[Bug 204628] ZFS dies when another dataset appears with same mountpoint as existing

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=204628

            Bug ID: 204628
           Summary: ZFS dies when another dataset appears with same
                    mountpoint as existing
           Product: Base System
           Version: 10.2-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: vlad-fbsd at acheronmedia.com

Importing a pool with mountpoints already existing for another, mounted
dataset, or doing send | receive of existing mounted datasets to, say, external
media, will cause ZFS to break instead of refusing to mount to existing
mountpoints.

What happens is that running any ZFS command fails with "internal error: failed
to initialize ZFS library".

In case of sending a backup of zroot (root dataset) snapshot to external media,
if the external media is then unplugged, the system freezes.

I have used -u on zfs receive but I apparently misunderstood the flag and will
file another PR about the manpage flag description ambiguity. Is it "is not
mounted" or "will not be mounted" (as I was lead to believe following an
article on zfs send,recv backups)?

I suspect this is also a gaping security hole if you import unknown pools and
forget to -N.

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