Allow zfs to send replication streams with missing snapshots

A tentative implementation and discussion was done in #5285.
According to it a send --skip-missing|-s flag has been added.
In a replication stream, when there are snapshots missing in
the hierarchy, if -s is provided print a warning and ignore
dataset (and its children) instead of throwing an error

Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pablo Correa Gómez <ablocorrea@hotmail.com>
Closes #11710

tests/runfiles/common.run

@@ -258,7 +258,7 @@ tags = ['functional', 'cli_root', 'zfs_rollback']
 tests = ['zfs_send_001_pos', 'zfs_send_002_pos', 'zfs_send_003_pos',
     'zfs_send_004_neg', 'zfs_send_005_pos', 'zfs_send_006_pos',
     'zfs_send_007_pos', 'zfs_send_encrypted', 'zfs_send_raw',
-    'zfs_send_sparse', 'zfs_send-b']
+    'zfs_send_sparse', 'zfs_send-b', 'zfs_send_skip_missing']
 tags = ['functional', 'cli_root', 'zfs_send']
 
 [tests/functional/cli_root/zfs_set]

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

@@ -13,7 +13,8 @@ dist_pkgdata_SCRIPTS = \
 	zfs_send_encrypted_unloaded.ksh \
 	zfs_send_raw.ksh \
 	zfs_send_sparse.ksh \
-	zfs_send-b.ksh
+	zfs_send-b.ksh \
+	zfs_send_skip_missing.ksh
 
 dist_pkgdata_DATA = \
 	zfs_send.cfg

tests/zfs-tests/tests/functional/cli_root/zfs_send/zfs_send_skip_missing.ksh

@@ -0,0 +1,77 @@
+#!/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 (c) 2016, loli10K. All rights reserved.
+# Copyright (c) 2021, Pablo Correa Gómez. All rights reserved.
+#
+
+. $STF_SUITE/tests/functional/cli_root/cli_common.kshlib
+. $STF_SUITE/tests/functional/cli_root/zfs_send/zfs_send.cfg
+
+#
+# DESCRIPTION:
+#	Verify 'zfs send' will avoid sending replication send
+#	streams when we're missing snapshots in the dataset
+#	hierarchy, unless -s|--skip-missing provided
+#
+# STRATEGY:
+#	1. Create a parent and child fs and then only snapshot the parent
+#	2. Verify sending with replication will fail
+#	3. Verify sending with skip-missing will print a warning but succeed
+#
+
+verify_runnable "both"
+
+function cleanup
+{
+	snapexists $SNAP && log_must zfs destroy -f $SNAP
+
+	datasetexists $PARENT && log_must zfs destroy -rf $PARENT
+
+	[[ -e $WARNF ]] && log_must rm -f $WARNF
+	rm -f $TEST_BASE_DIR/devnull
+}
+
+log_assert "Verify 'zfs send -Rs' works as expected."
+log_onexit cleanup
+
+PARENT=$TESTPOOL/parent
+CHILD=$PARENT/child
+SNAP=$PARENT@snap
+WARNF=$TEST_BASE_DIR/warn.2
+
+log_note "Verify 'zfs send -R' fails to generate replication stream"\
+	 " for datasets created before"
+
+log_must zfs create $PARENT
+log_must zfs create $CHILD
+log_must zfs snapshot $SNAP
+log_mustnot eval "zfs send -R $SNAP >$TEST_BASE_DIR/devnull"
+
+log_note "Verify 'zfs send -Rs' warns about missing snapshots, "\
+	 "but still succeeds"
+
+log_must eval "zfs send -Rs $SNAP 2> $WARNF >$TEST_BASE_DIR/devnull"
+log_must eval "[[ -s $WARNF ]]"
+
+log_pass "Verify 'zfs send -Rs' works as expected."