Add the ability to limit ContentCachingResponseWrapper cache size

[vpavic]

It would be nice if ContentCachingResponseWrapper could support setting a limit to its cache so that response body is cached only up to this limit. This could be beneficial for the existing ShallowEtagHeaderFilter and other use case that leverage ContentCachingResponseWrapper.

See spring-projects/spring-boot#43697 (comment) and the discussion around it for background of this enhancement request.

[bclozel]

If I understand correctly, this new feature means that developers would configure a threshold property (in bytes) on the filter instance and this would change its behavior at runtime. If the response body is larger than the threshold, the wrapper writes all cached content to the actual output with copyBodyToResponse and then resumes caching content.

From there, developers can only assume that the cached content is a piece of the actual response body and they have no clue about the total size of the response body. Right now, if the cached content exceeds 2Gb an exception is thrown. This also means that in this case the "Content-Length" response headers cannot be written to the response as the response would be committed already.

This would developers in a position where ContentCachingResponseWrapper has two very different behavior modes:

1. caching the entire content and writing content-length headers, failing otherwise
2. caching some part of the end of the content (this would not be a sliding window - if the response size is MAX + 1, then the cached content would be of size "1") and not setting the Content-Length header anymore.

I don't really like this proposal in this current state and I'm not sure we want to push ContentCachingResponseWrapper usage because of its existing limitations.