Clean up store hierarchy with IndirectRootStore

See the API doc comments for details.

src/libstore/indirect-root-store.hh

@@ -0,0 +1,48 @@
+#pragma once
+///@file
+
+#include "local-fs-store.hh"
+
+namespace nix {
+
+/**
+ * Mix-in class for implementing permanent roots as a pair of a direct
+ * (strong) reference and indirect weak reference to the first
+ * reference.
+ *
+ * See methods for details on the operations it represents.
+ */
+struct IndirectRootStore : public virtual LocalFSStore
+{
+    inline static std::string operationName = "Indirect GC roots registration";
+
+    /**
+     * Implementation of `LocalFSStore::addPermRoot` where the permanent
+     * root is a pair of
+     *
+     * - The user-facing symlink which all implementations must create
+     *
+     * - An additional weak reference known as the "indirect root" that
+     *   points to that symlink.
+     *
+     * The garbage collector will automatically remove the indirect root
+     * when it finds that the symlink has disappeared.
+     *
+     * The implementation of this method is concrete, but it delegates
+     * to `addIndirectRoot()` which is abstract.
+     */
+    Path addPermRoot(const StorePath & storePath, const Path & gcRoot) override final;
+
+    /**
+     * Add an indirect root, which is a weak reference to the
+     * user-facing symlink created by `addPermRoot()`.
+     *
+     * @param path user-facing and user-controlled symlink to a store
+     * path.
+     *
+     * The form this weak-reference takes is implementation-specific.
+     */
+    virtual void addIndirectRoot(const Path & path) = 0;
+};
+
+}