Implement scaling ranges

Scaling ranges keep the OpenFaaS CE gateway's requests to
within the intended bounds already implemented in other
components.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alex@openfaas.com>

gateway/scaling/ranges.go

@@ -1,5 +1,14 @@
 package scaling
 
+import (
+	"bytes"
+	"encoding/json"
+	"io/ioutil"
+	"net/http"
+
+	"github.com/openfaas/faas-provider/types"
+)
+
 const (
 	// DefaultMinReplicas is the minimal amount of replicas for a service.
 	DefaultMinReplicas = 1
@@ -21,3 +30,43 @@ const (
 	// ScalingFactorLabel label indicates the scaling factor for a function
 	ScalingFactorLabel = "com.openfaas.scale.factor"
 )
+
+func MakeHorizontalScalingHandler(next http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodPost {
+			http.Error(w, "Only POST is allowed", http.StatusMethodNotAllowed)
+			return
+		}
+
+		if r.Body == nil {
+			http.Error(w, "Error reading request body", http.StatusBadRequest)
+			return
+		}
+
+		body, err := ioutil.ReadAll(r.Body)
+		if err != nil {
+			http.Error(w, "Error reading request body", http.StatusBadRequest)
+			return
+		}
+
+		scaleRequest := types.ScaleServiceRequest{}
+		if err := json.Unmarshal(body, &scaleRequest); err != nil {
+			http.Error(w, "Error unmarshalling request body", http.StatusBadRequest)
+			return
+		}
+
+		if scaleRequest.Replicas < 1 {
+			scaleRequest.Replicas = 1
+		}
+
+		if scaleRequest.Replicas > DefaultMaxReplicas {
+			scaleRequest.Replicas = DefaultMaxReplicas
+		}
+
+		upstreamReq, _ := json.Marshal(scaleRequest)
+		// Restore the io.ReadCloser to its original state
+		r.Body = ioutil.NopCloser(bytes.NewBuffer(upstreamReq))
+
+		next.ServeHTTP(w, r)
+	}
+}