Best way to read & rewind input body in Rack 3.0 ?

[aglushkov]

Question comes from Rollbar issue rollbar/rollbar-gem#1106

As #rewind has gone in 3.0 there are some issues:

• Rollbar can't read body to track it (if it was already read by the app).
• It can't rewind body to allow other middlewares to read it

I propose one hack to rewind body but it looks fragile if internals change

     body = rack_req.body #  InputWrapper
     if body.respond_to?(:rewind)
       body.rewind
     else
       body_input = body.instance_variable_get(:@input) 
       body_input.rewind if body_input.respond_to?(:rewind) # additional check if its not rewindable
    end

Any advises ?

[jeremyevans]

If you need rewindable input in Rack 3, use Rack::RewindableInput::Middleware

[jeremyevans]

env['rack.input'].rewind if env['rack.input'].respond_to?(:rewind) (where env is the rack environment hash).

[aglushkov]

Is something wrong with my app?

 env['rack.input'] => #<Rack::Lint::Wrapper::InputWrapper:0x00007fcd652d6bc8 @input=#<StringIO:0x00007fcd652b1d50>>

env['rack.input'] does not respond to #rewind ever as it is a Rack::Lint::Wrapper::InputWrapper

[ioquatix]

I guess https://github.com/rack/rack/blob/main/lib/rack/lint.rb#L405 does not support rewind, or any other method that isn't specified by the spec. This might be a bug with the lint wrapper.

[ioquatix]

@jeremyevans do you think we should allow the InputWrapper to proxy all methods to the underlying input body or do you think this is a valid interface to enforce at this point?

[jeremyevans]

I don't think we should proxy all methods. The purpose of Lint is to ensure the application follows the rack SPEC, and calling methods on the body other than those mentioned in SPEC is outside SPEC. @aglushkov's example is why we shouldn't do this. They are using rewind, which is no longer required. The code calling rewind is not compatible with Rack 3, and if Lint starts proxying all methods, that issue would not be caught.

[ioquatix]

Okay, so the correct way for a middleware to handle this would be:

• Read the input into a buffer.
• Replace rack.input with the buffered input.

Now it has both the input and the downstream middleware can also read it.

However, this has to be done carefully - i.e. if the input was expected to be streaming - like a websocket connection or a huge upload (multi-GB), etc.

[jeremyevans]

I think the recommended way in Rack 3 if you want rewindable input is to use Rack::RewindableInput::Middleware. This should not be done if you want to support arbitrary streaming input in the same application.

[dblock]

Same problem in ruby-grape/grape#2388, still discussing the best fix.