Print fsync warning only once

The repo.Handler is freshly instantiated for every request such that it
forget that the fsync warning was already printed. Use a single instance
in the Server instead.

handlers.go

@@ -7,6 +7,7 @@ import (
 	"path"
 	"path/filepath"
 	"strings"
+	"sync"
 
 	"github.com/restic/rest-server/quota"
 	"github.com/restic/rest-server/repo"
@@ -34,6 +35,7 @@ type Server struct {
 
 	htpasswdFile *HtpasswdFile
 	quotaManager *quota.Manager
+	fsyncWarning sync.Once
 }
 
 // MaxFolderDepth is the maxDepth param passed to splitURLPath.
@@ -91,6 +93,7 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		QuotaManager:   s.quotaManager, // may be nil
 		PanicOnError:   s.PanicOnError,
 		NoVerifyUpload: s.NoVerifyUpload,
+		FsyncWarning:   &s.fsyncWarning,
 	}
 	if s.Prometheus {
 		opt.BlobMetricFunc = makeBlobMetricFunc(username, folderPath)

repo/repo.go

@@ -39,6 +39,7 @@ type Options struct {
 
 	BlobMetricFunc BlobMetricFunc
 	QuotaManager   *quota.Manager
+	FsyncWarning   *sync.Once
 }
 
 // DefaultDirMode is the file mode used for directory creation if not
@@ -74,8 +75,6 @@ func New(path string, opt Options) (*Handler, error) {
 type Handler struct {
 	path string // filesystem path of repo
 	opt  Options
-
-	fsyncWarning sync.Once
 }
 
 // httpDefaultError write a HTTP error with the default description
@@ -639,7 +638,7 @@ func (h *Handler) saveBlob(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if syncNotSup {
-		h.fsyncWarning.Do(func() {
+		h.opt.FsyncWarning.Do(func() {
 			log.Print("WARNING: fsync is not supported by the data storage. This can lead to data loss, if the system crashes or the storage is unexpectedly disconnected.")
 		})
 	} else {