base/test/scoped_feature_list.h

// Copyright 2016 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_TEST_SCOPED_FEATURE_LIST_H_
#define BASE_TEST_SCOPED_FEATURE_LIST_H_

#include <memory>
#include <string>
#include <vector>

#include "base/compiler_specific.h"
#include "base/containers/flat_map.h"
#include "base/feature_list.h"
#include "base/memory/raw_ptr.h"
#include "base/memory/raw_ref.h"
#include "base/metrics/field_trial.h"
#include "base/metrics/field_trial_params.h"
#include "base/types/pass_key.h"

namespace base::test {

// A reference to a base::Feature and field trial params that should be force
// enabled and overwritten for test purposes.
struct FeatureRefAndParams {
  FeatureRefAndParams(const Feature& feature LIFETIME_BOUND,
                      const FieldTrialParams& params);

  FeatureRefAndParams(const FeatureRefAndParams& other);

  ~FeatureRefAndParams();

  const raw_ref<const Feature> feature;
  const FieldTrialParams params;
};

// A lightweight wrapper for a reference to a base::Feature. Allows lists of
// features to be enabled/disabled to be easily passed without actually copying
// the underlying base::Feature. Actual C++ references do not work well for this
// purpose, as vectors of references are disallowed.
class FeatureRef {
 public:
  // NOLINTNEXTLINE(google-explicit-constructor)
  FeatureRef(const Feature& feature LIFETIME_BOUND) : feature_(feature) {}

  const Feature& operator*() const { return *feature_; }
  const Feature* operator->() const { return &*feature_; }

 private:
  friend bool operator==(const FeatureRef& lhs, const FeatureRef& rhs) {
    return &*lhs == &*rhs;
  }
  friend bool operator<(const FeatureRef& lhs, const FeatureRef& rhs) {
    return &*lhs < &*rhs;
  }

  raw_ref<const Feature> feature_;
};

// ScopedFeatureList resets the global FeatureList instance to a new instance
// and restores the original instance upon destruction. Whether the existing
// FeatureList state is kept or discarded depends on the `Init` method called.
// When using the non-deprecated APIs, a corresponding FieldTrialList is also
// created.
//
// Note: Re-using the same object is allowed. To reset the feature list and
// initialize it anew, call `Reset` and then one of the `Init` methods.
//
// If multiple instances of this class are used in a nested fashion, they
// should be destroyed in the opposite order of their Init*() methods being
// called.
//
// ScopedFeatureList needs to be initialized on the main thread (via one of
// Init*() methods) before running code that inspects the state of features,
// such as in the constructor of the test harness.
//
// WARNING: To be clear, in multithreaded test environments (such as browser
// tests) there may background threads using FeatureList before the test body is
// even entered. In these cases it is imperative that ScopedFeatureList be
// initialized BEFORE those threads are started, hence the recommendation to do
// initialization in the test harness's constructor.
class ScopedFeatureList final {
 public:
  struct Features;
  struct FeatureWithStudyGroup;

  // Constructs the instance in a non-initialized state.
  ScopedFeatureList();

  // Shorthand for immediately initializing with InitAndEnableFeature().
  explicit ScopedFeatureList(const Feature& enable_feature);

  ScopedFeatureList(const ScopedFeatureList&) = delete;
  ScopedFeatureList& operator=(const ScopedFeatureList&) = delete;

  ~ScopedFeatureList();

  // Resets the instance to a non-initialized state.
  void Reset();

  // Initializes and registers a FeatureList instance without any additional
  // enabled or disabled features. Existing state, if any, will be kept.
  // This is equivalent to calling InitWithFeatures({}, {}).
  void Init();

  // Initializes a FeatureList instance without any additional enabled or
  // disabled features. Existing state, if any, will be discarded.
  // Using this function is not generally recommended, as doing so in a test
  // removes the ability to run the test while passing additional
  // --enable-features flags from the command line.
  void InitWithEmptyFeatureAndFieldTrialLists();

  // Initializes a FeatureList instance and FieldTrialLists to be null and
  // clear all field trial parameters.
  // WARNING: This should not be generally used except for tests that require
  // manually instantiating objects like FieldTrialList, for example when
  // mocking an EntropyProvider.
  void InitWithNullFeatureAndFieldTrialLists();

