[PAL/vm-common] Emulate `PAL_HANDLE_FD_HANG_UP`/`PAL_WAIT_HANG_UP`

[dimakuv]

The PAL_HANDLE_FD_HANG_UP/PAL_WAIT_HANG_UP events correspond to normal POSIX POLLHUP events (hang up, e.g. in case of a pipe/stream socket when one end of the channel was shutdown).

Currently the VM PALs never return these events, instead they simply return PAL_HANDLE_FD_ERROR/PAL_WAIT_ERROR.

See related code:

• gramine-tdx/pal/src/host/vm-common/pal_common_object.c

  Lines 66 to 71 in f4405d3

  	if (!pipe_buf->readable && !pipe_buf->writable) {
  	/* pipe was shutdown */
  	handle->flags |= PAL_HANDLE_FD_ERROR; /* TODO: maybe PAL_HANDLE_FD_HANG_UP? */
  	revents = PAL_WAIT_ERROR; /* TODO: maybe PAL_WAIT_HANG_UP? */
  	goto out;
  	}

• gramine-tdx/pal/src/host/vm-common/pal_common_object.c

  Lines 103 to 110 in f4405d3

  	long peeked = virtio_vsock_peek(handle->sock.fd);
  	if (peeked < 0) {
  	/* socket is invalid or was shutdown or in the process of closing */
  	handle->flags |= PAL_HANDLE_FD_ERROR;
  	*out_events = PAL_WAIT_ERROR;
  	spinlock_unlock(&handle->sock.lock);
  	return 0;
  	}