btrfs: test ENOSPC caused by many orphan items

Btrfs has a bug where we can prematurely ENOSPC if we have lots of
orphaned files, i.e., deleted files which are still open. Add a test
which repeatedly creates and deletes a file while keeping all of the
file descriptors open. This should succeed but doesn't on Btrfs
without the fix.

[Eryu: use multi_open_unlink command to keep files open & unlinked
instead of shell scripts]

Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/generic/488

@@ -0,0 +1,62 @@
+#! /bin/bash
+# FS QA Test 488
+#
+# Test having many file descriptors referring to deleted files open. Regression
+# test for patch "Btrfs: fix ENOSPC caused by orphan items reservations".
+#
+#-----------------------------------------------------------------------
+# Copyright (c) 2018 Omar Sandoval.  All Rights Reserved.
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it would be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write the Free Software Foundation,
+# Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+#-----------------------------------------------------------------------
+#
+
+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.*
+}
+
+. ./common/rc
+. ./common/filter
+
+rm -f $seqres.full
+
+_supported_fs generic
+_supported_os Linux
+_require_scratch
+_require_test_program "multi_open_unlink"
+
+_scratch_mkfs_sized $((1024 * 1024 * 1024)) >> $seqres.full 2>&1
+_scratch_mount
+
+test_file="$SCRATCH_MNT/$seq"
+
+ulimit -n $((16 * 1024))
+# ~10000 files on a 1 GB filesystem should be no problem.
+$here/src/multi_open_unlink -f $SCRATCH_MNT/$seq -n 10000 -s 0
+
+echo "Silence is golden"
+
+status=0
+exit

tests/generic/488.out

@@ -0,0 +1,2 @@
+QA output created by 488
+Silence is golden

tests/generic/group

@@ -490,3 +490,4 @@
 485 auto quick insert
 486 auto quick attr
 487 auto quick
+488 auto quick