ZPublisher events before streaming a response

Hi,

In some cases (e.g. large OFS.File/Image responses), Zope 2 will use 
response.write() to stream the response.

We have events that fire before and after a "regular" response is 
returned, but none that allow us to set headers (caching headers, in 
this case) before such a streaming response is calculated. The normal 
events fire too late.

We'd therefore like to add a new event in the HTTPResponse class (in 
ZServer, though I think it makes sense to add to the ZPublisher base 
class version as well). It'd hook in something like this:

     if not self._wrote:

         # new event code
         site = getSite()
         request = getattr(site, 'REQUEST', None)
         notify(PubBeforeStreaming(request))

         # continue as before...

(I couldn't find a better way to get hold of the request from a method 
in the response, without adding a dependency on five.globalrequest, 
which I assume is not desirable).

Any objections? We need this in Zope 2.12, though I'll obviously merge 
to trunk, too.

Martin

-- 
Author of `Professional Plone Development`, a book for developers who
want to work with Plone. See http://martinaspeli.net/plone-book

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