fix: return provider response during fnc listing errors

gateway/metrics/add_metrics.go

@@ -34,15 +34,13 @@ func AddMetricsHandler(handler http.HandlerFunc, prometheusQuery PrometheusQuery
 			log.Printf("List functions responded with code %d, body: %s",
 				recorder.Code,
 				string(upstreamBody))
-
-			http.Error(w, "Metrics handler: unexpected status code from provider listing functions", http.StatusInternalServerError)
+			http.Error(w, string(upstreamBody), recorder.Code)
 			return
 		}
 
 		var functions []types.FunctionStatus
 
 		err := json.Unmarshal(upstreamBody, &functions)
-
 		if err != nil {
 			log.Printf("Metrics upstream error: %s, value: %s", err, string(upstreamBody))
 
@@ -63,8 +61,8 @@ func AddMetricsHandler(handler http.HandlerFunc, prometheusQuery PrometheusQuery
 
 			results, err := prometheusQuery.Fetch(url.QueryEscape(q))
 			if err != nil {
+				// log the error but continue, the mixIn will correctly handle the empty results.
 				log.Printf("Error querying Prometheus: %s\n", err.Error())
-				return
 			}
 			mixIn(&functions, results)
 		}

gateway/metrics/add_metrics_test.go

@@ -5,6 +5,7 @@ import (
 	"log"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
 	types "github.com/openfaas/faas-provider/types"
@@ -55,6 +56,34 @@ func Test_PrometheusMetrics_MixedInto_Services(t *testing.T) {
 	}
 }
 
+func Test_MetricHandler_ForwardsErrors(t *testing.T) {
+	functionsHandler := func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusConflict)
+		w.Write([]byte("test error case"))
+	}
+	// explicitly set the query fetcher to nil because it should
+	// not be called when a non-200 response is returned from the
+	// functions handler, if it is called then the test will panic
+	handler := AddMetricsHandler(functionsHandler, nil)
+
+	rr := httptest.NewRecorder()
+	request, _ := http.NewRequest(http.MethodGet, "/system/functions", nil)
+	handler.ServeHTTP(rr, request)
+
+	if status := rr.Code; status != http.StatusConflict {
+		t.Errorf("handler returned wrong status code: got %v want %v", status, http.StatusConflict)
+	}
+
+	if rr.Header().Get("Content-Type") != "text/plain; charset=utf-8" {
+		t.Errorf("Want 'text/plain; charset=utf-8' content-type, got: %s", rr.Header().Get("Content-Type"))
+	}
+	body := strings.TrimSpace(rr.Body.String())
+	if body != "test error case" {
+		t.Errorf("Want 'test error case', got: %q", body)
+	}
+
+}
+
 func Test_FunctionsHandler_ReturnsJSONAndOneFunction(t *testing.T) {
 	functionsHandler := makeFunctionsHandler()