Add DummyStore (dummy://)

DummyStore does not allow building or adding paths. This is useful for
evaluation tests when you don't want to initialize a "proper" store.

src/libstore/dummy-store.cc

@@ -0,0 +1,59 @@
+#include "store-api.hh"
+
+namespace nix {
+
+static std::string uriScheme = "dummy://";
+
+struct DummyStore : public Store
+{
+    DummyStore(const Params & params)
+        : Store(params)
+    { }
+
+    string getUri() override
+    {
+        return uriScheme;
+    }
+
+    void queryPathInfoUncached(const StorePath & path,
+        Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept override
+    {
+        callback(nullptr);
+    }
+
+    std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) override
+    { unsupported("queryPathFromHashPart"); }
+
+    void addToStore(const ValidPathInfo & info, Source & source,
+        RepairFlag repair, CheckSigsFlag checkSigs) override
+    { unsupported("addToStore"); }
+
+    StorePath addToStore(const string & name, const Path & srcPath,
+        FileIngestionMethod method, HashType hashAlgo,
+        PathFilter & filter, RepairFlag repair) override
+    { unsupported("addToStore"); }
+
+    StorePath addTextToStore(const string & name, const string & s,
+        const StorePathSet & references, RepairFlag repair) override
+    { unsupported("addTextToStore"); }
+
+    void narFromPath(const StorePath & path, Sink & sink) override
+    { unsupported("narFromPath"); }
+
+    void ensurePath(const StorePath & path) override
+    { unsupported("ensurePath"); }
+
+    BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
+        BuildMode buildMode) override
+    { unsupported("buildDerivation"); }
+};
+
+static RegisterStoreImplementation regStore([](
+    const std::string & uri, const Store::Params & params)
+    -> std::shared_ptr<Store>
+{
+    if (uri != uriScheme) return nullptr;
+    return std::make_shared<DummyStore>(params);
+});
+
+}

src/nix/main.cc

@@ -185,7 +185,7 @@ void mainWrapped(int argc, char * * argv)
     }
 
     if (argc == 2 && std::string(argv[1]) == "__dump-builtins") {
-        EvalState state({}, openStore("ssh://foo.invalid/"));
+        EvalState state({}, openStore("dummy://"));
         auto res = nlohmann::json::object();
         auto builtins = state.baseEnv.values[0]->attrs;
         for (auto & builtin : *builtins) {

tests/lang.sh

@@ -1,6 +1,7 @@
 source common.sh
 
 export TEST_VAR=foo # for eval-okay-getenv.nix
+export NIX_REMOTE=dummy://
 
 nix-instantiate --eval -E 'builtins.trace "Hello" 123' 2>&1 | grep -q Hello
 (! nix-instantiate --show-trace --eval -E 'builtins.addErrorContext "Hello" 123' 2>&1 | grep -q Hello)