Remove dependency on sharetab file and refactor sharing logic

== Motivation and Context

The current implementation of 'sharenfs' and 'sharesmb' relies on
the use of the sharetab file. The use of this file is os-specific
and not required by linux or freebsd. Currently the code must
maintain updates to this file which adds complexity and presents
a significant performance impact when sharing many datasets. In
addition, concurrently running 'zfs sharenfs' command results in
missing entries in the sharetab file leading to unexpected failures.

== Description

This change removes the sharetab logic from the linux and freebsd
implementation of 'sharenfs' and 'sharesmb'. It still preserves an
os-specific library which contains the logic required for sharing
NFS or SMB. The following entry points exist in the vastly simplified
libshare library:

- sa_enable_share -- shares a dataset but may not commit the change
- sa_disable_share -- unshares a dataset but may not commit the change
- sa_is_shared -- determine if a dataset is shared
- sa_commit_share -- notify NFS/SMB subsystem to commit the shares
- sa_validate_shareopts -- determine if sharing options are valid

The sa_commit_share entry point is provided as a performance enhancement
and is not required. The sa_enable_share/sa_disable_share may commit
the share as part of the implementation. Libshare provides a framework
for both NFS and SMB but some operating systems may not fully support
these protocols or all features of the protocol.

NFS Operation:
For linux, libshare updates /etc/exports.d/zfs.exports to add
and remove shares and then commits the changes by invoking
'exportfs -r'. This file, is automatically read by the kernel NFS
implementation which makes for better integration with the NFS systemd
service. For FreeBSD, libshare updates /etc/zfs/exports to add and
remove shares and then commits the changes by sending a SIGHUP to
mountd.

SMB Operation:
For linux, libshare adds and removes files in /var/lib/samba/usershares
by calling the 'net' command directly. There is no need to commit the
changes. FreeBSD does not support SMB.

== Performance Results

To test sharing performance we created a pool with an increasing number
of datasets and invoked various zfs actions that would enable and
disable sharing. The performance testing was limited to NFS sharing.
The following tests were performed on an 8 vCPU system with 128GB and
a pool comprised of 4 50GB SSDs:

Scale testing:
- Share all filesystems in parallel -- zfs sharenfs=on <dataset> &
- Unshare all filesystems in parallel -- zfs sharenfs=off <dataset> &

Functional testing:
- share each filesystem serially -- zfs share -a
- unshare each filesystem serially -- zfs unshare -a
- reset sharenfs property and unshare -- zfs inherit -r sharenfs <pool>

For 'zfs sharenfs=on' scale testing we saw an average reduction in time
of 89.43% and for 'zfs sharenfs=off' we saw an average reduction in time
of 83.36%.

Functional testing also shows a huge improvement:
- zfs share -- 97.97% reduction in time
- zfs unshare -- 96.47% reduction in time
- zfs inhert -r sharenfs -- 99.01% reduction in time

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Bryant G. Ly <bryangly@gmail.com>
Signed-off-by: George Wilson <gwilson@delphix.com>
External-Issue: DLPX-68690
Closes #1603
Closes #7692
Closes #7943
Closes #10300

tests/zfs-tests/include/libtest.shlib

@@ -21,7 +21,7 @@
 
 #
 # Copyright (c) 2009, Sun Microsystems Inc. All rights reserved.
-# Copyright (c) 2012, 2018, Delphix. All rights reserved.
+# Copyright (c) 2012, 2020, Delphix. All rights reserved.
 # Copyright (c) 2017, Tim Chase. All rights reserved.
 # Copyright (c) 2017, Nexenta Systems Inc. All rights reserved.
 # Copyright (c) 2017, Lawrence Livermore National Security LLC.
@@ -1373,6 +1373,80 @@ function is_shared
 	esac
 }
 
