Add osutil.AtomicWriter

This captures the common pattern of writing to a temp file and moving it
to it's real name only if everything went well. It reduces the amount of
code in some places where we do this, but maybe not as much as I
expected because the upgrade thing is still a special snowflake...

internal/config/wrapper.go

@@ -7,9 +7,7 @@
 package config
 
 import (
-	"io/ioutil"
 	"os"
-	"path/filepath"
 
 	"github.com/syncthing/protocol"
 	"github.com/syncthing/syncthing/internal/events"
@@ -283,24 +281,20 @@ func (w *Wrapper) IgnoredDevice(id protocol.DeviceID) bool {
 
 // Save writes the configuration to disk, and generates a ConfigSaved event.
 func (w *Wrapper) Save() error {
-	fd, err := ioutil.TempFile(filepath.Dir(w.path), "cfg")
+	fd, err := osutil.CreateAtomic(w.path, 0600)
 	if err != nil {
 		return err
 	}
-	defer os.Remove(fd.Name())
 
-	err = w.cfg.WriteXML(fd)
-	if err != nil {
+	if err := w.cfg.WriteXML(fd); err != nil {
 		fd.Close()
 		return err
 	}
 
-	err = fd.Close()
-	if err != nil {
+	if err := fd.Close(); err != nil {
 		return err
 	}
 
 	events.Default.Log(events.ConfigSaved, w.cfg)
-
-	return osutil.Rename(fd.Name(), w.path)
+	return nil
 }