libzfs: add zfs_mount_at

This will be used in libbe in place of the internal zmount(); libbe only
wants to be able to mount a dataset at an arbitrary mountpoint without
altering dataset/pool properties. The natural way to do this in a portable
way is by creating a zfs_mount_at() interface that's effectively zfs_mount()
+ a mountpoint parameter. zfs_mount() is now a light wrapper around the new
method.

The interface and implementation have already been accepted into ZFS On
Linux, and the next commit to switch libbe() over to this new interface will
solve the last compatibility issue with ZoL.  The next sysutils/openzfs
rebase against ZoL should be able to build libbe/bectl with only minor
adjustments to build glue.

Reviewed by:	Ryan Moeller <ryan freqlabs com>
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D23132

2 files changed, 27 insertions, 5 deletions

diff --git a/cddl/contrib/opensolaris/lib/libzfs/common/libzfs.h b/cddl/contrib/opensolaris/lib/libzfs/common/libzfs.h
index a1344f324729..0e53743b1a00 100644
--- a/cddl/contrib/opensolaris/lib/libzfs/common/libzfs.h
+++ b/cddl/contrib/opensolaris/lib/libzfs/common/libzfs.h
@@ -769,6 +769,7 @@ extern ulong_t get_system_hostid(void);
 extern boolean_t is_mounted(libzfs_handle_t *, const char *special, char **);
 extern boolean_t zfs_is_mounted(zfs_handle_t *, char **);
 extern int zfs_mount(zfs_handle_t *, const char *, int);
+extern int zfs_mount_at(zfs_handle_t *, const char *, int, const char *);
 extern int zfs_unmount(zfs_handle_t *, const char *, int);
 extern int zfs_unmountall(zfs_handle_t *, int);
 
diff --git a/cddl/contrib/opensolaris/lib/libzfs/common/libzfs_mount.c b/cddl/contrib/opensolaris/lib/libzfs/common/libzfs_mount.c
index 0efeba631aff..9d4948cc7173 100644
--- a/cddl/contrib/opensolaris/lib/libzfs/common/libzfs_mount.c
+++ b/cddl/contrib/opensolaris/lib/libzfs/common/libzfs_mount.c
@@ -301,6 +301,17 @@ zfs_is_mounted(zfs_handle_t *zhp, char **where)
 	return (is_mounted(zhp->zfs_hdl, zfs_get_name(zhp), where));
 }
 
+static boolean_t
+zfs_is_mountable_internal(zfs_handle_t *zhp, const char *mountpoint)
+{
+
+	if (zfs_prop_get_int(zhp, ZFS_PROP_ZONED) &&
+	    getzoneid() == GLOBAL_ZONEID)
+		return (B_FALSE);
+
+	return (B_TRUE);
+}
+
 /*
  * Returns true if the given dataset is mountable, false otherwise.  Returns the
  * mountpoint in 'buf'.
@@ -325,8 +336,7 @@ zfs_is_mountable(zfs_handle_t *zhp, char *buf, size_t buflen,
 	if (zfs_prop_get_int(zhp, ZFS_PROP_CANMOUNT) == ZFS_CANMOUNT_OFF)
 		return (B_FALSE);
 
-	if (zfs_prop_get_int(zhp, ZFS_PROP_ZONED) &&
-	    getzoneid() == GLOBAL_ZONEID)
+	if (!zfs_is_mountable_internal(zhp, buf))
 		return (B_FALSE);
 
 	if (source)
@@ -341,8 +351,19 @@ zfs_is_mountable(zfs_handle_t *zhp, char *buf, size_t buflen,
 int
 zfs_mount(zfs_handle_t *zhp, const char *options, int flags)
 {
-	struct stat buf;
 	char mountpoint[ZFS_MAXPROPLEN];
+
+	if (!zfs_is_mountable(zhp, mountpoint, sizeof (mountpoint), NULL))
+		return (0);
+
+	return (zfs_mount_at(zhp, options, flags, mountpoint));
+}
+
+int
+zfs_mount_at(zfs_handle_t *zhp, const char *options, int flags,
+    const char *mountpoint)
+{
+	struct stat buf;
 	char mntopts[MNT_LINE_MAX];
 	libzfs_handle_t *hdl = zhp->zfs_hdl;
 
@@ -357,8 +378,8 @@ zfs_mount(zfs_handle_t *zhp, const char *options, int flags)
 	if (zpool_get_prop_int(zhp->zpool_hdl, ZPOOL_PROP_READONLY, NULL))
 		flags |= MS_RDONLY;
 
-	if (!zfs_is_mountable(zhp, mountpoint, sizeof (mountpoint), NULL))
-		return (0);
+	if (!zfs_is_mountable_internal(zhp, mountpoint))
+		return (B_FALSE);
 
 	/* Create the directory if it doesn't already exist */
 	if (lstat(mountpoint, &buf) != 0) {