#19705: CommonMiddleware handles If-Modified-Since incorrectly
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Reporter: aaugustin | Owner: nobody
Type: Bug | Status: new
Component: HTTP handling | Version: master
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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Two middleware check ETags for unmodified responses: `CommonMiddleware`
and `ConditionalGetMiddleware` and they do it inconsistently.
If the response's ETag matches the request's If-Modified-Since:
- `ConditionalGetMiddleware` changes the response code to 304, preserving
all headers; the content gets removed later on
- `CommonMiddleware` creates a new `HttpResponseNotModified` without
content and simply restores the cookies.
As a consequence, `CommonMiddleware` returns a response without ETag,
which is wrong. I detected this with RedBot on a Django site I run. Any
site with `USE_ETAGS = True` has this problem.
In general, wiping headers sounds like a bad idea. A 304 is supposed to
have the same headers as the 200. (Well, the RFC is more complicated, but
I think it's the general idea. [http://tools.ietf.org/html/draft-ietf-
httpbis-p1-messaging-21#section-3.3.2 Future versions of HTTP] will likely
require the Content-Length not to be 0.)
----
I believe that `CommonMiddleware` should simply generate the ETag and not
handle conditional content removal; that's the job of
`ConditionalGetMiddleware`.
For example, if one is using GzipMiddleware, the correct response chain
is:
- `CommonMiddleware` computes the ETag,
- `GzipMiddleware` compresses the content and modifies the ETag,
- `ConditionalGetMiddleware` uses the modified ETag to decide if the
response was modified or not.
This is a good reason to keep "ETag generation" and "Etag checking"
concerns separate. The same argument applies to any middleware that sets
or modifies ETags.
Unfortunately, `CommonMiddleware` is documented to "take care of sending
Not Modified responses, if appropriate", so this would be a backwards
incompatible change.
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Ticket URL: <https://code.djangoproject.com/ticket/19705>
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