Subrequests from body filters

Marat Dakota dakota at
Mon Mar 25 15:41:16 UTC 2013

>> >> >> But is it ok to call next body filter in subrequest's body filter to
>> >> >> produce output to main request?
>> >> >> I mean ngx_http_next_body_filter(r->main, out).
>> >> >
>> >> > No.  You should call next body filter of the request you are
>> >> > working with.  It's postpone filter responsibility to manage
>> >> > subrequests output, and if you try to do this yourself instead -
>> >> > result will be undefined.
>> >>
>> >> It seems to work as expected for me. How can I cause problems with this?
>> >
>> > Undefined behaviour sometimes appear to work as expected.  This
>> > doesn't mean it's correct though.
>> >
>> > Depending on the exact place in a filter chain where you did it
>> > and various other factors like timings, results may vary from
>> > "nothing bad might happen, as r == r->main anyway" to "response
>> > will completely incorrect as wrong filters will be applied to the
>> > response body".
>> >
>> > Most trivial thing to test is probably a subrequest order, which
>> > likely will be wrong in your case if first subrequest will take
>> > longer to handle than second one.
>> Subrequests order doesn't matter much for me. I feed my library (the
>> one I write a Nginx module for) with a subrequests results in a
>> whatever order and my library returns next chunk of response only when
>> it is ready.
>> My library has just one function to call. This function returns the
>> next chunk of data (if any) to send as a response and/or a list of
>> subrequests to make. In every call to subrequest body filter I pass
>> subrequest's response to my library and get a new list of subrequests
>> (if any) and a new chunk of final response (if any). And so on, until
>> my library says it's done.
>> And if I really do something wrong in terms of Nginx architecture,
>> please, could you give me more details about how to achieve my goals
>> correctly?
> I don't really see why you don't call ngx_http_next_body_filter(r,
> out), which is perfectly correct.

Because I'm getting a mess with the chunk order.

Let's suppose I've made two subrequests in a handler.

ngx_http_subrequest(...); // first
ngx_http_subrequest(...); // second

If I call ngx_http_next_body_filter(r, out) in a subrequest body
filter, I get the following.
Let's suppose we've received a response from the second subrequest
before any data from the first subrequest. And my library said I
should send "aaa" as a response (and I passed "aaa" to
Then, we've got a response from the first subrequest and my library
said I should send "bbb" as a response. I would expect "aaabbb" as the
final result. But real result would be "bbbaaa" (because
ngx_http_subrequest for first subrequest is earlier).

How to deal with this?


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