Feature Request: In lua http filter add ability to set the contents of the body

[DevoKun]

Feature Request: In lua http filter add ability to set the contents of the body

The idea is to be able to set the contents of the response body from lua.
This feature is helpful for providing a custom response in the event of an upstream failure.
For example, if a circuit breaker fails for an API that is expected to be JSON, a response of an empty json array could [] be returned instead of the message upstream connect error or disconnect/reset before headers. reset reason: connection failure which would break an app expecting JSON.

filter_chains:
- filters:
  - name: envoy.http_connection_manager
    config:
      http_filters:
        - name: envoy.lua
          config:
            inline_code: |
              function envoy_on_response(response_handle)
                if response_handle:headers():get(":status") != "200" then
                  response_handle:body():set('[]')
                end
              end

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or other activity occurs. Thank you for your contributions.

[DevoKun]

Please do not close this feature request. The request is still relevant.

[dio]

@DevoKun gotcha!

[nmnellis]

I was just trying to implement this in order to fix the issue with grpc-json transcoder not placing error messages and codes in the body. #3383

[steveyang95]

Hi, are there any updates to this issue? I found a use case for this feature as well and would love to know what the status is for this feature request

[marrrcin]

+1
What is the current status?

[bluetechy]

The use case that I need this feature for is: User specifies the HTTP header Accept-Charset to cause the API to return back the response in the specified encoding (and update HTTP response header Content-Length, accordingly).

Similarly, I would like to update the request body so that I can allow users to submit requests using different encodings and handle them all as UTF-8 internally.

[dio]

One way to fix this is to make BufferWrapper to be mutable (or probably introduce another class that allows mutation to the buffered data). A quick sketch on making wrapped buffer to be mutable: https://github.com/envoyproxy/envoy/compare/master...dio:mutable-buffer?expand=1#diff-dc19b516207df88d31bc3ee355d58ecaR41

Hence we can do:

          - name: envoy.filters.http.lua
            typed_config:
              "@type": type.googleapis.com/envoy.config.filter.http.lua.v2.Lua
              inline_code: |
                function envoy_on_request(request_handle)
                end
                function envoy_on_response(response_handle)
                  response_handle:headers():replace("content-length", 6)
                  response_handle:body():set("hello\n")
                end

Notice that we need to know the content-length in advance since when calling response_handle:body() the headers is already continued. One might expect to have the following API instead:

content_length = response_handle:body():set("hello\n")
response_handle:headers():replace("content-length", content_length)

But it is not possible for now.

Another way is to implement full-blown handle:respond() but this requires checking on some internal states in the filter.

cc. @ceastman-ibm

[dio]

@mattklein123 do you think it is OK to let BufferWrapper to wrap mutable buffer (as exhibited in #5998 (comment))? Or probably you have other suggestions? Thanks!

[mattklein123]

From a quick glance it seems reasonable. Let's discuss in code review? re: content-length, you should also be able to clear it and switch to chunk encoding if necessary. I would definitely have documentation/example about that.

[dio]

I have submitted a PR (#13172) for this. Not entirely sure, probably we can repurpose the respond API as a wrapper for the low-level codes exhibited in the PR.