generic: add test for zero range over a file range with many small extents

Test a fallocate() zero range operation against a large file range for which
there are many small extents allocated. Verify the operation does not fail
and the respective range return zeroes on subsequent reads.

This test is motivated by a bug found on btrfs. The patch that fixes the
bug on btrfs has the following subject:

 "btrfs: fix metadata reservation for fallocate that leads to transaction aborts"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/generic/610

@@ -0,0 +1,61 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2020 SUSE Linux Products GmbH. All Rights Reserved.
+#
+# FS QA Test No. 610
+#
+# Test a fallocate() zero range operation against a large file range for which
+# there are many small extents allocated. Verify the operation does not fail
+# and the respective range return zeroes on subsequent reads.
+#
+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
+_supported_os Linux
+_require_scratch
+_require_xfs_io_command "fzero"
+_require_xfs_io_command "fpunch"
+_require_test_program "punch-alternating"
+
+rm -f $seqres.full
+
+_scratch_mkfs >>$seqres.full 2>&1
+_scratch_mount
+
+# Create a file with many small extents. To speed up file creation, do
+# buffered writes and then punch a hole on every other block.
+$XFS_IO_PROG -f -c "pwrite -S 0xab -b 10M 0 100M" \
+	$SCRATCH_MNT/foobar >>$seqres.full
+$here/src/punch-alternating $SCRATCH_MNT/foobar >>$seqres.full
+
+# For btrfs, trigger a transaction commit to force metadata COW for the
+# following fallocate zero range operation.
+sync
+
+$XFS_IO_PROG -c "fzero 0 100M" $SCRATCH_MNT/foobar
+
+# Check the file content after umounting and mounting again the fs, to verify
+# everything was persisted.
+_scratch_cycle_mount
+
+echo "File content after zero range operation:"
+od -A d -t x1 $SCRATCH_MNT/foobar
+
+status=0
+exit

tests/generic/610.out

@@ -0,0 +1,5 @@
+QA output created by 610
+File content after zero range operation:
+0000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+*
+104857600

tests/generic/group

@@ -612,3 +612,4 @@
 607 auto attr quick dax
 608 auto attr quick dax
 609 auto quick rw
+610 auto quick prealloc zero