Something

#!/usr/sbin/dtrace -s

#pragma D option flowindent

syscall::write:entry
/execname == "scp"/
{
        self->spec = speculation();
}

fbt:::
/self->spec/
{
        speculate(self->spec);
}

syscall::write:return
/self->spec && (int)arg0 < 0/
{
        commit(self->spec);
        self->spec = 0;
        self->doexit = 1;
}

syscall::write:return
/self->doexit/
{
        exit(0);
}

syscall::write:return
/self->spec && (int)arg0 >= 0/
{
        discard(self->spec);
        self->spec = 0;
}