Attempting to send a custom chunked response after skipping access phase results in only partial response&hanging connection

Hi there, I'm using Openresty 1.15.8.3 with nginx 1.16.

I've been trying to return a custom chunked response, after skipping the access phase/upstream request. This is a trimmed down version of how I've tried to achieve this:

access_by_lua_block {
    return ngx.exit(ngx.HTTP_OK) -- should skip upstream request
 }

 header_filter_by_lua_block {
     ngx.header["content-length"] = nil -- should enable response streaming
 }

 body_filter_by_lua_block {
     ngx.arg[1] = fetch_some_data()  -- fetches the next chunk of data to return to the client
     ngx.arg[2] = ngx.arg[1] == nil      -- set EOF if no more data
 }

Here, I'd expect body_filter_by_lua_block to be repeatedly called until EOF was set. Instead, what actually happens is body_filter_by_lua_block is only called once, then the next phase is called. The connection to the client (curl in this case) is left open, and is not closed.

Interestingly, if I allow the upstream request to go ahead by removing the access_by_lua_block block, then body_filter_by_lua_block is actually called repeatedly as expected. But I'd like to entirely avoid an upstream connection, so it's a no-go.

Is this a bug, or if not are there any workarounds?

Thanks!

Start with the provided minimal configuration using access_by_lua_block, header_filter_by_lua_block, and body_filter_by_lua_block, then compare it with the version that permits the upstream request. Trace why the body filter runs only once after ngx.exit(ngx.HTTP_OK). Done means the custom chunks continue until EOF and the client connection closes without an upstream request.