fix: Include expires header on 304 cache hit (#438)

When configured with a TTL, the relay proxy will include an `expires`
header when a flag or segment is first requested. This informs the PHP
SDK how long it can cache the response.

Once the SDK makes another request, it is possible the flag still hasn't
changed. In that situation, we should return a new `expires` header.
Otherwise, the PHP SDK will have no idea how long it should cache this
new response for, resulting in it re-fetching on demand going forward.

relay/endpoints_php_test.go

@@ -68,6 +68,7 @@ func TestEndpointsPHPPolling(t *testing.T) {
 							result, _ := st.DoRequest(r, p.relay)
 
 							assert.Equal(t, http.StatusNotModified, result.StatusCode)
+							assert.NotEmpty(t, result.Header.Get("Expires"))
 						})
 
 						t.Run("query with different ETag is cached", func(t *testing.T) {
@@ -76,6 +77,7 @@ func TestEndpointsPHPPolling(t *testing.T) {
 							result, _ := st.DoRequest(r, p.relay)
 
 							assert.Equal(t, http.StatusOK, result.StatusCode)
+							assert.NotEmpty(t, result.Header.Get("Expires"))
 						})
 					}
 				}

relay/relay_endpoints.go

@@ -297,15 +297,6 @@ func pollFlagOrSegment(clientContext relayenv.EnvContext, kind ldstoretypes.Data
 
 func writeCacheableJSONResponse(w http.ResponseWriter, req *http.Request, clientContext relayenv.EnvContext,
 	bytes []byte, etagValue string) {
-	etag := fmt.Sprintf("relay-%s", etagValue) // just to make it extra clear that these are relay-specific etags
-	if cachedEtag := req.Header.Get("If-None-Match"); cachedEtag != "" {
-		if cachedEtag == etag {
-			w.WriteHeader(http.StatusNotModified)
-			return
-		}
-	}
-	w.Header().Set("Content-Type", "application/json")
-	w.Header().Set("Etag", etag)
 	ttl := clientContext.GetTTL()
 	if ttl > 0 {
 		w.Header().Set("Vary", "Authorization")
@@ -315,7 +306,17 @@ func writeCacheableJSONResponse(w http.ResponseWriter, req *http.Request, client
 		// HTTP cache in front of ld-relay, multiple clients hitting the cache at different times
 		// will all see the same expiration time.
 	}
+
+	etag := fmt.Sprintf("relay-%s", etagValue) // just to make it extra clear that these are relay-specific etags
+	if cachedEtag := req.Header.Get("If-None-Match"); cachedEtag == etag {
+		w.WriteHeader(http.StatusNotModified)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	w.Header().Set("Etag", etag)
 	w.WriteHeader(http.StatusOK)
+
 	_, _ = w.Write(bytes)
 }