blob: 0d38babfe554310eb6367a152f86c9e34f09da03 [file] [log] [blame]
// Copyright 2015 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include <atomic>
#include <functional>
#include <map>
#include <memory>
#include <string>
#include <utility>
#include <vector>
#include "base/base_export.h"
#include "base/compiler_specific.h"
#include "base/containers/flat_map.h"
#include "base/dcheck_is_on.h"
#include "base/feature_list_buildflags.h"
#include "base/gtest_prod_util.h"
#include "base/logging.h"
#include "base/memory/raw_ptr.h"
#include "base/strings/string_piece.h"
#include "base/synchronization/lock.h"
#include "third_party/abseil-cpp/absl/types/optional.h"
namespace base {
class FieldTrial;
class FieldTrialList;
class PersistentMemoryAllocator;
// Specifies whether a given feature is enabled or disabled by default.
// NOTE: The actual runtime state may be different, due to a field trial or a
// command line switch.
enum FeatureState {
// Recommended macros for declaring and defining features:
// - `kFeature` is the C++ identifier that will be used for the `base::Feature`.
// - `name` is the feature name, which must be globally unique. This name is
// used to enable/disable features via experiments and command-line flags.
// Names should use CamelCase-style naming, e.g. "MyGreatFeature".
// - `default_state` is the default state to use for the feature, i.e.
// As noted above, the actual runtime state may differ from the default state,
// due to field trials or command-line switches.
// Provides a forward declaration for `kFeature` in a header file, e.g.
// If the feature needs to be marked as exported, i.e. it is referenced by
// multiple components, then write:
#define BASE_DECLARE_FEATURE(kFeature) \
extern CONSTINIT const base::Feature kFeature
// Provides a definition for `kFeature` with `name` and `default_state`, e.g.
// Features should *not* be defined in header files; do not use this macro in
// header files.
#define BASE_FEATURE(feature, name, default_state) \
CONSTINIT const base::Feature feature(name, default_state)
// The Feature struct is used to define the default state for a feature. There
// must only ever be one struct instance for a given feature name—generally
// defined as a constant global variable or file static. Declare and define
// features using the `BASE_DECLARE_FEATURE()` and `BASE_FEATURE()` macros
// above, as there are some subtleties involved.
// Feature constants are internally mutable, as this allows them to contain a
// mutable member to cache their override state, while still remaining declared
// as const. This cache member allows for significantly faster IsEnabled()
// checks.
// However, the "Mutable Constants" check [1] detects this as a regression,
// because this usually means that a readonly symbol is put in writable memory
// when readonly memory would be more efficient.
// The performance gains of the cache are large enough to offset the downsides
// to having the symbols in bssdata rather than rodata. Use LOGICALLY_CONST to
// suppress the "Mutable Constants" check.
// [1]:
constexpr Feature(const char* name, FeatureState default_state)
: name(name), default_state(default_state) {
if (StringPiece(name).find(BUILDFLAG(BANNED_BASE_FEATURE_PREFIX)) == 0) {
LOG(FATAL) << "Invalid feature name " << name << " starts with "
// This object needs to be copyable because of some signatures in
// ScopedFeatureList, but generally isn't copied anywhere except unit tests.
// The `cached_value` doesn't get copied and copies will trigger a lookup if
// their state is queried.
Feature(const Feature& other)
: name(, default_state(other.default_state), cached_value(0) {}
// The name of the feature. This should be unique to each feature and is used
// for enabling/disabling features via command line flags and experiments.
// It is strongly recommended to use CamelCase style for feature names, e.g.
// "MyGreatFeature".
const char* const name;
// The default state (i.e. enabled or disabled) for this feature.
// NOTE: The actual runtime state may be different, due to a field trial or a
// command line switch.
const FeatureState default_state;
friend class FeatureList;
// A packed value where the first 8 bits represent the `OverrideState` of this
// feature, and the last 16 bits are a caching context ID used to allow
// ScopedFeatureLists to invalidate these cached values in testing. A value of
// 0 in the caching context ID field indicates that this value has never been
// looked up and cached, a value of 1 indicates this value contains the cached
// `OverrideState` that was looked up via `base::FeatureList`, and any other
// value indicate that this cached value is only valid for a particular
// ScopedFeatureList instance.
// Packing these values into a uint32_t makes it so that atomic operations
// performed on this fields can be lock free.
// The override state stored in this field is only used if the current
// `FeatureList::caching_context_` field is equal to the lower 16 bits of the
// packed cached value. Otherwise, the override state is looked up in the
// feature list and the cache is updated.
mutable std::atomic<uint32_t> cached_value = 0;
// DCHECKs have been built-in, and are configurable at run-time to be fatal, or
// not, via a DcheckIsFatal feature. We define the Feature here since it is
// checked in FeatureList::SetInstance(). See
// The FeatureList class is used to determine whether a given feature is on or
// off. It provides an authoritative answer, taking into account command-line
// overrides and experimental control.
