zfs: Add option for forcible unmounting dataset while receiving snapshot.

Currently when the dataset is in use we can't receive snapshots.

zfs send test/1@asd | zfs recv -FM test/2
cannot unmount '/test/2': Device busy

This commits add option 'M' which attempts to forcibly unmount the
dataset.  Thanks to this we can enforce receiving snapshots in a
single step.

Note that this functionality is not supported on Linux because the
VFS will prevent active mounted filesystems from being unmounted,
even with the force option.  This is the intended VFS behavior.

Discussed-with: Pawel Jakub Dawidek <pjd@FreeBSD.org>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allanjude@freebsd.org>
Differential Revision:	https://reviews.freebsd.org/D22306

openzfs/zfs@a57d3d45d6efdff935421e2ef3f97e3dc089d93d

2 files changed, 14 insertions, 8 deletions

diff --git a/cddl/contrib/opensolaris/cmd/zfs/zfs.8 b/cddl/contrib/opensolaris/cmd/zfs/zfs.8
index e060db67867c..74b94881a64a 100644
--- a/cddl/contrib/opensolaris/cmd/zfs/zfs.8
+++ b/cddl/contrib/opensolaris/cmd/zfs/zfs.8
@@ -32,7 +32,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 16, 2019
+.Dd February 16, 2020
 .Dt ZFS 8
 .Os
 .Sh NAME
@@ -201,12 +201,12 @@
 .Fl t Ar receive_resume_token
 .Nm
 .Cm receive Ns | Ns Cm recv
-.Op Fl vnsFu
+.Op Fl vnsFMu
 .Op Fl o Sy origin Ns = Ns Ar snapshot
 .Ar filesystem Ns | Ns Ar volume Ns | Ns Ar snapshot
 .Nm
 .Cm receive Ns | Ns Cm recv
-.Op Fl vnsFu
+.Op Fl vnsFMu
 .Op Fl d | e
 .Op Fl o Sy origin Ns = Ns Ar snapshot
 .Ar filesystem
@@ -2909,14 +2909,14 @@ for more details.
 .It Xo
 .Nm
 .Cm receive Ns | Ns Cm recv
-.Op Fl vnsFu
+.Op Fl vnsFMu
 .Op Fl o Sy origin Ns = Ns Ar snapshot
 .Ar filesystem Ns | Ns Ar volume Ns | Ns Ar snapshot
 .Xc
 .It Xo
 .Nm
 .Cm receive Ns | Ns Cm recv
-.Op Fl vnsFu
+.Op Fl vnsFMu
 .Op Fl d | e
 .Op Fl o Sy origin Ns = Ns Ar snapshot
 .Ar filesystem
@@ -3016,6 +3016,9 @@ performing the receive operation. If receiving an incremental replication
 stream (for example, one generated by
 .Qq Nm Cm send Fl R Bro Fl i | Fl I Brc ) ,
 destroy snapshots and file systems that do not exist on the sending side.
+.It Fl M
+Force an unmount of the file system while receiving a snapshot.
+This option is not supported on Linux.
 .It Fl s
 If the receive is interrupted, save the partially received state, rather
 than deleting it.  Interruption may be due to premature termination of
diff --git a/cddl/contrib/opensolaris/cmd/zfs/zfs_main.c b/cddl/contrib/opensolaris/cmd/zfs/zfs_main.c
index 3f389d1a51ff..851ba3f9523f 100644
--- a/cddl/contrib/opensolaris/cmd/zfs/zfs_main.c
+++ b/cddl/contrib/opensolaris/cmd/zfs/zfs_main.c
@@ -274,9 +274,9 @@ get_usage(zfs_help_t idx)
 	case HELP_PROMOTE:
 		return (gettext("\tpromote <clone-filesystem>\n"));
 	case HELP_RECEIVE:
-		return (gettext("\treceive|recv [-vnsFu] <filesystem|volume|"
+		return (gettext("\treceive|recv [-vnsFMu] <filesystem|volume|"
 		    "snapshot>\n"
-		    "\treceive|recv [-vnsFu] [-o origin=<snapshot>] [-d | -e] "
+		    "\treceive|recv [-vnsFMu] [-o origin=<snapshot>] [-d | -e] "
 		    "<filesystem>\n"
 		    "\treceive|recv -A <filesystem|volume>\n"));
 	case HELP_RENAME:
@@ -4078,7 +4078,7 @@ zfs_do_receive(int argc, char **argv)
 		nomem();
 
 	/* check options */
-	while ((c = getopt(argc, argv, ":o:denuvFsA")) != -1) {
+	while ((c = getopt(argc, argv, ":o:denuvMFsA")) != -1) {
 		switch (c) {
 		case 'o':
 			if (parseprop(props, optarg) != 0)
@@ -4106,6 +4106,9 @@ zfs_do_receive(int argc, char **argv)
 		case 'F':
 			flags.force = B_TRUE;
 			break;
+		case 'M':
+			flags.forceunmount = B_TRUE;
+			break;
 		case 'A':
 			abort_resumable = B_TRUE;
 			break;