Locking around osutil.Rename and some descriptive text

osutil/osutil.go

@@ -9,9 +9,22 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"sync"
 )
 
+// Try to keep this entire operation atomic-like. We shouldn't be doing this
+// often enough that there is any contention on this lock.
+var renameLock sync.Mutex
+
+// Rename renames a file, while trying hard to succeed on various systems by
+// temporarily tweaking directory permissions and removing the destination
+// file when necessary. Will make sure to delete the from file if the
+// operation fails, so use only for situations like committing a temp file to
+// it's final location.
 func Rename(from, to string) error {
+	renameLock.Lock()
+	defer renameLock.Unlock()
+
 	// Make sure the destination directory is writeable
 	toDir := filepath.Dir(to)
 	if info, err := os.Stat(toDir); err == nil {