Do not allow the SNPSTTY ioctl for the snoop device that has a tty
attached. Otherwise, the snp->snp_tty would be overwritten, while the
tty line discipline still set to the snpdisc. Then snplwrite() causes
panic because ttytosnp() cannot find the snp. (And do it at the proper
place, from rev. 1.107).
MFC rev. 1.107:
The snp_target == NULL implies the snp_tty == NULL. Remove the code
that is put under snp_target == NULL and snp_tty != NULL clause.
In snpclose(), do the snp_detach() before scheduling the snp device
destruction. Otherwise, after the return from snpclose(), the snp
device is already removed from the snp_list, but tty is still in
snooped state. Any attempt to do i/o on such tty cause panic because
ttytosnp() returns NULL.
Approved by: re (kensmith)