libutil: disallow enum serialization by default

allow opting in to serialization as integers via a trait type instead,
and add string-list serializers for the feature flag set enumerations.

fixes #738

Change-Id: I2746eb5ef1f15c01b4e681f9ba1615b6c6e64f44

doc/manual/rl-next/feature-enum-json.md

@@ -0,0 +1,9 @@
+---
+synopsis: "Fix experimental and deprecated features showing as integers in `nix config show --json`"
+issues: [fj#738]
+cls: [2882]
+category: "Fixes"
+credits: ["horrors"]
+---
+
+Internal changes in 2.92 caused `nix config show --json` to show deprecated and experimental features not as the list of named features 2.91 and earlier produced, but as integers. This has been fixed.

lix/libstore/store-api.hh

@@ -4,6 +4,7 @@
 #include "lix/libutil/archive.hh"
 #include "lix/libutil/async-io.hh"
 #include "lix/libutil/async.hh"
+#include "lix/libutil/json-fwd.hh"
 #include "lix/libutil/logging.hh"
 #include "lix/libstore/nar-info.hh"
 #include "lix/libstore/realisation.hh"
@@ -108,6 +109,9 @@ BuildMode buildModeFromInteger(int);
 
 enum TrustedFlag : bool { NotTrusted = false, Trusted = true };
 
+template<>
+struct json::is_integral_enum<TrustedFlag> : std::true_type {};
+
 struct BuildResult;
 struct KeyedBuildResult;
 

lix/libutil/deprecated-features.cc

@@ -102,4 +102,20 @@ void from_json(const JSON & j, DeprecatedFeature & feature)
         throw Error("Unknown deprecated feature '%s' in JSON input", input);
 }
 
+void to_json(JSON & j, const DeprecatedFeatures & f)
+{
+    StringSet res;
+    for (auto & depFeature : depFeatureDetails) {
+        if ((f & depFeature.tag) == (DeprecatedFeatures{} | depFeature.tag)) {
+            res.emplace(depFeature.name);
+        }
+    }
+    j = res;
+}
+
+void from_json(const JSON & j, DeprecatedFeatures & f)
+{
+    f = parseDeprecatedFeatures(j.get<StringSet>());
+}
+
 }

lix/libutil/deprecated-features.hh

@@ -4,6 +4,7 @@
 #include "lix/libutil/error.hh"
 #include "lix/libutil/json-fwd.hh"
 #include "lix/libutil/types.hh"
+#include <type_traits>
 
 namespace nix {
 
@@ -19,6 +20,10 @@ enum struct DeprecatedFeature
     NumDepFeatures, // number of available deprecated features, do not use
 };
 
+template<>
+struct json::avoids_null<DeprecatedFeature> : std::true_type
+{};
+
 enum struct DeprecatedFeatures {};
 
 inline DeprecatedFeatures operator| (DeprecatedFeatures a, DeprecatedFeatures b) {
@@ -78,6 +83,9 @@ JSON documentDeprecatedFeatures();
 void to_json(JSON &, const DeprecatedFeature &);
 void from_json(const JSON &, DeprecatedFeature &);
 
+void to_json(JSON &, const DeprecatedFeatures &);
+void from_json(const JSON &, DeprecatedFeatures &);
+
 /**
  * A deprecated feature used for some
  * operation, but was not enabled.

lix/libutil/error.hh

@@ -15,6 +15,7 @@
  * See libutil/tests/logging.cc for usage examples.
  */
 
+#include "lix/libutil/json-fwd.hh"
 #include "lix/libutil/suggestions.hh"
 #include "lix/libutil/fmt.hh"
 
@@ -27,6 +28,7 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <system_error>
+#include <type_traits>
 
 namespace nix {
 
@@ -42,6 +44,9 @@ typedef enum {
     lvlVomit
 } Verbosity;
 
+template<>
+struct json::is_integral_enum<Verbosity> : std::true_type {};
+
 Verbosity verbosityFromIntClamped(int val);
 
 /**

lix/libutil/experimental-features.cc

@@ -102,4 +102,20 @@ void from_json(const JSON & j, ExperimentalFeature & feature)
         throw Error("Unknown experimental feature '%s' in JSON input", input);
 }
 
+void to_json(JSON & j, const ExperimentalFeatures & f)
+{
+    StringSet res;
+    for (auto & xpFeature : xpFeatureDetails) {
+        if ((f & xpFeature.tag) == (ExperimentalFeatures{} | xpFeature.tag)) {
+            res.emplace(xpFeature.name);
+        }
+    }
+    j = res;
+}
+
+void from_json(const JSON & j, ExperimentalFeatures & f)
+{
+    f = parseFeatures(j.get<StringSet>());
+}
+
 }

lix/libutil/experimental-features.hh

@@ -4,6 +4,7 @@
 #include "lix/libutil/error.hh"
 #include "lix/libutil/json-fwd.hh"
 #include "lix/libutil/types.hh"
+#include <type_traits>
 
 namespace nix {
 
@@ -16,6 +17,10 @@ enum struct ExperimentalFeature
     NumXpFeatures, // number of available experimental features, do not use
 };
 
+template<>
+struct json::avoids_null<ExperimentalFeature> : std::true_type
+{};
+
 enum struct ExperimentalFeatures {};
 
 inline ExperimentalFeatures operator| (ExperimentalFeatures a, ExperimentalFeatures b) {
@@ -75,6 +80,9 @@ JSON documentExperimentalFeatures();
 void to_json(JSON &, const ExperimentalFeature &);
 void from_json(const JSON &, ExperimentalFeature &);
 
+void to_json(JSON &, const ExperimentalFeatures &);
+void from_json(const JSON &, ExperimentalFeatures &);
+
 /**
  * An experimental feature was required for some (experimental)
  * operation, but was not enabled.

lix/libutil/json-fwd.hh

@@ -2,6 +2,7 @@
 ///@file Lix-specific JSON handling (forward declarations only).
 
