Make zfs-share service resilient to stale exports

The are a few cases where stale entries in /etc/exports.d/zfs.exports 
will cause the nfs-server service to fail when starting up.

Since the nfs-server startup consumes /etc/exports.d/zfs.exports, the 
zfs-share service (which rebuilds the list of zfs exports) should run 
before the nfs-server service.

To make the zfs-share service resilient to stale exports, this change 
truncates the zfs config file as part of the zfs share -a operation.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #13775

lib/libshare/libshare.c

@@ -22,7 +22,7 @@
 /*
  * Copyright (c) 2002, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2011 Gunnar Beutner
- * Copyright (c) 2018, 2020 by Delphix. All rights reserved.
+ * Copyright (c) 2018, 2022 by Delphix. All rights reserved.
  */
 
 #include <stdio.h>
@@ -96,6 +96,16 @@ sa_commit_shares(enum sa_protocol protocol)
 	fstypes[protocol]->commit_shares();
 }
 
+void
+sa_truncate_shares(enum sa_protocol protocol)
+{
+	/* CSTYLED */
+	VALIDATE_PROTOCOL(protocol, );
+
+	if (fstypes[protocol]->truncate_shares != NULL)
+		fstypes[protocol]->truncate_shares();
+}
+
 int
 sa_validate_shareopts(const char *options, enum sa_protocol protocol)
 {

lib/libshare/libshare_impl.h

@@ -22,7 +22,7 @@
 /*
  * Copyright (c) 2002, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2011 Gunnar Beutner
- * Copyright (c) 2019, 2020 by Delphix. All rights reserved.
+ * Copyright (c) 2019, 2022 by Delphix. All rights reserved.
  */
 #ifndef _LIBSPL_LIBSHARE_IMPL_H
 #define	_LIBSPL_LIBSHARE_IMPL_H
@@ -39,6 +39,7 @@ typedef struct {
 	boolean_t (*const is_shared)(sa_share_impl_t share);
 	int (*const validate_shareopts)(const char *shareopts);
 	int (*const commit_shares)(void);
+	void (*const truncate_shares)(void);
 } sa_fstype_t;
 
 extern const sa_fstype_t libshare_nfs_type, libshare_smb_type;

lib/libshare/nfs.c

@@ -28,6 +28,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include <libshare.h>
+#include <unistd.h>
 #include "nfs.h"
 
 
@@ -281,6 +282,17 @@ fullerr:
 	return (error);
 }
 
+void
+nfs_reset_shares(const char *lockfile, const char *exports)
+{
+	int nfs_lock_fd = -1;
+
+	if (nfs_exports_lock(lockfile, &nfs_lock_fd) == 0) {
+		(void) ! truncate(exports, 0);
+		nfs_exports_unlock(lockfile, &nfs_lock_fd);
+	}
+}
+
 static boolean_t
 nfs_is_shared_cb(void *userdata, char *line, boolean_t found_mountpoint)
 {

lib/libshare/nfs.h

@@ -22,6 +22,7 @@
 /*
  * Copyright (c) 2002, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2011 Gunnar Beutner
+ * Copyright (c) 2022 by Delphix. All rights reserved.
  */
 
 #include "libshare_impl.h"
@@ -33,3 +34,4 @@ boolean_t nfs_is_shared_impl(const char *exports, sa_share_impl_t impl_share);
 int nfs_toggle_share(const char *lockfile, const char *exports,
     const char *expdir, sa_share_impl_t impl_share,
     int(*cbk)(sa_share_impl_t impl_share, FILE *tmpfile));
+void nfs_reset_shares(const char *lockfile, const char *exports);

lib/libshare/os/freebsd/nfs.c

@@ -23,7 +23,7 @@
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
- * Copyright (c) 2020 by Delphix. All rights reserved.
+ * Copyright (c) 2020, 2022 by Delphix. All rights reserved.
  */
 
 #include <sys/cdefs.h>
@@ -195,6 +195,12 @@ start:
 	return (SA_OK);
 }
 
+static void
+nfs_truncate_shares(void)
+{
+	nfs_reset_shares(ZFS_EXPORTS_LOCK, ZFS_EXPORTS_FILE);
+}
+
 const sa_fstype_t libshare_nfs_type = {
 	.enable_share = nfs_enable_share,
 	.disable_share = nfs_disable_share,
@@ -202,4 +208,5 @@ const sa_fstype_t libshare_nfs_type = {
 
 	.validate_shareopts = nfs_validate_shareopts,
 	.commit_shares = nfs_commit_shares,
+	.truncate_shares = nfs_truncate_shares,
 };

lib/libshare/os/linux/nfs.c

@@ -23,7 +23,7 @@
  * Copyright (c) 2002, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2011 Gunnar Beutner
  * Copyright (c) 2012 Cyril Plisko. All rights reserved.
- * Copyright (c) 2019, 2020 by Delphix. All rights reserved.
+ * Copyright (c) 2019, 2022 by Delphix. All rights reserved.
  */
 
 #include <dirent.h>
@@ -495,6 +495,12 @@ nfs_commit_shares(void)
 	return (libzfs_run_process(argv[0], argv, 0));
 }
 
+static void
+nfs_truncate_shares(void)
+{
+	nfs_reset_shares(ZFS_EXPORTS_LOCK, ZFS_EXPORTS_FILE);
+}
+
 const sa_fstype_t libshare_nfs_type = {
 	.enable_share = nfs_enable_share,
 	.disable_share = nfs_disable_share,
@@ -502,6 +508,7 @@ const sa_fstype_t libshare_nfs_type = {
 
 	.validate_shareopts = nfs_validate_shareopts,
 	.commit_shares = nfs_commit_shares,
+	.truncate_shares = nfs_truncate_shares,
 };
 
 static boolean_t