test(libexpr-tests): add tests for nix_get_external

Regression test for the check_value_out → check_value_in fix: nix_get_external
reads from an already-initialized value and must use check_value_in, not
check_value_out (which asserts the value is NOT initialized).

Added:
- nix_get_external_roundtrip: create external → init → force → get back
- nix_get_external_invalid: null and uninitialized value error paths
- nix_get_external_value_content_null: null ExternalValue* gracefully

Author: Lillecarl

src/libexpr-tests/nix_api_external.cc

@@ -104,4 +104,34 @@ TEST_F(nix_api_expr_test, nix_external_printValueAsJSON_can_use_state)
     nix_gc_decref(ctx, toJsonFn);
 }
 
+TEST_F(nix_api_expr_test, nix_get_external_roundtrip)
+{
+    int content = 42;
+    NixCExternalValueDesc desc{};
+    ExternalValue * ext = nix_create_external_value(ctx, &desc, &content);
+    assert_ctx_ok();
+    ASSERT_NE(nullptr, ext);
+
+    nix_init_external(ctx, value, ext);
+    assert_ctx_ok();
+
+    nix_value_force(ctx, state, value);
+    assert_ctx_ok();
+
+    ExternalValue * retrieved = nix_get_external(ctx, value);
+    assert_ctx_ok();
+    ASSERT_NE(nullptr, retrieved);
+
+    void * content_ptr = nix_get_external_value_content(ctx, retrieved);
+    assert_ctx_ok();
+    ASSERT_EQ(&content, content_ptr);
+
+    nix_gc_decref(ctx, ext);
+}
+
+TEST_F(nix_api_expr_test, nix_get_external_value_content_null)
+{
+    ASSERT_EQ(nullptr, nix_get_external_value_content(ctx, nullptr));
+}
+
 } // namespace nixC

src/libexpr-tests/nix_api_value.cc

@@ -119,6 +119,14 @@ TEST_F(nix_api_expr_test, nix_value_set_get_path)
     ASSERT_EQ(NIX_TYPE_PATH, nix_get_type(ctx, value));
 }
 
+TEST_F(nix_api_expr_test, nix_get_external_invalid)
+{
+    ASSERT_EQ(nullptr, nix_get_external(ctx, nullptr));
+    assert_ctx_err();
+    ASSERT_EQ(nullptr, nix_get_external(ctx, value));
+    assert_ctx_err();
+}
+
 TEST_F(nix_api_expr_test, nix_build_and_init_list_invalid)
 {
     ASSERT_EQ(nullptr, nix_get_list_byidx(ctx, nullptr, state, 0));