Check reference also in norwfs SetRef

Signed-off-by: Javi Fontan <jfontan@gmail.com>

Author: jfontan

storage/filesystem/internal/dotgit/dotgit_setref_norwfs.go

@@ -2,16 +2,39 @@
 
 package dotgit
 
-import "gopkg.in/src-d/go-git.v4/plumbing"
+import (
+	"fmt"
 
-// There are some filesystems tha don't support opening files in RDWD mode.
+	"gopkg.in/src-d/go-git.v4/plumbing"
+)
+
+// There are some filesystems that don't support opening files in RDWD mode.
 // In these filesystems the standard SetRef function can not be used as i
 // reads the reference file to check that it's not modified before updating it.
 //
 // This version of the function writes the reference without extra checks
 // making it compatible with these simple filesystems. This is usually not
 // a problem as they should be accessed by only one process at a time.
 func (d *DotGit) setRef(fileName, content string, old *plumbing.Reference) error {
+	_, err := d.fs.Stat(fileName)
+	if err == nil && old != nil {
+		fRead, err := d.fs.Open(fileName)
+		if err != nil {
+			return err
+		}
+
+		ref, err := d.readReferenceFrom(fRead, old.Name().String())
+		fRead.Close()
+
+		if err != nil {
+			return err
+		}
+
+		if ref.Hash() != old.Hash() {
+			return fmt.Errorf("reference has changed concurrently")
+		}
+	}
+
 	f, err := d.fs.Create(fileName)
 	if err != nil {
 		return err