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 <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Reviewed-by: Bryant G. Ly <[email protected]>
Signed-off-by: George Wilson <[email protected]>
External-Issue: DLPX-68690
Closes #1603
Closes #7692
Closes #7943
Closes #10300 

3 files changed, 7 insertions, 14 deletions

diff --git a/include/libzfs.h b/include/libzfs.h
index a93f694a1..6f797c0a3 100644
--- a/include/libzfs.h
+++ b/include/libzfs.h
@@ -837,6 +837,10 @@ extern int zfs_unshareall_bytype(zfs_handle_t *, const char *, const char *);
 extern int zfs_unshareall(zfs_handle_t *);
 extern int zfs_deleg_share_nfs(libzfs_handle_t *, char *, char *, char *,
     void *, void *, int, zfs_share_op_t);
+extern void zfs_commit_nfs_shares(void);
+extern void zfs_commit_smb_shares(void);
+extern void zfs_commit_all_shares(void);
+extern void zfs_commit_shares(const char *);
 
 extern int zfs_nicestrtonum(libzfs_handle_t *, const char *, uint64_t *);
 
diff --git a/include/libzfs_impl.h b/include/libzfs_impl.h
index 35b92f6d9..0f1116797 100644
--- a/include/libzfs_impl.h
+++ b/include/libzfs_impl.h
@@ -49,7 +49,6 @@ struct libzfs_handle {
 	int libzfs_error;
 	int libzfs_fd;
 	FILE *libzfs_mnttab;
-	FILE *libzfs_sharetab;
 	zpool_handle_t *libzfs_pool_handles;
 	uu_avl_pool_t *libzfs_ns_avlpool;
 	uu_avl_t *libzfs_ns_avl;
@@ -59,8 +58,6 @@ struct libzfs_handle {
 	char libzfs_desc[1024];
 	int libzfs_printerr;
 	int libzfs_storeerr; /* stuff error messages into buffer */
-	void *libzfs_sharehdl; /* libshare handle */
-	uint_t libzfs_shareflags;
 	boolean_t libzfs_mnttab_enable;
 	/*
 	 * We need a lock to handle the case where parallel mount
@@ -76,8 +73,6 @@ struct libzfs_handle {
 	regex_t libzfs_urire;
 };
 
-#define	ZFSSHARE_MISS	0x01	/* Didn't find entry in cache */
-
 struct zfs_handle {
 	libzfs_handle_t *zfs_hdl;
 	zpool_handle_t *zpool_hdl;
@@ -206,12 +201,6 @@ int zfs_validate_name(libzfs_handle_t *hdl, const char *path, int type,
 
 void namespace_clear(libzfs_handle_t *);
 
-/*
- * libshare (sharemgr) interfaces used internally.
- */
-
-extern int zfs_init_libshare(libzfs_handle_t *, int);
-extern void zfs_uninit_libshare(libzfs_handle_t *);
 extern int zfs_parse_options(char *, zfs_share_proto_t);
 
 extern int zfs_unshare_proto(zfs_handle_t *,
@@ -256,13 +245,14 @@ extern int unshare_one(libzfs_handle_t *hdl, const char *name,
     const char *mountpoint, zfs_share_proto_t proto);
 extern boolean_t zfs_is_mountable(zfs_handle_t *zhp, char *buf, size_t buflen,
     zprop_source_t *source, int flags);
-extern zfs_share_type_t is_shared_impl(libzfs_handle_t *hdl,
-    const char *mountpoint, zfs_share_proto_t proto);
+extern zfs_share_type_t is_shared(const char *mountpoint,
+    zfs_share_proto_t proto);
 extern int libzfs_load_module(void);
 extern int zpool_relabel_disk(libzfs_handle_t *hdl, const char *path,
     const char *msg);
 extern int find_shares_object(differ_info_t *di);
 extern void libzfs_set_pipe_max(int infd);
+extern void zfs_commit_proto(zfs_share_proto_t *);
 
 #ifdef	__cplusplus
 }
diff --git a/include/sys/fs/zfs.h b/include/sys/fs/zfs.h
index 1bfd7a485..ef87c24a4 100644
--- a/include/sys/fs/zfs.h
+++ b/include/sys/fs/zfs.h
@@ -1170,7 +1170,6 @@ typedef struct ddt_histogram {
 #define	ZVOL_DRIVER	"zvol"
 #define	ZFS_DRIVER	"zfs"
 #define	ZFS_DEV		"/dev/zfs"
-#define	ZFS_SHARETAB	"/etc/dfs/sharetab"
 
 #define	ZFS_SUPER_MAGIC	0x2fc12fc1