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

[oshogbo]

Summary

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

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

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

If we think that this is a good idea I can submit pull request to
the OpenZFS as well.

[oshogbo]

Update description

[bcr]

Minor correction to the man page and (this is new to me) the code.

cddl/contrib/opensolaris/cmd/zfs/zfs.8

3005

s/receiving snapshot/receiving a snapshot/

cddl/contrib/opensolaris/cmd/zfs/zfs_main.c

4165

This is debug output, right? ;)

cddl/contrib/opensolaris/lib/libzfs/common/libzfs_sendrecv.c

3831

Another debug output here and below?

[oshogbo]

Thx bcr@ !

[pjd]

cddl/contrib/opensolaris/lib/libzfs/common/libzfs_sendrecv.c

2897

flags->forceunmount?

3846

I think mflags is never used.

[oshogbo]

Thank you @pjd.

[allanjude]

cddl/contrib/opensolaris/lib/libzfs/common/libzfs_sendrecv.c

2897

is this meant to be flags->force or flags->forceunmount ?

[oshogbo]

cddl/contrib/opensolaris/lib/libzfs/common/libzfs_sendrecv.c

2897

flags->forceunmount. Fixed.

[oshogbo]

Ping.

[oshogbo]

Like Matt suggested I checked the behavior on Linux as well.

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 #26-Ubuntu SMP Mon Jun 24 09:32:08 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
oshogbo@u-wing:/test$

After closing file descriptor in test/2:

oshogbo@u-wing:/test$ sudo sudo zfs send test/1@b | sudo zfs recv -F test/2
oshogbo@u-wing:/test$

[allanjude]

Reviewed by: allanjude