lib/osutil: Fix "atomic" rename on Windows (ref #6495, ref #6493) (#6510)

So, in a funny plot twist, it turns out that WriteFile in Go 1.13
doesn't actually set the read only bit on Windows when called with
permissions 0444 so my test was broken. With an improved test it turns
out that Rename does not, in fact, overwrite a read-only file on
Windows. This adds a fix for that.

(Rename might get improved in Go 1.15...)

lib/osutil/atomic.go

@@ -9,6 +9,7 @@ package osutil
 import (
 	"errors"
 	"path/filepath"
+	"runtime"
 
 	"github.com/syncthing/syncthing/lib/fs"
 )
@@ -92,7 +93,15 @@ func (w *AtomicWriter) Close() error {
 		return err
 	}
 
-	if err := w.fs.Rename(w.next.Name(), w.path); err != nil {
+	err := w.fs.Rename(w.next.Name(), w.path)
+	if runtime.GOOS == "windows" && fs.IsPermission(err) {
+		// On Windows, we might not be allowed to rename over the file
+		// because it's read-only. Get us some write permissions and try
+		// again.
+		_ = w.fs.Chmod(w.path, 0644)
+		err = w.fs.Rename(w.next.Name(), w.path)
+	}
+	if err != nil {
 		w.err = err
 		return err
 	}