zfs: add option for forcible unmounting dataset while receiving snaps…

…hot.

Currently when the dataset is in use we can't receive snapshot.
zfs send test/1@asd | zfs recv -FM test/2
cannot unmount '/test/2': Device busy

The same goes for the Linux version:
oshogbo@u-wing:/test$ sudo sudo zfs send test/1@b | sudo zfs recv -F test/2
umount: /test/2: target is busy.
cannot unmount '/test/2': umount failed
oshogbo@u-wing:/test$ uname -a
Linux u-wing 4.18.0-25-generic openzfs#26-Ubuntu SMP Mon Jun 24 09:32:08 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

This commits add option 'M' which forcible unmounting the dataset.
Thanks to to that we can enforce receiving snapshot in single step.

Discussed with: pjd
Reviewed by:	AllanJude (FreeBSD version)
FreeBSD review:	https://reviews.freebsd.org/D22306

Signed-off-by: Mariusz Zaborski <oshogbo@vexillium.org>

cmd/zfs/zfs_main.c

@@ -297,10 +297,10 @@ get_usage(zfs_help_t idx)
 	case HELP_PROMOTE:
 		return (gettext("\tpromote <clone-filesystem>\n"));
 	case HELP_RECEIVE:
-		return (gettext("\treceive [-vnsFhu] "
+		return (gettext("\treceive [-vMnsFhu] "
 		    "[-o <property>=<value>] ... [-x <property>] ...\n"
 		    "\t    <filesystem|volume|snapshot>\n"
-		    "\treceive [-vnsFhu] [-o <property>=<value>] ... "
+		    "\treceive [-vMnsFhu] [-o <property>=<value>] ... "
 		    "[-x <property>] ... \n"
 		    "\t    [-d | -e] <filesystem>\n"
 		    "\treceive -A <filesystem|volume>\n"));
@@ -4550,7 +4550,7 @@ zfs_do_receive(int argc, char **argv)
 		nomem();
 
 	/* check options */
-	while ((c = getopt(argc, argv, ":o:x:dehnuvFsA")) != -1) {
+	while ((c = getopt(argc, argv, ":o:x:dehMnuvFsA")) != -1) {
 		switch (c) {
 		case 'o':
 			if (!parseprop(props, optarg)) {
@@ -4585,6 +4585,9 @@ zfs_do_receive(int argc, char **argv)
 		case 'h':
 			flags.skipholds = B_TRUE;
 			break;
+		case 'M':
+			flags.forceunmount = B_TRUE;
+			break;
 		case 'n':
 			flags.dryrun = B_TRUE;
 			break;

include/libzfs.h

@@ -755,6 +755,9 @@ typedef struct recvflags {
 
 	/* mount the filesystem unless nomount is specified */
 	boolean_t domount;
+
+	/* force unmount while recv snapshot (private) */
+	boolean_t forceunmount;
 } recvflags_t;
 
 extern int zfs_receive(libzfs_handle_t *, const char *, nvlist_t *,

lib/libzfs/libzfs_sendrecv.c

@@ -4059,7 +4059,8 @@ zfs_receive_package(libzfs_handle_t *hdl, int fd, const char *destname,
 				    ZFS_TYPE_FILESYSTEM);
 				if (zhp != NULL) {
 					clp = changelist_gather(zhp,
-					    ZFS_PROP_MOUNTPOINT, 0, 0);
+					    ZFS_PROP_MOUNTPOINT, 0,
+					    flags->forceunmount ? MS_FORCE : 0);
 					zfs_close(zhp);
 					if (clp != NULL) {
 						softerr |=
@@ -4876,7 +4877,8 @@ zfs_receive_one(libzfs_handle_t *hdl, int infd, const char *tosnap,
 		if (!flags->dryrun && zhp->zfs_type == ZFS_TYPE_FILESYSTEM &&
 		    stream_wantsnewfs) {
 			/* We can't do online recv in this case */
-			clp = changelist_gather(zhp, ZFS_PROP_NAME, 0, 0);
+			clp = changelist_gather(zhp, ZFS_PROP_NAME, 0,
+			    flags->forceunmount ? MS_FORCE : 0);
 			if (clp == NULL) {
 				zfs_close(zhp);
 				err = -1;
@@ -5556,7 +5558,8 @@ zfs_receive(libzfs_handle_t *hdl, const char *tosnap, nvlist_t *props,
 			}
 
 			clp = changelist_gather(zhp, ZFS_PROP_MOUNTPOINT,
-			    CL_GATHER_MOUNT_ALWAYS, 0);
+			    CL_GATHER_MOUNT_ALWAYS,
+			    flags->forceunmount ? MS_FORCE : 0);
 			zfs_close(zhp);
 			if (clp == NULL) {
 				err = -1;

man/man8/zfs-receive.8

@@ -30,7 +30,7 @@
 .\" Copyright 2018 Nexenta Systems, Inc.
 .\" Copyright 2019 Joyent, Inc.
 .\"
-.Dd June 30, 2019
+.Dd January 27, 2020
 .Dt ZFS-RECEIVE 8
 .Os Linux
 .Sh NAME
@@ -39,14 +39,14 @@
 .Sh SYNOPSIS
 .Nm
 .Cm receive
-.Op Fl Fhnsuv
+.Op Fl FhMnsuv
 .Op Fl o Sy origin Ns = Ns Ar snapshot
 .Op Fl o Ar property Ns = Ns Ar value
 .Op Fl x Ar property
 .Ar filesystem Ns | Ns Ar volume Ns | Ns Ar snapshot
 .Nm
 .Cm receive
-.Op Fl Fhnsuv
+.Op Fl FhMnsuv
 .Op Fl d Ns | Ns Fl e
 .Op Fl o Sy origin Ns = Ns Ar snapshot
 .Op Fl o Ar property Ns = Ns Ar value
@@ -61,7 +61,7 @@
 .It Xo
 .Nm
 .Cm receive
-.Op Fl Fhnsuv
+.Op Fl FhMnsuv
 .Op Fl o Sy origin Ns = Ns Ar snapshot
 .Op Fl o Ar property Ns = Ns Ar value
 .Op Fl x Ar property
@@ -70,7 +70,7 @@
 .It Xo
 .Nm
 .Cm receive
-.Op Fl Fhnsuv
+.Op Fl FhMnsuv
 .Op Fl d Ns | Ns Fl e
 .Op Fl o Sy origin Ns = Ns Ar snapshot
 .Op Fl o Ar property Ns = Ns Ar value
@@ -229,6 +229,8 @@ that element to determine the name of the target file system for the new
 snapshot as described in the paragraph above.
 .It Fl h
 Skip the receive of holds.  There is no effect if holds are not sent.
+.It Fl M
+Force an unmount of the file system while receiving a snapshot.
 .It Fl n
 Do not actually receive the stream.
 This can be useful in conjunction with the