extend cluster health to return errors for IAM, and Bucket metadata (#19995)

Bonus: make API freeze to be opt-in instead of default

cmd/healthcheck-handler.go

@@ -29,14 +29,35 @@ import (
 
 const unavailable = "offline"
 
-// ClusterCheckHandler returns if the server is ready for requests.
-func ClusterCheckHandler(w http.ResponseWriter, r *http.Request) {
-	ctx := newContext(r, w, "ClusterCheckHandler")
-
+func checkHealth(w http.ResponseWriter) ObjectLayer {
 	objLayer := newObjectLayerFn()
 	if objLayer == nil {
 		w.Header().Set(xhttp.MinIOServerStatus, unavailable)
 		writeResponse(w, http.StatusServiceUnavailable, nil, mimeNone)
+		return nil
+	}
+
+	if !globalBucketMetadataSys.Initialized() {
+		w.Header().Set(xhttp.MinIOServerStatus, "bucket-metadata-offline")
+		writeResponse(w, http.StatusServiceUnavailable, nil, mimeNone)
+		return nil
+	}
+
+	if !globalIAMSys.Initialized() {
+		w.Header().Set(xhttp.MinIOServerStatus, "iam-offline")
+		writeResponse(w, http.StatusServiceUnavailable, nil, mimeNone)
+		return nil
+	}
+
+	return objLayer
+}
+
+// ClusterCheckHandler returns if the server is ready for requests.
+func ClusterCheckHandler(w http.ResponseWriter, r *http.Request) {
+	ctx := newContext(r, w, "ClusterCheckHandler")
+
+	objLayer := checkHealth(w)
+	if objLayer == nil {
 		return
 	}
 
@@ -72,10 +93,8 @@ func ClusterCheckHandler(w http.ResponseWriter, r *http.Request) {
 func ClusterReadCheckHandler(w http.ResponseWriter, r *http.Request) {
 	ctx := newContext(r, w, "ClusterReadCheckHandler")
 
-	objLayer := newObjectLayerFn()
+	objLayer := checkHealth(w)
 	if objLayer == nil {
-		w.Header().Set(xhttp.MinIOServerStatus, unavailable)
-		writeResponse(w, http.StatusServiceUnavailable, nil, mimeNone)
 		return
 	}