Add a new test to confirm that the ZSTD feature flag is activated

When the compression property on a dataset is set to zstd, the feature
flag is not changed from 'enabled' to 'active' until the first block
is born.

If the pool is exported after `zfs set` but before a block is written
then imported on a system with an older version of ZFS that does not
understand ZSTD, it will cause the userland utilities to panic.

Signed-off-by: Allan Jude <allanjude@freebsd.org>

tests/zfs-tests/tests/functional/cli_root/zfs_set/zfs_set_feature_activation.ksh

@@ -0,0 +1,83 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# The contents of this file are subject to the terms of the
+# Common Development and Distribution License (the "License").
+# You may not use this file except in compliance with the License.
+#
+# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+# or http://www.opensolaris.org/os/licensing.
+# See the License for the specific language governing permissions
+# and limitations under the License.
+#
+# When distributing Covered Code, include this CDDL HEADER in each
+# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+# If applicable, add the following below this CDDL HEADER, with the
+# fields enclosed by brackets "[]" replaced with your own identifying
+# information: Portions Copyright [yyyy] [name of copyright owner]
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2020 The FreeBSD Foundation [1]
+#
+# [1] Portions of this software were developed by Allan Jude
+#     under sponsorship from the FreeBSD Foundation.
+
+. $STF_SUITE/include/libtest.shlib
+
+#
+# DESCRIPTION:
+# Setting the compression property to any of the zstd levels should activate
+# the zstd feature flag. Destroying the last dataset using the zstd feature flag
+# should revert the feature to the 'enabled' state.
+#
+# STRATEGY:
+# 1. Create pool, then create a file system within it.
+# 2. Check that the zstd feature flag is 'enabled'.
+# 3. Setting the compression property to zstd.
+# 4. Check that the zstd feature flag is now 'active'.
+# 5. Destroy the dataset
+# 6. Confirm that the feature flag reverts to the 'enabled' state.
+#
+
+verify_runnable "both"
+
+log_assert "Setting compression=zstd should activate the"\
+	"org.freebsd:zstd_compress feature flag, and destroying the last"\
+	"dataset using that property, should revert the feature flag to"\
+	"the enabled state."
+
+featureval="$(get_pool_prop feature@zstd_compress $TESTPOOL)"
+
+[[ "$featureval" == "disabled" ]] && \
+	log_unsupported "ZSTD feature flag unsupposed"
+
+[[ "$featureval" == "active" ]] && \
+	log_unsupported "ZSTD feature already active before test"
+
+random_level=$((RANDOM%19 + 1))
+log_note "Randomly selected ZSTD level: $random_level"
+
+log_must zfs create -o compress=zstd-$random_level $TESTPOOL/$TESTFS-zstd
+
+featureval="$(get_pool_prop feature@zstd_compress $TESTPOOL)"
+
+log_note "After zfs set, feature flag value is: $featureval"
+
+[[ "$featureval" == "active" ]] ||
+	log_fail "ZSTD feature flag not activated"
+
+log_must zfs destroy $TESTPOOL/$TESTFS-zstd
+
+featureval="$(get_pool_prop feature@zstd_compress $TESTPOOL)"
+
+log_note "After zfs destroy, feature flag value is: $featureval"
+
+[[ "$featureval" == "enabled" ]] ||
+	log_fail "ZSTD feature flag not deactivated"
+
+log_pass "Setting compression=zstd activated the feature flag, and"\
+	"destroying the dataset deactivated it."