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Thu Feb 24 17:58:36 CET 2011

means that if the entire request body was likely to be forced into
RAM. If a Request contained file uploads, then the entirely file
contents would be forced into RAM. That is clearly not acceptable.

If the request body is just a field in the Request like:

 data Request =3D Request { body :: Lazy.ByteString, ... }

then there is no way to avoid forcing the whole request body into RAM.
Even if you use Lazy.writeFile to write the file to disk, the Request
itself will still have a reference to the body and so none of the data
can be garbage collected until after the Response is sent and the
whole Request gets garbage collected.

So, instead we need to introduce an MVar which allows us to detach the
body contents from the Request type so that it can be garbage
collected separately. So that is why we have:

data Request =3D Request { rqBody :: MVar RqBody, ... }

To get the request body. You need only call:

> takeRequestBody ::  MonadIO m =3D> Request -> m (Maybe RqBody)

Which will get the RqBody from the Request and leave the MVar empty.
That ensures that as you can lazily consume the input as it comes over
the network and everything will be nicely garbage collected.

Sometimes the request body contains form data that we want to decode
so we can examine it using the look* functions. decodeBody will use
takeRequestBody to get the request body and stick the decoded data in:

data Request =3D Request { ..., rqInputsBody :: MVar [(String, Input)] }

Since takeRequestBody can only be called once, that means our
application can either have access to the raw request body or it can
have access to the decoded form data. This is to avoid a space leak.

Fortunately, decodeBody will only call takeRequestBody if the
content-type is multipart/form-data or
application/x-www-form-urlencoded (and the request method is PUT or
POST). Usually you are going to be calling takeRequestBody when the
content-type is something else -- so you won't have a problem calling
decodeBody and then calling takeRequestBody later. (If you really need
access to the raw request and the decoded form data you can do a
takeRequestBody and then put the request body back and call
decodeBody.. but that will almost certainly force the entire response
into RAM.. there is no avoiding that).

So, here is why I believe you are having an error with 'lookCookieValue'.

It is my guess that lookCookieValue is failing for a POST or PUT
request where the content-type is *not* multipart/form-data or

If we look at the definition for askRqEnv we  have:

> instance (MonadIO m) =3D> HasRqData (ServerPartT m) where
>     askRqEnv =3D
>         do rq <- askRq
>            mbi <- liftIO $ if ((rqMethod rq) =3D=3D POST) || ((rqMethod r=
q) =3D=3D PUT)
>                            then readInputsBody rq
>                            else return (Just [])
>            case mbi of
>              Nothing   -> escape $ internalServerError (toResponse "askRq=
Env failed because the request body has not been decoded yet. Try using 'de=
>              (Just bi) -> return (rqInputsQuery rq, bi, rqCookies rq)

We see here that when you call askRqEnv and the request method is POST
or PUT, it expects to be able to find the decoded request body data in

But, that is not right. If the content-type is not
multipart/form-data or application/x-www-form-urlencoded, then there
isn't any request body inputs to decode. But that should not preclude
you from look at the request parameters or the cookies.

I am not sure what the best fix here is. Here are some options:

 1. avoid checking if decodeBody has been called at all. We can simply
try to call readInputsBody. If it returns Nothing then we just set the
body inputs to []. Unfortunately, if you fail to call decodeBody, then
even though your form-data is submitted, look won't be able to find it
and will not tell you why. That seems really confusing and unfriendly.

 2. modify the check so that it also checks the content-type. That
would fix the problem most of the time. But, it does not handle the
case where the request body *is* multipart/form-data, but you don't
want to decode it for some reason. For example, maybe you want to look
at the cookie and validate the user before decoding the request body
because different users have different maximum upload size file

 3. modify the check so that it only runs when you call look functions
that actually examine the request body. Normally, functions like
lookText will look at the query string and the body. But, doing,
queryString $ lookText, allows you to limit that search to just the
query string.

 4. when decodeBody is called, but the content-type is not for
form-data, it should set rqBodyInput to [].

I think the right solution might be a combination of 3 & 4. We only
report an error when you try to access fields from the body, but have
made no attempt to actually decode the body.

With that change your code should work fine. With that change, your
code should also work fine even if you don't call decodeBody. That
should give you the behavior that you expect (and was intended).

I will fix that now.

- jeremy

On Wed, Jul 27, 2011 at 4:46 AM, Sebastiaan Visser <haskell at> wro=
> Hello all,
> We have some problems upgrading one of our server applications to Happsta=
ck 6, lots of things related to request bodies and request parameters have =
changed. Unfortunately, the new situation doesn't seem that straightforward=
 to me.
> In the previous version of Happstack we were able to access the request b=
ody directly using the `rqBody` function. In the new situation we have to f=
irst decode the body using `decodeBody` and than access it by reading from =
an MVar. The utility functions accessing this variables ensure the body is =
only request once, all consecutive calls will error.
> We're pretty sure we're accessing the body only once and after decoding i=
t, but we still get the following error: "askRqEnv failed because the reque=
st body has not been decoded yet. Try using 'decodeBody'." This happens whe=
n accessing the cookie using 'readCookieValue'. Isn't strange that accessin=
g the cookie value uses the request body at all?
> So the pattern we use is:
>> =A0 do decodeBody (defaultBodyPolicy "/tmp/" (10 * 1024 * 1024) (10 * 10=
24 * 1024) (1024 * 1024))
>> =A0 =A0 =A0liftIO (print 1)
>> =A0 =A0 =A0c <- lookCookieValue "tid"
>> =A0 =A0 =A0liftIO (print 2)
>> =A0 =A0 =A0b <- takeRequestBody
>> =A0 =A0 =A0...
> And the result is:
>> =A0 1
>> =A0 askRqEnv failed because the request body has not been decoded yet. T=
ry using 'decodeBody'.
> (this happens using happstack-server-6.0.3, in 6.1.6 the same error strin=
g is sent to the client)
> And then it stops, the handler crashes and the request fails. The migrati=
on guide[1] tells me "To simulate the old behavior, simple call decodeBody =
in your top-level handler for every request.", but this is clearly not enou=
> Anyone knows how to fix this in the correct way? I cannot help to think i=
t is a bit strange that it takes me so much time to convince a web framewor=
k to hand me over the request body. This should be easy stuff right?
> Thanks,
> Sebastiaan
> [1]
> _______________________________________________
> web-devel mailing list
> web-devel at

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