[PATCH 2 of 3] Headers filter: add "add_trailer" directive

[Piotr Sikora]

Hey Maxim,

> (Just for the record, with the first patch fixed to avoid using
> chunked with HTTP/1.0, the "Trailer" header is expectedly still
> added with HTTP/1.0.  This confirms the idea that the approach
> choosen is somewhat fragile.)

It confirms no such thing. The only thing it confirms is that making
major changes during code review for a code that was written almost a
year ago is an error-prone process.

> The question is: if we need this indicator to be sent to a
> particular client.
>
> For example, if you are using trailers to pass additional logging
> information to your own frontends, and use something like
>
>     geo $mine {
>         127.0.0.1/8 1;
>     }
>
>     map $mine $x_request_time {
>         1     $request_time;
>     }
>
>     add_trailer X-Response-Time $x_request_time;
>
> to send the information to your frontends, but not other clients,
> you probably don't want the X-Response-Time trailer to be
> indicated to other clients.

In such setup, clients would probably talk to frontends.

> Acutally I don't see how it's a problem, given that "Trailer" is
> not something required.  Moreover, it seems to be not needed or
> even harmful in most of the use cases discussed.

I don't think it's harmful, but I'm not aware of any clients that
_require_ "Trailer" header, so I'm going to skip this whole discussion
and just remove it. You might always re-add it later if needed.

Best regards,
Piotr Sikora