Varnish 4.0 and IMS oddity

[Mark Moseley]

Hi. I'm testing out Varnish 4.0 finally. I'm super excited about the IMS
stuff and am dying to use it. And a big congratulations on the 4.0 release.

I was playing with porting our shared hosting configuration to 4.0 and ran
into a slight weirdness with IMS. Keeping in mind that we do a number of
things to make things play nicely with the fact that we have no idea what
our customers might be doing (and therefore have to jump through a bunch of
crazy hoops to make sure we don't return things like authenticated content
to unauthenticated users), this could very easily be something weird with
our particular setup. I've re-run the test a bunch of times and seen the
same thing each time.

Here's the scenario:

* I have IMS up and running and working. Other than this one particular
oddity, everything else IMS-related seems to be working great and I'm
greatly looking forward to using it. The test page I'm using deliberately
returns a TTL of 1 second to make testing easier.

* As a mockup of a customer doing something like cookie-base
authentication, or IP-based .htaccess authentication, I wrote up a simple
rewrite rule to return a 403 if a certain cookie was missing.

* I turn off the rewrite rule

* Do a request to that page a few times with the expected 200 from the
backend. On the 2nd and subsequent reqs, the IMS stuff kicks in. BTW, is
the client getting a "HTTP/1.1 200 Not Modified" the expected behavior? I
know the strings after the status code are completely arbitrary but it
looked a bit odd.

* I turn the rewrite rule back *on*

* Do the request again. Here's where it gets odd.

* Varnish does an IMS request to the backend

* The backend responds with a 403 as expected.

* Varnish replies to the client with a HTTP/1.1 200 Not Modified

I would expect an error status (or really anything that's not a 304) to
fail the IMS test on Varnish's side and that it would then return the 403
to the client. Something weird about what I'm doing/abusing?
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