Avoid ListBuckets() call instead rely on simple HTTP GET (#8475)

This is to avoid making calls to backend and requiring
gateways to allow permissions for ListBuckets() operation
just for Liveness checks, we can avoid this and make
our liveness checks to be more performant.

cmd/gateway-common.go

@@ -17,8 +17,10 @@
 package cmd
 
 import (
+	"context"
 	"net/http"
 	"strings"
+	"time"
 
 	"github.com/minio/minio/cmd/config"
 	xhttp "github.com/minio/minio/cmd/http"
@@ -286,6 +288,28 @@ func ToMinioClientCompleteParts(parts []CompletePart) []minio.CompletePart {
 	return mparts
 }
 
+// IsBackendOnline - verifies if the backend is reachable
+// by performing a GET request on the URL. returns 'true'
+// if backend is reachable.
+func IsBackendOnline(ctx context.Context, clnt *http.Client, urlStr string) bool {
+	ctx, cancel := context.WithTimeout(ctx, 1*time.Second)
+	defer cancel()
+
+	// never follow redirects
+	clnt.CheckRedirect = func(*http.Request, []*http.Request) error {
+		return http.ErrUseLastResponse
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, urlStr, nil)
+	if err != nil {
+		return false
+	}
+	if _, err = clnt.Do(req); err != nil {
+		return !xnet.IsNetworkOrHostDown(err)
+	}
+	return true
+}
+
 // ErrorRespToObjectError converts MinIO errors to minio object layer errors.
 func ErrorRespToObjectError(err error, params ...string) error {
 	if err == nil {