Prepare for firstGeq to be safe on foreign threads

RootSet.h

@@ -0,0 +1,46 @@
+#pragma once
+
+#include <atomic>
+#include <memory>
+#include <stdint.h>
+
+struct RootSet {
+
+  /// Register the root node for version after adding mutations
+  void add(uint32_t node, int64_t version);
+
+  /// Inform that there will be no calls to rootForVersion with a version less
+  /// than `oldestVersion`
+  void setOldestVersion(int64_t oldestVersion);
+
+  /// Foreign threads may freely interact with a `ThreadSafeHandle`, as long as
+  /// the latest version as of obtaining the handle is greater than
+  /// `oldestVersion`.
+  struct ThreadSafeHandle {
+
+    /// Get a root node that can correctly be used for `version`
+    uint32_t rootForVersion(int64_t version) const;
+
+    /// @private
+    struct Impl;
+
+  private:
+    Impl *impl; // not owned
+  };
+
+  /// Safe to call from foreign threads
+  ThreadSafeHandle getThreadSafeHandle() const;
+
+  const uint32_t *roots() const;
+  int rootCount() const;
+
+  RootSet();
+
+  ~RootSet();
+
+  /// @private
+  struct Impl;
+
+private:
+  Impl *impl;
+};