  // WARNING: This method will reset any globally configured features to their
  // default values, which can hide feature interaction bugs. Please use
  // sparingly.  https://crbug.com/713390
  // Initializes and registers the given FeatureList instance.
  void InitWithFeatureList(std::unique_ptr<FeatureList> feature_list);

  // Initializes and registers a FeatureList instance based on the current
  // FeatureList and overridden with the given enabled features and the
  // specified field trial parameters, and the given disabled features
  // with the given enabled and disabled features (comma-separated names).
  // Note: This creates a scoped global field trial list if there is not
  // currently one.
  void InitFromCommandLine(const std::string& enable_features,
                           const std::string& disable_features);

  // Initializes and registers a FeatureList instance based on the current
  // FeatureList and overridden with the given enabled and disabled features.
  // Any feature overrides already present in the global FeatureList will
  // continue to apply, unless they conflict with the overrides passed into this
  // method. This is important for testing potentially unexpected feature
  // interactions.
  void InitWithFeatures(const std::vector<FeatureRef>& enabled_features,
                        const std::vector<FeatureRef>& disabled_features);

  // Initializes and registers a FeatureList instance based on the current
  // FeatureList and overridden with single enabled feature.
  void InitAndEnableFeature(const Feature& feature);

  // Initializes and registers a FeatureList instance based on the current
  // FeatureList and overridden with single enabled feature and associated field
  // trial parameters.
  // Note: this creates a scoped global field trial list if there is not
  // currently one.
  void InitAndEnableFeatureWithParameters(
      const Feature& feature,
      const FieldTrialParams& feature_parameters);

  // Initializes and registers a FeatureList instance based on the current
  // FeatureList and overridden with the given enabled features and the
  // specified field trial parameters, and the given disabled features.
  // Note: This creates a scoped global field trial list if there is not
  // currently one.
  void InitWithFeaturesAndParameters(
      const std::vector<FeatureRefAndParams>& enabled_features,
      const std::vector<FeatureRef>& disabled_features);

  // Initializes and registers a FeatureList instance based on the current
  // FeatureList and overridden with single disabled feature.
  void InitAndDisableFeature(const Feature& feature);

  // Initializes and registers a FeatureList instance based on the current
  // FeatureList and overridden with a single feature either enabled or
  // disabled depending on |enabled|.
  void InitWithFeatureState(const Feature& feature, bool enabled);

  // Same as `InitWithFeatureState()`, but supports multiple features at a time.
  // `feature_states` - a map where the keys are features and the values are
  //                    their overridden states (`false` for force-disabled,
  //                    `true` for force-enabled).
  void InitWithFeatureStates(const flat_map<FeatureRef, bool>& feature_states);

 private:
  using PassKey = base::PassKey<ScopedFeatureList>;

  // Initializes and registers a FeatureList instance based on the current
  // FeatureList and overridden with the given enabled and disabled features.
  // Any feature overrides already present in the global FeatureList will
  // continue to apply, unless they conflict with the overrides passed into this
  // method.
  // Features to enable may be specified through either |enabled_features| or
  // |enabled_feature_and_params|, but not both (i.e. one of these must be
  // empty).
  void InitWithFeaturesImpl(
      const std::vector<FeatureRef>& enabled_features,
      const std::vector<FeatureRefAndParams>& enabled_features_and_params,
      const std::vector<FeatureRef>& disabled_features,
      bool keep_existing_states = true);

  // Initializes and registers a FeatureList instance based on the current
  // FeatureList and overridden with the given enabled and disabled features.
  // Any feature overrides already present in the global FeatureList will
  // continue to apply, unless they conflict with the overrides passed into this
  // method.
  // If |create_associated_field_trials| is true, associated field trials are
  // always created independent of feature parameters. If false, field trials
  // for features whose parameters are specified will be created.
  // If |keep_existing_states| is true, keep all states and override them
  // according to the |merged_features|. Otherwise, clear all states and
  // newly initialize all states with |merged_features|.
  void InitWithMergedFeatures(Features&& merged_features,
                              bool create_associated_field_trials,
                              bool keep_existing_states);

  bool init_called_ = false;
  std::unique_ptr<FeatureList> original_feature_list_;
  raw_ptr<base::FieldTrialList> original_field_trial_list_ = nullptr;
  std::string original_params_;
  std::unique_ptr<base::FieldTrialList> field_trial_list_;
};

}  // namespace base::test

#endif  // BASE_TEST_SCOPED_FEATURE_LIST_H_