Set headers in body_filter_by_lua*

[adrianke]

When trying to set headers via ngx.headers.HEADER in body_filter_by_lua*, I get the following error:

*311 attempt to set ngx.header.HEADER after sending out response headers while sending to client

I need to set headers based on what's in the response. Is there a way to delay the first byte of the response until I've set my headers? Kind of like lua_need_request_body.

[agentzh]

@adrianke Nginx's output body filters always run after its output header filters, which means the response header has already been sent out. You'll need a time machine to change the sent-out response header which may already be on the wire. Please note that, nginx tries to output data as soon as possible to reduce memory footprint (as opposed to full-buffering).

[agentzh]

@adrianke If you need full buffering, then you shouldn't use the output filters in the first place since output filters are for the nonbuffered outputting mode which is the most optimal approach.

[ruslantalpa]

looking at this right now,
@agentzh if what you are saying here is true (and it is :))

then the docs are wrong/misleading

ngx.header.HEADER
syntax: ngx.header.HEADER = VALUE
syntax: value = ngx.header.HEADER
context: rewrite_by_lua*, access_by_lua*, content_by_lua*, header_filter_by_lua*, body_filter_by_lua*, log_by_lua*

notice it says it can be used in body_filter_by_lua*

so, absolutely no way to set a response header based on the body from the upstream (i am buffering the response body as you showed in some issue using ngx.arg[1] = nil)?
Any way to have nginx buffer the upstream response and send out the response (headers + body) only when manually triggered by some lua code, for example playing with proxy_buffering?

Thank you