DTrace EAGAIN returns from read(2)

read-eagain.d

#!/usr/sbin/dtrace -s

syscall::open:entry
/arg0 != NULL/
{
    self->pathptr = arg0;
    self->oflag = arg1;
}

syscall::open:return
/self->pathptr != NULL/
{
    self->path = copyinstr(self->pathptr);
    self->pathptr = 0;
    printf("open(%s, %d) = %d (%d)\n",
           self->path, self->oflag, arg1, errno);
    self->path = 0;
    self->oflag = 0;
}

syscall::read:entry
{
    self->filedes = arg1;
}

syscall::read:return
/self->filedes != NULL && errno == 11/
{
    printf("read(%d, buf) = %d (%d)\n",
           self->filedes, arg1, errno);
    self->filedes = 0;
    ustack();
}

Fix invalid address by guarding against null path in open(2).

Fix dynamic variables dropped by explicitly releasing thread-locals by assigning them to 0 as they are no longer needed.