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From Bryan McQuade <>
Subject Re: mod_deflate handling of empty initial brigade
Date Tue, 01 Jun 2010 14:53:59 GMT
The guide to writing output filters says:

"An output filter should never pass an empty brigade down the filter
chain. But, for good defensive programming, filters should be prepared
to accept an empty brigade, and do nothing."

According to this, mod_deflate should not pass the empty brigade
along, and your module also should not be passing and empty brigade to

Eric, others, should we submit a patch to mod_deflate to change its
behavior to be consistent with the output filter guide?

On Tue, Jun 1, 2010 at 10:39 AM, Brian Pane <> wrote:
> In a filter module I'm writing, it's possible for the first pass
> through the output filter to end up calling:
>    ap_pass_brigade(f->next, an_empty_brigade)
> If mod_deflate's output filter appears later in the output filter
> chain, bad things happen:
> 1. On the first trip through the output filter chain, mod_deflate just
> passes the empty brigade through to the next filter:
>    /* Do nothing if asked to filter nothing. */
>    if (APR_BRIGADE_EMPTY(bb)) {
>        return ap_pass_brigade(f->next, bb);
>    }
> 2. Control eventually reaches core_output_filter, which sends the
> response headers.
> 3. On a subsequent trip through the output filter chain, my module calls:
>    ap_pass_brigade(f->next, a non_empty_brigade)
> 4. Upon seeing the non empty brigade, mod_deflate's output filter does
> all the initialization that it would normally do at the start of a
> response.  If the response is compressible, deflate_out_filter adds
> "Content-Encoding: gzip" to the output headers.  The output headers
> have been sent already, though, so the Content-Encoding never gets
> sent.
> 5. The client receives a compressed response without a Content-Encoding header.
> I can prevent this by not calling ap_pass_brigade in my module until I
> have the first non-empty brigade for the rest of the output filter
> chain to work with.
> I'm curious, though: which module is doing the wrong thing:
> - My module, by sending an empty brigade through the rest of the
> output filter chain prior to sending the first non-empty brigade?
> - Or mod_deflate, by adding fields to the response header after
> calling ap_pass_brigade?
> Thanks,
> -Brian

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