zfs create -V should wait for udev

[behlendorf]

System information

Type	Version/Name
Distribution Name	any
Distribution Version	any
Linux Kernel	any
Architecture	any
ZFS Version	master
SPL Version	master

Describe the problem you're observing

Creation of the /dev/ device file for a volume is handled asynchronously on Linux by udev. This means that it's possible for the zfs create -V to return before udev has populated /dev/ with the required entry. This behavior complicates scripting since the caller is responsible for waiting on udev.

The proposed solution is to update zfs create -V to use libudev to block until udev has completed and created the entry. This should be the default behavior, and we should provide a way to request the asynchronous behavior for callers which can take advantage of it.

Describe how to reproduce the problem

zfs create -V 10G tank/volume

See issue #7863

[bprotopopov]

There are a few cases in addition to zfs create that could result in interaction with udev and creation of device nodes, e.g. zfs recv and zpool import. It would also be nice to address similar issues with destruction of device nodes that occurs as a result of zfs destroy and zpool export.

[jgallag88]

It looks like only root can listen for udev events. Does ZoL support, or plan on someday supporting, delegating the ability to create volumes? If non-root users can create volumes, that would make using libudev trickier.

[loli10K]

@jgallag88 #7381 "[WIP] Permit ZVOL creation by unprivileged users"

[behlendorf]

That's definitely a possibility as @loli10K mentioned.

It looks like only root can listen for udev events.

@jgallag88 I'm pretty sure that isn't the case, at least on CentOS 7. You can see this for yourself by running udevadm monitor in a second terminal when creating a new volume.

$ sudo zfs create -V 10G tank/vol

$ udevadm monitor
monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
KERNEL - the kernel uevent
...
KERNEL[126.605833] add      /devices/virtual/bdi/230:0 (bdi)
UDEV  [126.608094] add      /devices/virtual/bdi/230:0 (bdi)
KERNEL[126.626682] add      /devices/virtual/block/zd0 (block)
UDEV  [126.646514] add      /devices/virtual/block/zd0 (block)

[stale]

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