A decorator for caching views and handling etag conditional requests.

    The decorator adds headers to GET requests that help with caching: Last-
    Modified, Expires and ETag. It also handles conditional requests, when the
    client send an If-Matches header.

    If a cache is set, the decorator will cache GET responses, bypassing the
    dataframe serialization. POST requests will still benefit from the
    dataframe cache for requests that produce the same SQL.

def etag_cache(max_age, check_perms=bool):
    """
    A decorator for caching views and handling etag conditional requests.

    The decorator adds headers to GET requests that help with caching: Last-
    Modified, Expires and ETag. It also handles conditional requests, when the
    client send an If-Matches header.

    If a cache is set, the decorator will cache GET responses, bypassing the
    dataframe serialization. POST requests will still benefit from the
    dataframe cache for requests that produce the same SQL.

    """
    def decorator(f):
        @wraps(f)
        def wrapper(*args, **kwargs):
            # check if the user can access the resource
            check_perms(*args, **kwargs)

            # for POST requests we can't set cache headers, use the response
            # cache nor use conditional requests; this will still use the
            # dataframe cache in `superset/viz.py`, though.
            if request.method == 'POST':
                return f(*args, **kwargs)

            response = None
            if cache:
                try:
                    # build the cache key from the function arguments and any
                    # other additional GET arguments (like `form_data`, eg).
                    key_args = list(args)
                    key_kwargs = kwargs.copy()
                    key_kwargs.update(request.args)
                    cache_key = wrapper.make_cache_key(f, *key_args, **key_kwargs)
                    response = cache.get(cache_key)
                except Exception:  # pylint: disable=broad-except
                    if app.debug:
                        raise
                    logging.exception('Exception possibly due to cache backend.')

            # if no response was cached, compute it using the wrapped function
            if response is None:
                response = f(*args, **kwargs)

                # add headers for caching: Last Modified, Expires and ETag
                response.cache_control.public = True
                response.last_modified = datetime.utcnow()
                expiration = max_age if max_age != 0 else FAR_FUTURE
                response.expires = \
                    response.last_modified + timedelta(seconds=expiration)
                response.add_etag()

                # if we have a cache, store the response from the request
                if cache:
                    try:
                        cache.set(cache_key, response, timeout=max_age)
                    except Exception:  # pylint: disable=broad-except
                        if app.debug:
                            raise
                    logging.exception('Exception possibly due to cache backend.')

            return response.make_conditional(request)

        if cache:
            wrapper.uncached = f
            wrapper.cache_timeout = max_age
            wrapper.make_cache_key = \
                cache._memoize_make_cache_key(  # pylint: disable=protected-access
                    make_name=None, timeout=max_age)

        return wrapper

    return decorator