Init local overlay store

src/libstore/local-overlay-store.cc

@@ -0,0 +1,20 @@
+#include "local-overlay-store.hh"
+
+namespace nix {
+
+LocalOverlayStore::LocalOverlayStore(const Params & params)
+    : StoreConfig(params)
+    , LocalFSStoreConfig(params)
+    , LocalStoreConfig(params)
+    , LocalOverlayStoreConfig(params)
+    , Store(params)
+    , LocalFSStore(params)
+    , LocalStore(params)
+    , lowerStore(openStore(lowerStoreUri).dynamic_pointer_cast<LocalFSStore>())
+{
+}
+
+
+static RegisterStoreImplementation<LocalOverlayStore, LocalOverlayStoreConfig> regLocalOverlayStore;
+
+}

src/libstore/local-overlay-store.hh

@@ -0,0 +1,74 @@
+#include "local-store.hh"
+
+namespace nix {
+
+/**
+ * Configuration for `LocalOverlayStore`.
+ */
+struct LocalOverlayStoreConfig : virtual LocalStoreConfig
+{
+    // FIXME why doesn't this work?
+    // using LocalStoreConfig::LocalStoreConfig;
+
+    LocalOverlayStoreConfig(const StringMap & params)
+        : StoreConfig(params)
+        , LocalFSStoreConfig(params)
+        , LocalStoreConfig(params)
+    { }
+
+    const Setting<std::string> lowerStoreUri{(StoreConfig*) this, "", "lower-store",
+        R"(
+          [Store URL](@docroot@/command-ref/new-cli/nix3-help-stores.md#store-url-format)
+          for the lower store. The default is `auto` (i.e. use the Nix daemon or `/nix/store` directly).
+
+          Must be a store with a store dir on the file system.
+        )"};
+
+    const std::string name() override { return "Experimental Local Overlay Store"; }
+
+    std::string doc() override
+    {
+        return
+          ""
+          // FIXME write docs
+          //#include "local-overlay-store.md"
+          ;
+    }
+};
+
+/**
+ * Variation of local store using overlayfs for the store dir.
+ */
+class LocalOverlayStore : public virtual LocalOverlayStoreConfig, public virtual LocalStore
+{
+    /**
+     * The store beneath us.
+     *
+     * Our store dir should be an overlay fs where the lower layer
+     * is that store's store dir, and the upper layer is some
+     * scratch storage just for us.
+     */
+    ref<LocalFSStore> lowerStore;
+
+public:
+    LocalOverlayStore(const Params & params);
+
+    LocalOverlayStore(std::string scheme, std::string path, const Params & params)
+        : LocalOverlayStore(params)
+    {
+        throw UnimplementedError("LocalOverlayStore");
+    }
+
+    static std::set<std::string> uriSchemes()
+    { return {}; }
+
+    std::string getUri() override
+    {
+        return "local-overlay";
+    }
+
+private:
+    // Overridden methods…
+};
+
+}

src/libstore/store-api.cc

@@ -10,6 +10,7 @@
 #include "archive.hh"
 #include "callback.hh"
 #include "remote-store.hh"
+#include "local-overlay-store.hh"
 
 #include <nlohmann/json.hpp>
 #include <regex>
@@ -1391,6 +1392,8 @@ std::shared_ptr<Store> openFromNonUri(const std::string & uri, const Store::Para
         return std::make_shared<UDSRemoteStore>(params);
     } else if (uri == "local") {
         return std::make_shared<LocalStore>(params);
+    } else if (uri == "local-overlay") {
+        return std::make_shared<LocalOverlayStore>(params);
     } else if (isNonUriPath(uri)) {
         Store::Params params2 = params;
         params2["root"] = absPath(uri);