lib/osutil: Don't remove before rename on Windows (ref #6493) (#6495)

This was needed in ancient times but not currently.

lib/osutil/atomic.go

@@ -9,7 +9,6 @@ package osutil
 import (
 	"errors"
 	"path/filepath"
-	"runtime"
 
 	"github.com/syncthing/syncthing/lib/fs"
 )
@@ -93,16 +92,6 @@ func (w *AtomicWriter) Close() error {
 		return err
 	}
 
-	// Remove the destination file, on Windows only. If it fails, and not due
-	// to the file not existing, we won't be able to complete the rename
-	// either. Return this error because it may be more informative. On non-
-	// Windows we want the atomic rename behavior so we don't attempt remove.
-	if runtime.GOOS == "windows" {
-		if err := w.fs.Remove(w.path); err != nil && !fs.IsNotExist(err) {
-			return err
-		}
-	}
-
 	if err := w.fs.Rename(w.next.Name(), w.path); err != nil {
 		w.err = err
 		return err

lib/osutil/atomic_test.go

@@ -52,6 +52,15 @@ func TestCreateAtomicCreate(t *testing.T) {
 }
 
 func TestCreateAtomicReplace(t *testing.T) {
+	testCreateAtomicReplace(t, 0644)
+}
+func TestCreateAtomicReplaceReadOnly(t *testing.T) {
+	testCreateAtomicReplace(t, 0400)
+}
+
+func testCreateAtomicReplace(t *testing.T, oldPerms os.FileMode) {
+	t.Helper()
+
 	os.RemoveAll("testdata")
 	defer os.RemoveAll("testdata")
 
@@ -59,7 +68,7 @@ func TestCreateAtomicReplace(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	if err := ioutil.WriteFile("testdata/file", []byte("some old data"), 0644); err != nil {
+	if err := ioutil.WriteFile("testdata/file", []byte("some old data"), oldPerms); err != nil {
 		t.Fatal(err)
 	}