libexpr-tests: Add unit tests for broken readDir /. for pure eval

A very unfortunate interaction of current filtering with pure eval is
that the following actually leads to `lib.a = {}`. This just adds a unit
test for this broken behavior. This is really good to be done as a unit test
via the in-memory store.

{
  outputs =
    { ... }:
    {
      lib.a = builtins.readDir /.;
    };
}

src/libexpr-test-support/include/nix/expr/tests/libexpr.hh

@@ -26,11 +26,20 @@ public:
     }
 
 protected:
-    LibExprTest()
+    LibExprTest(ref<Store> store, auto && makeEvalSettings)
         : LibStoreTest()
+        , evalSettings(makeEvalSettings(readOnlyMode))
         , state({}, store, fetchSettings, evalSettings, nullptr)
     {
-        evalSettings.nixPath = {};
+    }
+
+    LibExprTest()
+        : LibExprTest(openStore("dummy://"), [](bool & readOnlyMode) {
+            EvalSettings settings{readOnlyMode};
+            settings.nixPath = {};
+            return settings;
+        })
+    {
     }
 
     Value eval(std::string input, bool forceValue = true)

src/libexpr-tests/eval.cc

@@ -3,6 +3,7 @@
 
 #include "nix/expr/eval.hh"
 #include "nix/expr/tests/libexpr.hh"
+#include "nix/util/memory-source-accessor.hh"
 
 namespace nix {
 
@@ -174,4 +175,41 @@ TEST_F(EvalStateTest, getBuiltin_fail)
     ASSERT_THROW(state.getBuiltin("nonexistent"), EvalError);
 }
 
+class PureEvalTest : public LibExprTest
+{
+public:
+    PureEvalTest()
+        : LibExprTest(openStore("dummy://", {{"read-only", "false"}}), [](bool & readOnlyMode) {
+            EvalSettings settings{readOnlyMode};
+            settings.pureEval = true;
+            settings.restrictEval = true;
+            return settings;
+        })
+    {
+    }
+};
+
+TEST_F(PureEvalTest, pathExists)
+{
+    ASSERT_THAT(eval("builtins.pathExists /."), IsFalse());
+    ASSERT_THAT(eval("builtins.pathExists /nix"), IsFalse());
+    ASSERT_THAT(eval("builtins.pathExists /nix/store"), IsFalse());
+
+    {
+        std::string contents = "Lorem ipsum";
+
+        StringSource s{contents};
+        auto path = state.store->addToStoreFromDump(
+            s, "source", FileSerialisationMethod::Flat, ContentAddressMethod::Raw::Text, HashAlgorithm::SHA256);
+        auto printed = store->printStorePath(path);
+
+        ASSERT_THROW(eval(fmt("builtins.readFile %s", printed)), RestrictedPathError);
+        ASSERT_THAT(eval(fmt("builtins.pathExists %s", printed)), IsFalse());
+
+        ASSERT_THROW(eval("builtins.readDir /."), RestrictedPathError);
+        state.allowPath(path); // FIXME: This shouldn't behave this way.
+        ASSERT_THAT(eval("builtins.readDir /."), IsAttrsOfSize(0));
+    }
+}
+
 } // namespace nix

src/libstore-test-support/include/nix/store/tests/libstore.hh

@@ -19,14 +19,13 @@ public:
     }
 
 protected:
+    LibStoreTest(ref<Store> store)
+        : store(std::move(store))
+    {
+    }
+
     LibStoreTest()
-        : store(openStore({
+        : LibStoreTest(openStore("dummy://"))
-              .variant =
-                  StoreReference::Specified{
-                      .scheme = "dummy",
-                  },
-              .params = {},
-          }))
     {
     }