Support auth against OpenSearch (#1030)

Little hackish, as it involves additional call to the backend (in order
to decide whether use OpenSearch or Elasticsearch security endpoint)
**but** given that we have auth header cache I think we might get away
with that 😉


FWIW, Elasticsearch 7 uses the same [endpoint
(`_security/_authenticate`)](https://www.elastic.co/guide/en/elasticsearch/reference/current/security-api-authenticate.html)
as Elasticsearch 8 for auth (just verified 😌)

quesma/elasticsearch/client.go

@@ -13,7 +13,11 @@ import (
 	"time"
 )
 
-const esRequestTimeout = 5 * time.Second
+const (
+	esRequestTimeout              = 5 * time.Second
+	elasticsearchSecurityEndpoint = "_security/_authenticate"
+	openSearchSecurityEndpoint    = "_plugins/_security/api/account"
+)
 
 type SimpleClient struct {
 	client *http.Client
@@ -41,7 +45,22 @@ func (es *SimpleClient) RequestWithHeaders(ctx context.Context, method, endpoint
 }
 
 func (es *SimpleClient) Authenticate(ctx context.Context, authHeader string) bool {
-	resp, err := es.doRequest(ctx, "GET", "_security/_authenticate", nil, http.Header{"Authorization": {authHeader}})
+	var authEndpoint string
+	// This is really suboptimal, and we should find a better way to set this systematically (config perhaps?)
+	// OTOH, since we have auth cache in place, I am not concerned about this additional backend call - at least for the time being.
+	r, err := es.doRequest(ctx, "GET", "/", nil, http.Header{"Authorization": {authHeader}})
+	if err != nil {
+		logger.ErrorWithCtx(ctx).Msgf("error sending request: %v", err)
+		return false
+	}
+	defer r.Body.Close()
+
+	if isResponseFromElasticsearch(r) {
+		authEndpoint = elasticsearchSecurityEndpoint
+	} else {
+		authEndpoint = openSearchSecurityEndpoint
+	}
+	resp, err := es.doRequest(ctx, "GET", authEndpoint, nil, http.Header{"Authorization": {authHeader}})
 	if err != nil {
 		logger.ErrorWithCtx(ctx).Msgf("error sending request: %v", err)
 		return false
@@ -66,3 +85,7 @@ func (es *SimpleClient) doRequest(ctx context.Context, method, endpoint string,
 	}
 	return es.client.Do(req)
 }
+
+func isResponseFromElasticsearch(resp *http.Response) bool {
+	return resp.Header.Get("X-Elastic-Product") != ""
+}