+function is_exported_illumos
+{
+	typeset fs=$1
+	typeset mtpt
+
+	for mtpt in `awk '{print $1}' /etc/dfs/sharetab` ; do
+		if [[ $mtpt == $fs ]] ; then
+			return 0
+		fi
+	done
+
+	return 1
+}
+
+function is_exported_freebsd
+{
+	typeset fs=$1
+	typeset mtpt
+
+	for mtpt in `awk '{print $1}' /etc/zfs/exports` ; do
+		if [[ $mtpt == $fs ]] ; then
+			return 0
+		fi
+	done
+
+	return 1
+}
+
+function is_exported_linux
+{
+	typeset fs=$1
+	typeset mtpt
+
+	for mtpt in `awk '{print $1}' /etc/exports.d/zfs.exports` ; do
+		if [[ $mtpt == $fs ]] ; then
+			return 0
+		fi
+	done
+
+	return 1
+}
+
+#
+# Given a mountpoint, or a dataset name, determine if it is exported via
+# the os-specific NFS exports file.
+#
+# Returns 0 if exported, 1 otherwise.
+#
+function is_exported
+{
+	typeset fs=$1
+	typeset mtpt
+
+	if [[ $fs != "/"* ]] ; then
+		if datasetnonexists "$fs" ; then
+			return 1
+		else
+			mtpt=$(get_prop mountpoint "$fs")
+			case $mtpt in
+				none|legacy|-) return 1
+					;;
+				*)	fs=$mtpt
+					;;
+			esac
+		fi
+	fi
+
+	case $(uname) in
+	FreeBSD)	is_exported_freebsd "$fs"	;;
+	Linux)		is_exported_linux "$fs"	;;
+	*)		is_exported_illumos "$fs"	;;
+	esac
+}
+
 #
 # Given a dataset name determine if it is shared via SMB.
 #
@@ -1397,7 +1471,7 @@ function is_shared_smb
 		done
 		return 1
 	else
-		log_unsupported "Currently unsupported by the test framework"
+		log_note "Currently unsupported by the test framework"
 		return 1
 	fi
 }
@@ -1445,7 +1519,7 @@ function unshare_fs #fs
 
 	is_shared $fs || is_shared_smb $fs
 	if (($? == 0)); then
-		log_must zfs unshare $fs
+		zfs unshare $fs || log_fail "zfs unshare $fs failed"
 	fi
 
 	return 0
@@ -1523,6 +1597,21 @@ function showshares_smb
 	return 0
 }
 
+function check_nfs
+{
+	if is_linux; then
+		share -s
+	elif is_freebsd; then
+		showmount -e
+	else
+		log_unsupported "Unknown platform"
+	fi
+
+	if [[ $? -ne 0 ]]; then
+		log_unsupported "The NFS utilities are not installed"
+	fi
+}
+
 #
 # Check NFS server status and trigger it online.
 #
@@ -1535,13 +1624,18 @@ function setup_nfs_server
 		return
 	fi
 
-	if is_linux || is_freebsd; then
+	if is_linux; then
 		#
 		# Re-synchronize /var/lib/nfs/etab with /etc/exports and
 		# /etc/exports.d./* to provide a clean test environment.
 		#
 		log_must share -r
 
+		log_note "NFS server must be started prior to running ZTS."
+		return
+	elif is_freebsd; then
+		kill -s HUP $(cat /var/run/mountd.pid)
+
 		log_note "NFS server must be started prior to running ZTS."
 		return
 	fi
@@ -4078,3 +4172,23 @@ function get_arcstat # stat
 		;;
 	esac
 }
+
+#
+# Given an array of pids, wait until all processes
+# have completed and check their return status.
+#
+function wait_for_children #children
+{
+	rv=0
+	children=("$@")
+	for child in "${children[@]}"
+	do
+		child_exit=0
+		wait ${child} || child_exit=$?
+		if [ $child_exit -ne 0 ]; then
+			echo "child ${child} failed with ${child_exit}"
+			rv=1
+		fi
+	done
+	return $rv
+}