/null
+++ b/tests/generic/545
@@ -0,0 +1,70 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2019 SUSE Linux Products GmbH. All Rights Reserved.
+#
+# FS QA Test 545
+#
+# Test when a fs is full we can still:
+# - Do buffered write into a unpopulated preallocated extent
+# - Clone the untouched part of that preallocated extent
+# - Fsync
+# All operations above should not fail.
+#
+# Xfs passes the test while btrfs fails. The fix for btrfs is:
+# "btrfs: Flush before reflinking any extent to prevent NOCOW write falling
+#  back to CoW without data reservation"
+#
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1	# failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+_cleanup()
+{
+	cd /
+	rm -f $tmp.*
+}
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+. ./common/reflink
+
+# remove previous $seqres.full before test
+rm -f $seqres.full
+
+# real QA test starts here
+
+# Modify as appropriate.
+_supported_fs generic
+_supported_os Linux
+_require_scratch
+_require_scratch_reflink
+
+_scratch_mkfs_sized $((512 * 1024 * 1024)) >> $seqres.full 2>&1
+_scratch_mount
+
+# Create preallocated extent where we can write into
+$XFS_IO_PROG -f -c 'falloc 8k 64m' "$SCRATCH_MNT/foobar" >> $seqres.full
+
+# Use up all data space, to test later write-into-preallocate behavior
+_pwrite_byte 0x00 0 512m "$SCRATCH_MNT/padding" >> $seqres.full 2>&1
+
+# This should not fail
+_pwrite_byte 0xcd 1m 16m "$SCRATCH_MNT/foobar" >> $seqres.full
+
+# Do reflink here, we shouldn't use extra data space, thus it should not fail
+$XFS_IO_PROG -c "reflink ${SCRATCH_MNT}/foobar 8k 0 4k" "$SCRATCH_MNT/foobar" \
+	>> $seqres.full
+
+# Fsync to check if writeback is ok
+$XFS_IO_PROG -c 'fsync'  "$SCRATCH_MNT/foobar"
+
+echo "Silence is golden"
+# success, all done
+status=0
+exit
diff --git a/tests/generic/545.out b/tests/generic/545.out
new file mode 100644
index 00000000..920d7244
--- /dev/null
+++ b/tests/generic/545.out
@@ -0,0 +1,2 @@
+QA output created by 545
+Silence is golden
diff --git a/tests/generic/group b/tests/generic/group
index 40deb4d0..f26b91fe 100644
--- a/tests/generic/group
+++ b/tests/generic/group
@@ -547,3 +547,4 @@
 542 auto quick clone
 543 auto quick clone
 544 auto quick clone
+545 auto quick clone enospc
-- 
2.21.0