'zfs share -a' should clean noauto exports

This is a follow on to PR #10688 where `zfs share -a` allows the 
sharing of canmount=noauto datasets if they are mounted.  However, 
when a dataset with canmount=noauto is not mounted, the command 
should also purge any existing entries from the exports file. 
Otherwise, after a reboot, the nfs server attempts to export the 
underlying mountpath, not the dataset. This can lead to a hard hang 
for existing client mounts.

Instead of just skipping the adding of an export if not mounted 
and canmount=noauto, have it also remove an existing export of the 
dataset so that, after a reboot, we don't export an unmounted dataset.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <gwilson@delphix.com>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #10747

cmd/zfs/zfs_main.c

@@ -6634,9 +6634,12 @@ share_mount_one(zfs_handle_t *zhp, int op, int flags, char *protocol,
 		 */
 		if (op == OP_MOUNT)
 			return (0);
-		if (op == OP_SHARE && !zfs_is_mounted(zhp, NULL))
+		if (op == OP_SHARE && !zfs_is_mounted(zhp, NULL)) {
+			/* also purge it from existing exports */
+			zfs_unshareall_bypath(zhp, mountpoint);
 			return (0);
 		}
+	}
 
 	/*
 	 * If this filesystem is encrypted and does not have

tests/runfiles/linux.run

@@ -52,7 +52,8 @@ tests = ['zfs_mount_006_pos', 'zfs_mount_008_pos', 'zfs_multi_mount']
 tags = ['functional', 'cli_root', 'zfs_mount']
 
 [tests/functional/cli_root/zfs_share:Linux]
-tests = ['zfs_share_005_pos', 'zfs_share_007_neg', 'zfs_share_009_neg']
+tests = ['zfs_share_005_pos', 'zfs_share_007_neg', 'zfs_share_009_neg',
+    'zfs_share_012_pos']
 tags = ['functional', 'cli_root', 'zfs_share']
 
 [tests/functional/cli_root/zfs_sysfs:Linux]

tests/zfs-tests/tests/functional/cli_root/zfs_share/Makefile.am

@@ -13,6 +13,7 @@ dist_pkgdata_SCRIPTS = \
 	zfs_share_009_neg.ksh \
 	zfs_share_010_neg.ksh \
 	zfs_share_011_pos.ksh \
+	zfs_share_012_pos.ksh \
 	zfs_share_concurrent_shares.ksh
 
 dist_pkgdata_DATA = \

tests/zfs-tests/tests/functional/cli_root/zfs_share/zfs_share_012_pos.ksh

@@ -0,0 +1,85 @@
+#!/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 2009 Sun Microsystems, Inc.  All rights reserved.
+# Use is subject to license terms.
+#
+
+#
+# Copyright (c) 2020 by Delphix. All rights reserved.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+#
+# DESCRIPTION: Unmounted canmount=noauto export is removed during zfs share -a
+#
+# STRATEGY:
+# 1. Share a dataset that also has canmount set to noauto
+# 2. Capture the zfs exports file when the dataset is mounted + shared
+# 3. Simulate a reboot by unmounting the dataset and restoring the exports file
+# 4. Verify that 'zfs share -a' removes the export since dataset is not mounted
+#
+
+verify_runnable "both"
+
+dataset="$TESTPOOL/$TESTFS"
+mountpt=$(get_prop mountpoint $dataset)
+
+function cleanup
+{
+	zfs set canmount=on $dataset
+	zfs set sharenfs=off $dataset
+	zfs mount -a
+
+	#
+	# unset __ZFS_POOL_EXCLUDE so that we include all file systems when
+	# rebuilding the exports file
+	#
+	unset __ZFS_POOL_EXCLUDE
+	rm /etc/exports.d/zfs.exports
+	zfs share -a
+}
+
+log_assert "Unmounted canmount=noauto export is removed during zfs share -a"
+log_onexit cleanup
+
+log_must zfs set canmount=noauto $dataset
+zfs mount $dataset > /dev/null 2>&1
+log_must mounted $dataset
+log_must zfs set sharenfs=on $dataset
+log_must is_exported $mountpt
+
+log_must cp /etc/exports.d/zfs.exports /etc/exports.d/zfs.exports.save
+log_must zfs umount $dataset
+log_must unmounted $dataset
+log_mustnot is_exported $mountpt
+
+# simulate a reboot condition
+log_must mv /etc/exports.d/zfs.exports.save /etc/exports.d/zfs.exports
+
+log_must is_exported $mountpt
+log_must zfs share -a
+log_mustnot is_exported $mountpt
+
+log_pass "Unmounted canmount=noauto export is removed during zfs share -a"