Add support to decode a resume token

Adding a new subcommand to zstream called token. This
now allows users to decode a resume token to retrieve the toname
field. This can be useful for tools that need this information.
The syntax works as follows zstream token <resume_token>.

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Zuchowski <pzuchowski@datto.com>
Signed-off-by: Tony Perkins <tperkins@datto.com>
Closes #10558

cmd/zstream/zstream_token.c

@@ -0,0 +1,78 @@
+/*
+ * 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 2010 Sun Microsystems, Inc.  All rights reserved.
+ * Use is subject to license terms.
+ *
+ * Portions Copyright 2012 Martin Matuska <martin@matuska.org>
+ */
+
+/*
+ * Copyright (c) 2020 by Datto Inc. All rights reserved.
+ */
+
+#include <ctype.h>
+#include <libnvpair.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <strings.h>
+#include <unistd.h>
+#include <stddef.h>
+
+#include <libzfs.h>
+#include <libzfs_core.h>
+
+#include <sys/dmu.h>
+#include <sys/zfs_ioctl.h>
+#include "zstream.h"
+
+int
+zstream_do_token(int argc, char *argv[])
+{
+	char *resume_token = NULL;
+
+	if (argc < 2) {
+		(void) fprintf(stderr, "Need to pass the resume token\n");
+		zstream_usage();
+	}
+
+	resume_token = argv[1];
+
+	libzfs_handle_t *hdl = libzfs_init();
+
+	nvlist_t *resume_nvl =
+	    zfs_send_resume_token_to_nvlist(hdl, resume_token);
+
+	if (resume_nvl == NULL) {
+		(void) fprintf(stderr,
+		    "Unable to parse resume token: %s\n",
+		    libzfs_error_description(hdl));
+		libzfs_fini(hdl);
+		return (1);
+	}
+
+	dump_nvlist(resume_nvl, 5);
+	nvlist_free(resume_nvl);
+
+	libzfs_fini(hdl);
+	return (0);
+}