Seems that mod_include can pass empty bucket brigades down the filter chain. For instance if the first line in an SSI is <!--#include... then SPLIT_AND_PASS_PRETAG_BUCKETS will pass an empty brigade as the first brigade.
This causes problems with the logic in mod_deflate which attempts to bypass deflate when a response has zero body length (e.g. proxied 304's)
What happens is that this empty brigade gets passed down the filter chain and ap_http_header_filter is called before the Content-Encoding header has been set (mod_deflate is holding off setting this until it knows it has some content).
As a defensive move, mod_deflate could just check for APR_BRIGADE_EMPTY and immediately return APR_SUCCESS. Or we could put checks in mod_include to prevent passing empty buckets in the first place (I think there was such a check at one time but were taken out for performance improving reasons?).
The latter might avoid problems with other modules but being defensive is probably a good idea also.
Comments?
Allan
