Double error message for incompatible filesystems

[ilovezfs]

filesystem 'foo' (v6) is not supported by this implementation of ZFS (max v5).
cannot mount 'foo': Resource temporarily unavailable

This happens because ENOTSUP is converted to MOUNT_SYSERR in mount_zfs.c, and then MOUNT_SYSERR is converted to EAGAIN in libzfs_mount.c's do_mount, which then gets printed for the user as "Resource temporarily unavailable" by libzfs_mount.c's zfs_mount.

It is worth noting that do_mount never returns ENOTSUP and yet zfs_mount has handling for ENOTSUP anyway, though it can never be reached:

https://github.com/zfsonlinux/zfs/blob/master/lib/libzfs/libzfs_mount.c#L471-L482

If we do return ENOTSUP from do_mount, then we still get a double error message:

filesystem 'foo' (v6) is not supported by this implementation of ZFS (max v5).
cannot mount 'foo': Can't mount a version 6 file system on a version 5000 pool. Pool must be upgraded to mount this file system.

Any thoughts on how to avoid the double error message? Perhaps the entirety of https://github.com/zfsonlinux/zfs/blob/master/lib/libzfs/libzfs_mount.c#L459-L489 should be moved to mount_zfs.c

On a related note, it strikes me that in light of 287be44 the error message in libzfs_mount.c's zfs_mount() for ENOTSUP should probably not say "Pool must be upgraded to mount this file system" since that is not in fact a solution when the file system is version 6.

[behlendorf]

Closing, we can live with a double warning in this case.