Support hard links in tarballs

Fixes #10395.

src/libutil/fs-sink.hh

@@ -41,6 +41,19 @@ struct FileSystemObjectSink
     virtual void createSymlink(const Path & path, const std::string & target) = 0;
 };
 
+/**
+ * An extension of `FileSystemObjectSink` that supports file types
+ * that are not supported by Nix's FSO model.
+ */
+struct ExtendedFileSystemObjectSink : FileSystemObjectSink
+{
+    /**
+     * Create a hard link. The target must be the path of a previously
+     * encountered file relative to the root of the FSO.
+     */
+    virtual void createHardlink(const Path & path, const CanonPath & target) = 0;
+};
+
 /**
  * Recursively copy file system objects from the source into the sink.
  */