Sometimes nmount(2) will fail without setting errmsg. The previous (ignored)
error would then be shown as the reason for the failed call if the next
nmount(2) also fails without [ENOENT,ENOTSUP].
An example is when there is a tmpfs mounted with -o size. vfs_filteropt() adds
'size' as an error in errmsg due to 'size' not being in tmpfs_updateopts. Then
tmpfs_mount returns [ENOTSUP] from nmount(2), which is then ignored. The next
call may race with an unmount causing an invalid [EINVAL] that then does log an
error, with the tmpfs errmsg.
The race itself is a separate issue to fix as it is expected to have an
[ENOENT] returned instead.
In this example the mount being shown is actually nullfs, not tmpfs that the
error is from.
mountd[740]: can't delete exports for /poudriere/data/.m/exp-head-commit-test-devel/04/.npkg: Invalid argument mount option <size> is unknown
It should only show:
mountd[740]: can't delete exports for /poudriere/data/.m/exp-head-commit-test-devel/04/.npkg: Invalid argument
MFC after: 2 weeks
805bdee05f