 #include <nlohmann/json_fwd.hpp>
+#include <type_traits>
 
 namespace nix {
 
@@ -16,7 +17,11 @@ template<typename T>
 struct avoids_null;
 
 template<typename T>
-struct is_integral_enum;
+struct is_integral_enum : std::false_type
+{};
+
+template<typename T>
+concept IntegralEnum = is_integral_enum<T>::value;
 
 template<typename T = void, typename SFINAE = void>
 struct adl_serializer;

lix/libutil/json.hh

@@ -4,6 +4,8 @@
 /// overrides on out-of-tree users of libutil, as we'd be required to when
 /// specializing templates in the nlohmann namespace. nlohmann::json can't
 /// deal with `std::optional<T>` types until 3.11.3 at, and we need those.
+/// We also customize enum serialization to not automatically cast to int;
+/// nlohmann can be told to disable this only via a special global define.
 
 #include "lix/libutil/json-fwd.hh" // IWYU pragma: keep
 #include <concepts>
@@ -30,7 +32,7 @@ namespace json {
  */
 template<typename T>
 struct avoids_null
-    : std::bool_constant<std::is_integral_v<T> || std::is_floating_point_v<T> || std::is_enum_v<T>>
+    : std::bool_constant<std::is_integral_v<T> || std::is_floating_point_v<T> || IntegralEnum<T>>
 {};
 
 template<>
@@ -104,6 +106,20 @@ struct adl_serializer<T, void>
     {
         return T::from_json(std::forward<Json>(j));
     }
+
+    template<typename Json>
+        requires IntegralEnum<T>
+    static void to_json(Json && json, const T & value)
+    {
+        json = static_cast<std::underlying_type_t<T>>(value);
+    }
+
+    template<typename Json>
+        requires IntegralEnum<T>
+    static void from_json(const Json & json, T & value)
+    {
+        value = static_cast<T>(json.template get<std::underlying_type_t<T>>());
+    }
 };
 
 template<typename T>

lix/libutil/logging.hh

@@ -44,6 +44,9 @@ typedef enum {
     actBuildWaiting = 111,
 } ActivityType;
 
+template<>
+struct json::is_integral_enum<ActivityType> : std::true_type {};
+
 typedef enum {
     /** Fields:
      * 0: int: bytes linked
@@ -82,6 +85,9 @@ typedef enum {
     resPostBuildLogLine = 107,
 } ResultType;
 
+template<>
+struct json::is_integral_enum<ResultType> : std::true_type {};
+
 typedef uint64_t ActivityId;
 
 struct LoggerSettings : Config

meson.build

@@ -518,6 +518,9 @@ add_project_arguments(
   '-Wdeprecated-copy',
   '-Wignored-qualifiers',
   '-Werror=suggest-override',
+  # don't let nlohmann automatically cast enums to ints. we don't set this
+  # inside libutil headers to not force this behavior on users of libutil.
+  '-DJSON_DISABLE_ENUM_SERIALIZATION=1',
   language : 'cpp',
 )
 

tests/unit/libutil/config.cc

@@ -1,5 +1,7 @@
 #include "lix/libutil/config.hh"
 #include "lix/libutil/args.hh"
+#include "lix/libutil/deprecated-features.hh"
+#include "lix/libutil/experimental-features.hh"
 #include "lix/libutil/file-system.hh"
 #include "lix/libutil/environment-variables.hh"
 #include "lix/libutil/json.hh"
@@ -323,4 +325,23 @@ namespace nix {
         ASSERT_THROW(config.applyConfig("value == key"), UsageError);
         ASSERT_THROW(config.applyConfig("value "), UsageError);
     }
+
+/* ----------------------------------------------------------------------------
+ * Config enums
+ * --------------------------------------------------------------------------*/
+TEST(Config, regression_738)
+{
+    // feature enums serialized as integers because bitmaks type serializers
+    // were forgotten and nlohmann casted then to ints automatically. -sigh-
+    ASSERT_EQ(JSON(ExperimentalFeature::AutoAllocateUids).dump(), "\"auto-allocate-uids\"");
+    ASSERT_EQ(
+        JSON((ExperimentalFeatures{} | ExperimentalFeature::AutoAllocateUids)).dump(),
+        "[\"auto-allocate-uids\"]"
+    );
+
+    ASSERT_EQ(JSON(DeprecatedFeature::AncientLet).dump(), "\"ancient-let\"");
+    ASSERT_EQ(
+        JSON((DeprecatedFeatures{} | DeprecatedFeature::AncientLet)).dump(), "[\"ancient-let\"]"
+    );
+}
 }