Hello.

I'm working on a zero-copy brotli compression filter. With zero-copy I wrap compressor output into a buffer and send it to next filter in a chain.

The problem is - it is not clear how to properly wait until this buffer is released.

If I just continue asking the next filter to do its work, until buffer is released, it is possible to get into infinite loop (see https://github.com/eustas/ngx_brotli/issues/9#issuecomment-373737792).

If I return NGX_AGAIN in a case the next filter is not able to use more of the buffer data, the previous filter never gives a chance to continue compression (https://github.com/eustas/ngx_brotli/issues/9#issuecomment-371513645).