oauth2 (cc) block does not use referenced backend

[johakoch]

Describe the bug
The oauth2 (cc) block does not use the referenced but a default backend

To Reproduce
Steps to reproduce the behavior:

1. Which Couper version? Run couper version or docker run avenga/couper version

$ couper version
0 2021-09-28 dev

2. Provide your configuration file *.hcl. Remove sensitive data.

# OAuth2 client
server "oauth2-cc-client" {
  hosts = ["*:8080"]
  api {
    endpoint "/**" {
      proxy {
        backend = "rs"
      }
    }
  }
}

# OAuth2 resource server
server "oauth2-rs" {
  hosts = ["*:8081"]
  api {
    access_control = ["jwt"]

    endpoint "/**" {
      response {
        json_body = request.context.jwt
      }
    }
  }
}

# OAuth2 authorization server
server "oauth2-as" {
  hosts = ["*:8082"]
  api {
    endpoint "/token" {
      response {
        json_body = {
          access_token = jwt_sign("jwt", { foo = request.headers.x-foo })
          expires_in = 120
        }
      }
    }
  }
}

definitions {
  # backend for resource server
  backend "rs" {
    origin = "http://localhost:8081"
    oauth2 {
      token_endpoint = "http://localhost:8082/token"
      backend = "as"
      client_id      = "id"
      client_secret  = "secret"
      grant_type     = "client_credentials"
    }
  }

  # backend for authorization server
  backend "as" {
    origin = "http://localhost:8082"
    add_request_headers = {
      x-foo = "bar"
    }
  }

  # jwt for signing (at authorization server) and signature validation (at resource server)
  jwt "jwt" {
    signature_algorithm = "HS256"
    header = "authorization"
    key = "$e(rEt"
    signing_ttl = "120s"
    claims = {
      foo = "bar"
    }
  }
}

3. Provide a curl call for reproduction

$ curl -si localhost:8080/foo

4. See an error response or error log.

$ curl -si localhost:8080/foo
HTTP/1.1 502 Bad Gateway
Content-Type: application/json
Couper-Error: backend error
Couper-Request-Id: c59enjl916bmb43dn3tg
Server: couper.io
Vary: Accept-Encoding
Date: Tue, 28 Sep 2021 10:17:50 GMT
Content-Length: 134

{
  "error": {
    "id":      "c59enjl916bmb43dn3tg",
    "message": "backend error",
    "path":    "/foo",
    "status":  502
  }
}

ERRO[0006] expression evaluation error: couper.hcl:29,65-71: Missing map element; This map does not have an element with the key "x-foo".  auth_user=id build=dev client_ip=127.0.0.1 endpoint=/token error_type=evaluation handler=api method=POST port=8082 request="map[bytes:29 headers:map[accept:application/json] host:localhost method:POST origin:localhost:8082 path:/token proto:http status:500 tls:false]" response="map[bytes:150 headers:map[content-type:application/json]]" server=oauth2-as status=500 timings="map[total:0.631]" type=couper_access uid=c59enjl916bmb43dn3u0 url="http://localhost:8082/token" version=0
INFO[0006]                                               auth_user=id backend=default build=dev method=POST request="map[bytes:29 headers:map[accept:application/json] host:localhost method:POST name:default origin:localhost:8082 path:/token port:8082 proto:]" response="map[headers:map[content-type:application/json] status:500]" status=500 timings="map[dns:5.014 tcp:0.658 total:12.556 ttfb:5.763]" token_request=oauth2 type=couper_backend uid=c59enjl916bmb43dn3tg url="http://localhost:8082/token" version=0
ERRO[0006] backend error: as: token request error: error=  build=dev client_ip=127.0.0.1 endpoint="/**" handler=api method=GET port=8080 request="map[headers:map[accept:*/* user-agent:curl/7.67.0] host:localhost method:GET origin:localhost:8080 path:/foo proto:http status:502 tls:false]" response="map[bytes:134 headers:map[content-type:application/json]]" server=oauth2-cc-client status=502 timings="map[total:14.752]" type=couper_access uid=c59enjl916bmb43dn3tg url="http://localhost:8080/foo" version=0

• The error message shows that there is no x-foo header.
• The upstream log of the token request shows that the used backed is a default backend, not the one labelled as.

Expected behavior

• The oauth2 cc client should use the as backend which adds a x-foo request header which is then used at the token endpoint to add an extra foo claim:

$ curl -si localhost:8080/foo
HTTP/1.1 200 OK
...

{"exp":...,"foo":"bar"}

• The log entry in the upstream log should have backend=as.