Range filter: fixed duplicate last buffers. In ngx_http_range_singlepart_body() special buffers where passed unmodified, including ones after the end of the range. As such, if the last buffer of a response was sent separately as a special buffer, two buffers with b->last_buf set were present in the response. In particular, this might result in a duplicate final chunk when using chunked transfer encoding (normally range filter and chunked transfer encoding are not used together, but this may happen if there are trailers in the response). This also likely to cause problems in HTTP/2. Fix is to skip all special buffers after we've sent the last part of the range requested. These special buffers are not meaningful anyway, since we set b->last_buf in the buffer with the last part of the range, and everything is expected to be flushed due to it. Additionally, ngx_http_next_body_filter() is now called even if no buffers are to be passed to it. This ensures that various write events are properly propagated through the filter chain. In particular, this fixes test failures observed with the above change and aio enabled.