cmcfarlen opened a new pull request, #12271:
URL: https://github.com/apache/trafficserver/pull/12271

   PR #11519 was a security related PR that removed support for URL param, but 
also introduced a change that altered the cache key produced from a URL, even 
if the URL did not have a param component.  While this doesn't affect the cache 
disk layout or cause the cache to reset, it does cause any objects stored by 
prior versions of ATS to not be found.  This has the same effect as resetting 
the cache when upgrading to ATS10.
   
   This PR mitigates this by introducing a cache key lookup compatibility 
option.  If this option is set, then when ATS encounters a cache miss, it will 
retry the lookup using the prior versions URL cachekey algorithm.  If it then 
gets a hit, it increments a metric and then proceeds as if the first attempt 
was found.  If it still gets a miss, the transaction will continue on as a 
miss.  In this mode, cache writes will always use the new algorithm and so over 
time, the need for having this mode enabled goes away.
   
   This route was chosen over reverting the PR because of the security nature 
of the change and it could be a strategy for future releases to be able to make 
similar changes to cache and offer a backward compatibility mode for lookups 
that can be used until the cache wraps.
   


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