Expose some core implementation details and write a basic unit test for the dummy store

This test currently doesn't use the new-exposed functionality, but with
future changes the tests will be expanded and they will be used.

src/libstore-tests/dummy-store.cc

@@ -0,0 +1,27 @@
+#include <gtest/gtest.h>
+
+#include "nix/store/dummy-store.hh"
+#include "nix/store/globals.hh"
+#include "nix/store/realisation.hh"
+
+namespace nix {
+
+TEST(DummyStore, realisation_read)
+{
+    initLibStore(/*loadConfig=*/false);
+
+    auto store = [] {
+        auto cfg = make_ref<DummyStoreConfig>(StoreReference::Params{});
+        cfg->readOnly = false;
+        return cfg->openStore();
+    }();
+
+    auto drvHash = Hash::parseExplicitFormatUnprefixed(
+        "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad", HashAlgorithm::SHA256, HashFormat::Base16);
+
+    auto outputName = "foo";
+
+    EXPECT_EQ(store->queryRealisation({drvHash, outputName}), nullptr);
+}
+
+} // namespace nix

src/libstore-tests/meson.build

@@ -61,6 +61,7 @@ sources = files(
   'derivation.cc',
   'derived-path.cc',
   'downstream-placeholder.cc',
+  'dummy-store.cc',
   'http-binary-cache-store.cc',
   'legacy-ssh-store.cc',
   'local-binary-cache-store.cc',

src/libstore/dummy-store.cc

@@ -2,7 +2,7 @@
 #include "nix/util/archive.hh"
 #include "nix/util/callback.hh"
 #include "nix/util/memory-source-accessor.hh"
-#include "nix/store/dummy-store.hh"
+#include "nix/store/dummy-store-impl.hh"
 
 #include <boost/unordered/concurrent_flat_map.hpp>
 
@@ -108,24 +108,15 @@ public:
 
 } // namespace
 
-struct DummyStore : virtual Store
+ref<Store> DummyStoreConfig::openStore() const
+{
+    return openDummyStore();
+}
+
+struct DummyStoreImpl : DummyStore
 {
     using Config = DummyStoreConfig;
 
-    ref<const Config> config;
-
-    struct PathInfoAndContents
-    {
-        UnkeyedValidPathInfo info;
-        ref<MemorySourceAccessor> contents;
-    };
-
-    /**
-     * This is map conceptually owns the file system objects for each
-     * store object.
-     */
-    boost::concurrent_flat_map<StorePath, PathInfoAndContents> contents;
-
     /**
      * This view conceptually just borrows the file systems objects of
      * each store object from `contents`, and combines them together
@@ -135,9 +126,9 @@ struct DummyStore : virtual Store
      */
     ref<WholeStoreViewAccessor> wholeStoreView = make_ref<WholeStoreViewAccessor>();
 
-    DummyStore(ref<const Config> config)
+    DummyStoreImpl(ref<const Config> config)
         : Store{*config}
-        , config(config)
+        , DummyStore{config}
     {
         wholeStoreView->setPathDisplay(config->storeDir);
     }
@@ -289,9 +280,9 @@ struct DummyStore : virtual Store
     }
 };
 
-ref<Store> DummyStore::Config::openStore() const
+ref<DummyStore> DummyStore::Config::openDummyStore() const
 {
-    return make_ref<DummyStore>(ref{shared_from_this()});
+    return make_ref<DummyStoreImpl>(ref{shared_from_this()});
 }
 
 static RegisterStoreImplementation<DummyStore::Config> regDummyStore;

src/libstore/include/nix/store/dummy-store-impl.hh

@@ -0,0 +1,40 @@
+#pragma once
+///@file
+
+#include "nix/store/dummy-store.hh"
+
+#include <boost/unordered/concurrent_flat_map.hpp>
+
+namespace nix {
+
+struct MemorySourceAccessor;
+
+/**
+ * Enough of the Dummy Store exposed for sake of writing unit tests
+ */
+struct DummyStore : virtual Store
+{
+    using Config = DummyStoreConfig;
+
+    ref<const Config> config;
+
+    struct PathInfoAndContents
+    {
+        UnkeyedValidPathInfo info;
+        ref<MemorySourceAccessor> contents;
+    };
+
+    /**
+     * This is map conceptually owns the file system objects for each
+     * store object.
+     */
+    boost::concurrent_flat_map<StorePath, PathInfoAndContents> contents;
+
+    DummyStore(ref<const Config> config)
+        : Store{*config}
+        , config(config)
+    {
+    }
+};
+
+} // namespace nix

src/libstore/include/nix/store/dummy-store.hh

@@ -5,6 +5,8 @@
 
 namespace nix {
 
+struct DummyStore;
+
 struct DummyStoreConfig : public std::enable_shared_from_this<DummyStoreConfig>, virtual StoreConfig
 {
     DummyStoreConfig(const Params & params)
@@ -42,6 +44,11 @@ struct DummyStoreConfig : public std::enable_shared_from_this<DummyStoreConfig>,
         return {"dummy"};
     }
 
+    /**
+     * Same as `openStore`, just with a more precise return type.
+     */
+    ref<DummyStore> openDummyStore() const;
+
     ref<Store> openStore() const override;
 
     StoreReference getReference() const override

src/libstore/include/nix/store/meson.build

@@ -34,6 +34,7 @@ headers = [ config_pub_h ] + files(
   'derived-path-map.hh',
   'derived-path.hh',
   'downstream-placeholder.hh',
+  'dummy-store-impl.hh',
   'dummy-store.hh',
   'export-import.hh',
   'filetransfer.hh',