diff --git a/src/libutil/fs-sink.hh b/src/libutil/fs-sink.hh
index 994f19960..32dcb2f01 100644
--- a/src/libutil/fs-sink.hh
+++ b/src/libutil/fs-sink.hh
@@ -45,7 +45,7 @@ struct FileSystemObjectSink
  * An extension of `FileSystemObjectSink` that supports file types
  * that are not supported by Nix's FSO model.
  */
-struct ExtendedFileSystemObjectSink : FileSystemObjectSink
+struct ExtendedFileSystemObjectSink : virtual FileSystemObjectSink
 {
     /**
      * Create a hard link. The target must be the path of a previously
diff --git a/tests/unit/libutil-support/tests/tracing-file-system-object-sink.cc b/tests/unit/libutil-support/tests/tracing-file-system-object-sink.cc
new file mode 100644
index 000000000..737e02213
--- /dev/null
+++ b/tests/unit/libutil-support/tests/tracing-file-system-object-sink.cc
@@ -0,0 +1,33 @@
+#include <iostream>
+#include "tracing-file-system-object-sink.hh"
+
+namespace nix::test {
+
+void TracingFileSystemObjectSink::createDirectory(const Path & path)
+{
+    std::cerr << "createDirectory(" << path << ")\n";
+    sink.createDirectory(path);
+}
+
+void TracingFileSystemObjectSink::createRegularFile(const Path & path, std::function<void(CreateRegularFileSink &)> fn)
+{
+    std::cerr << "createRegularFile(" << path << ")\n";
+    sink.createRegularFile(path, [&](CreateRegularFileSink & crf) {
+        // We could wrap this and trace about the chunks of data and such
+        fn(crf);
+    });
+}
+
+void TracingFileSystemObjectSink::createSymlink(const Path & path, const std::string & target)
+{
+    std::cerr << "createSymlink(" << path << ", target: " << target << ")\n";
+    sink.createSymlink(path, target);
+}
+
+void TracingExtendedFileSystemObjectSink::createHardlink(const Path & path, const CanonPath & target)
+{
+    std::cerr << "createHardlink(" << path << ", target: " << target << ")\n";
+    sink.createHardlink(path, target);
+}
+
+} // namespace nix::test
diff --git a/tests/unit/libutil-support/tests/tracing-file-system-object-sink.hh b/tests/unit/libutil-support/tests/tracing-file-system-object-sink.hh
new file mode 100644
index 000000000..9527b0be3
--- /dev/null
+++ b/tests/unit/libutil-support/tests/tracing-file-system-object-sink.hh
@@ -0,0 +1,41 @@
+#pragma once
+#include "fs-sink.hh"
+
+namespace nix::test {
+
+/**
+ * A `FileSystemObjectSink` that traces calls, writing to stderr.
+ */
+class TracingFileSystemObjectSink : public virtual FileSystemObjectSink
+{
+    FileSystemObjectSink & sink;
+public:
+    TracingFileSystemObjectSink(FileSystemObjectSink & sink)
+        : sink(sink)
+    {
+    }
+
+    void createDirectory(const Path & path) override;
+
+    void createRegularFile(const Path & path, std::function<void(CreateRegularFileSink &)> fn);
+
+    void createSymlink(const Path & path, const std::string & target);
+};
+
+/**
+ * A `ExtendedFileSystemObjectSink` that traces calls, writing to stderr.
+ */
+class TracingExtendedFileSystemObjectSink : public TracingFileSystemObjectSink, public ExtendedFileSystemObjectSink
+{
+    ExtendedFileSystemObjectSink & sink;
+public:
+    TracingExtendedFileSystemObjectSink(ExtendedFileSystemObjectSink & sink)
+        : TracingFileSystemObjectSink(sink)
+        , sink(sink)
+    {
+    }
+
+    void createHardlink(const Path & path, const CanonPath & target);
+};
+
+}