Trouble working with `requestBody` (or `getRequestBodyChunk`)

Question

Dependency versions

• Wai: 3.2.2.1

Synopsis

The working of getRequestBodyChunk (https://hackage.haskell.org/package/wai-3.2.2.1/docs/Network-Wai.html#v:getRequestBodyChunk) is quite unclear to me. I understand that it reads the next chunk of bytes available in a request; and finally yields a Data.ByteString.empty indicating no more bytes are available.

Thus, given a req :: Request; I'm trying to create a Conduit from the request body as:

streamBody :: Wai.Request -> ConduitM () ByteString IO ()
streamBody req = repeatWhileMC (Wai.getRequestBodyChunk) (Data.ByteString.empty /=)

However, this seems to terminate immediately, as I've seen tracing the outputs:

repeatWhileMC (traceShowId <$> Wai.getRequestBodyChunk req) (traceShowId . (Data.ByteString.empty /=))

The output is

""
False

And hence the stream terminates.

But I can verify that the request body is not empty by other means.

So I'm a little stumped about this and I have a few questions:

• Does this mean that the request body has been already consumed?

• Or does this mean that the request body wasn't chunked?

• Or does it mean that for smaller requests; the chunked bytes are always empty?

• Is Scotty overriding this especially for smaller sized requests (I cannot seem to find this anywhere in its code/docs.)

• If I do something like this:

streamBody req = 
  let req' = req { requestBody = pure "foobar" }
  in repeatWhileMC (Wai.getRequestBodyChunk req') (Data.ByteString.empty /=)

I do get a non terminating stream of bytes; which leads me to suspect that something before this part of the code consumes the request body: or that function returns an empty body to begin with.

A point to note is that this seems to happen with smaller requests only.

Another point to note that I get the Wai.Request via Web.Scotty.Trans.request; which also has the body related streaming helpers.

Can this also be documented? I believe the documentation for getRequestBodyChunk etc. can be improved with this information.