generic: test for non-zero used blocks while writing into a file

Test that if we keep overwriting an entire file, either with buffered
writes or direct IO writes, the number of used blocks reported by stat(2)
is never zero while the writes and writeback are in progress.

This is motivated by a bug in btrfs and currently fails on btrfs only. It
is fixed a patchset for btrfs that has the following patches:

  btrfs: fix missing delalloc new bit for new delalloc ranges
  btrfs: refactor btrfs_drop_extents() to make it easier to extend
  btrfs: fix race when defragging that leads to unnecessary IO
  btrfs: update the number of bytes used by an inode atomically

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

tests/generic/615

@@ -0,0 +1,84 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2020 SUSE Linux Products GmbH. All Rights Reserved.
+#
+# FS QA Test No. 615
+#
+# Test that if we keep overwriting an entire file, either with buffered writes
+# or direct IO writes, the number of used blocks reported by stat(2) is never
+# zero while writeback is in progress.
+#
+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()
+{
+	cd /
+	rm -f $tmp.*
+}
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+
+# real QA test starts here
+_supported_fs generic
+_require_scratch
+_require_odirect
+
+rm -f $seqres.full
+
+stat_loop()
+{
+	trap "wait; exit" SIGTERM
+	local filepath=$1
+	local blocks
+
+	while :; do
+		blocks=$(stat -c %b $filepath)
+		if [ $blocks -eq 0 ]; then
+		    echo "error: stat(2) reported zero blocks"
+		    break
+		fi
+	done
+}
+
+_scratch_mkfs >>$seqres.full 2>&1
+_scratch_mount
+
+$XFS_IO_PROG -f -s -c "pwrite -b 64K 0 64K" $SCRATCH_MNT/foo > /dev/null
+
+stat_loop $SCRATCH_MNT/foo &
+loop_pid=$!
+
+echo "Testing buffered writes"
+
+# Now keep overwriting the entire file, triggering writeback after each write,
+# while another process is calling stat(2) on the file. We expect the number of
+# used blocks reported by stat(2) to be always greater than 0.
+for ((i = 0; i < 2000; i++)); do
+	if ! kill -s 0 $loop_pid &> /dev/null; then
+	    break
+	fi
+	$XFS_IO_PROG -s -c "pwrite -b 64K 0 64K" $SCRATCH_MNT/foo > /dev/null
+done
+
+echo "Testing direct IO writes"
+
+# Now similar to what we did before but for direct IO writes.
+for ((i = 0; i < 2000; i++)); do
+	if ! kill -s 0 $loop_pid &> /dev/null; then
+	    break
+	fi
+	$XFS_IO_PROG -d -c "pwrite -b 64K 0 64K" $SCRATCH_MNT/foo > /dev/null
+done
+
+kill $loop_pid &> /dev/null
+wait
+
+status=0
+exit

tests/generic/615.out

@@ -0,0 +1,3 @@
+QA output created by 615
+Testing buffered writes
+Testing direct IO writes

tests/generic/group

@@ -617,3 +617,4 @@
 612 auto quick clone
 613 auto quick encrypt
 614 auto quick rw
+615 auto rw