Google Fronted, Content-Length and HEAD requests

[Piotr Sikora]

Hello,
I noticed that Google Frontend overrides user-supplied "Content-Length"
header with its own. This is of course good idea, but I find this
undesirable for HEAD requests, especially when combined with Blobstore. For
*every* HEAD request Google Frontend returns response with "Content-Length:
0".

Would it be possible to disable this for HEAD requests or at least provide
way to conditionaly disable this based on user-supplied header (ie.
"X-AppEngine-Override: Content-Length")?

According to RFC 2616:

9.4 HEAD
The HEAD method is identical to GET except that the server MUST NOT return a
message-body in the response. The metainformation contained in the HTTP
headers in response to a HEAD request SHOULD be identical to the information
sent in response to a GET request. This method can be used for obtaining
metainformation about the entity implied by the request without transferring
the entity-body itself. This method is often used for testing hypertext
links for validity, accessibility, and recent modification.

The response to a HEAD request MAY be cacheable in the sense that the
information contained in the response MAY be used to update a previously
cached entity from that resource. If the new field values indicate that the
cached entity differs from the current entity (as would be indicated by a
change in Content-Length, Content-MD5, ETag or Last-Modified), then the
cache MUST treat the cache entry as stale.

Best regards,
Piotr Sikora < piotr....@frickle.com >

[Mr. Rajiv Bakulesh Shah]

Hi, Piotr.

On Google App Engine, response headers are just a dictionary.  In order to delete the Content-Length response header, somewhere in your head request handler, add this line:

del self.response.headers['Content-Length']

Good luck,

Raj

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[Piotr Sikora]

> On Google App Engine, response headers are just a dictionary. In order to
> delete the Content-Length response header, somewhere in your head request
> handler, add this line:
>
> del self.response.headers['Content-Length']

Raj, thanks for reply, but you kind of missed my point. The problem is that
Google Frontend (the web server) discards "Content-Length" header that
you've set in your application (the dictionary) and it sets its own value
based on the length of response body. This works fine for GET and other
requests, but not for HEAD, since there is no body in its response and
Google Frontend sets "Content-Length" to 0.

[Danny Tuppeny]

This sounds like a bug more than a missing feature. If a HEAD request
is supposed to include the Content-Length of an equivalent GET
request, then returning a Content-Length of 0 would rarely be correct.

It might be worth raising in the bugtracker:

http://code.google.com/p/googleappengine/issues/list

> Piotr Sikora < piotr.sik...@frickle.com >