oidc discovery: encode metadata once and reuse

Signed-off-by: Monis Khan <mok@vmware.com>

internal/oidc/discovery/discovery_handler.go

@@ -5,6 +5,7 @@
 package discovery
 
 import (
+	"bytes"
 	"encoding/json"
 	"net/http"
 
@@ -40,14 +41,6 @@ type Metadata struct {
 
 // NewHandler returns an http.Handler that serves an OIDC discovery endpoint.
 func NewHandler(issuerURL string) http.Handler {
-	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/json")
-
-		if r.Method != http.MethodGet {
-			http.Error(w, `Method not allowed (try GET)`, http.StatusMethodNotAllowed)
-			return
-		}
-
 	oidcConfig := Metadata{
 		Issuer:                            issuerURL,
 		AuthorizationEndpoint:             issuerURL + oidc.AuthorizationEndpointPath,
@@ -60,8 +53,26 @@ func NewHandler(issuerURL string) http.Handler {
 		ScopesSupported:                   []string{"openid", "offline"},
 		ClaimsSupported:                   []string{"groups"},
 	}
-		if err := json.NewEncoder(w).Encode(&oidcConfig); err != nil {
+
+	var b bytes.Buffer
+	encodeErr := json.NewEncoder(&b).Encode(&oidcConfig)
+	encodedMetadata := b.Bytes()
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodGet {
+			http.Error(w, `Method not allowed (try GET)`, http.StatusMethodNotAllowed)
+			return
+		}
+
+		if encodeErr != nil {
+			http.Error(w, encodeErr.Error(), http.StatusInternalServerError)
+			return
+		}
+
+		w.Header().Set("Content-Type", "application/json")
+		if _, err := w.Write(encodedMetadata); err != nil {
 			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
 		}
 	})
 }