// The basic use case is for any feature that can be toggled (e.g. through
// command-line or an experiment) to have a defined Feature struct, e.g.:
// const base::Feature kMyGreatFeature {
// "MyGreatFeature", base::FEATURE_ENABLED_BY_DEFAULT
// };
// Then, client code that wishes to query the state of the feature would check:
// if (base::FeatureList::IsEnabled(kMyGreatFeature)) {
// // Feature code goes here.
// }
// Behind the scenes, the above call would take into account any command-line
// flags to enable or disable the feature, any experiments that may control it
// and finally its default state (in that order of priority), to determine
// whether the feature is on.
// Features can be explicitly forced on or off by specifying a list of comma-
// separated feature names via the following command-line flags:
// --enable-features=Feature5,Feature7
// --disable-features=Feature1,Feature2,Feature3
// To enable/disable features in a test, do NOT append --enable-features or
// --disable-features to the command-line directly. Instead, use
// ScopedFeatureList. See base/test/scoped_feature_list.h for details.
// After initialization (which should be done single-threaded), the FeatureList
// API is thread safe.
// Note: This class is a singleton, but does not use base/memory/singleton.h in
// order to have control over its initialization sequence. Specifically, the
// intended use is to create an instance of this class and fully initialize it,
// before setting it as the singleton for a process, via SetInstance().
class BASE_EXPORT FeatureList {
FeatureList(const FeatureList&) = delete;
FeatureList& operator=(const FeatureList&) = delete;
// Used by common test fixture classes to prevent abuse of ScopedFeatureList
// after multiple threads have started.
class BASE_EXPORT ScopedDisallowOverrides {
explicit ScopedDisallowOverrides(const char* reason);
ScopedDisallowOverrides(const ScopedDisallowOverrides&) = delete;
ScopedDisallowOverrides& operator=(const ScopedDisallowOverrides&) = delete;
const char* const previous_reason_;
// Specifies whether a feature override enables or disables the feature.
enum OverrideState {
// Accessor class, used to look up features by _name_ rather than by Feature
// object.
// Should only be used in limited cases. See ConstructAccessor() for details.
class BASE_EXPORT Accessor {
Accessor(const Accessor&) = delete;
Accessor& operator=(const Accessor&) = delete;
// Looks up the feature, returning only its override state, rather than
// falling back on a default value (since there is no default value given).
// Callers of this MUST ensure that there is a consistent, compile-time
// default value associated.
FeatureList::OverrideState GetOverrideStateByFeatureName(
StringPiece feature_name);
// Look up the feature, and, if present, populate |params|.
// See GetFieldTrialParams in field_trial_params.h for more documentation.
bool GetParamsByFeatureName(StringPiece feature_name,
std::map<std::string, std::string>* params);
// Allow FeatureList to construct this class.
friend class FeatureList;
explicit Accessor(FeatureList* feature_list);
// Unowned pointer to the FeatureList object we use to look up feature
// enablement.
raw_ptr<FeatureList> feature_list_;
// Describes a feature override. The first member is a Feature that will be
// overridden with the state given by the second member.
using FeatureOverrideInfo =
std::pair<const std::reference_wrapper<const Feature>, OverrideState>;
// Initializes feature overrides via command-line flags `--enable-features=`
// and `--disable-features=`, each of which is a comma-separated list of
// features to enable or disable, respectively. This function also allows
// users to set a feature's field trial params via `--enable-features=`. Must
// only be invoked during the initialization phase (before
// FinalizeInitialization() has been called).
// If a feature appears on both lists, then it will be disabled. If
// a list entry has the format "FeatureName<TrialName" then this
// initialization will also associate the feature state override with the
// named field trial, if it exists. If a list entry has the format
// "FeatureName:k1/v1/k2/v2", "FeatureName<TrialName:k1/v1/k2/v2" or
// "FeatureName<TrialName.GroupName:k1/v1/k2/v2" then this initialization will
// also associate the feature state override with the named field trial and
// its params. If the feature params part is provided but trial and/or group
// isn't, this initialization will also create a synthetic trial, named
// "Study" followed by the feature name, i.e. "StudyFeature", and group, named
// "Group" followed by the feature name, i.e. "GroupFeature", for the params.
// If a feature name is prefixed with the '*' character, it will be created
// with OVERRIDE_USE_DEFAULT - which is useful for associating with a trial
// while using the default state.
void InitializeFromCommandLine(const std::string& enable_features,
const std::string& disable_features);
// Initializes feature overrides through the field trial allocator, which
// we're using to store the feature names, their override state, and the name
// of the associated field trial.
void InitializeFromSharedMemory(PersistentMemoryAllocator* allocator);
// Returns true if the state of |feature_name| has been overridden (regardless
// of whether the overridden value is the same as the default value) for any
// reason (e.g. command line or field trial).
bool IsFeatureOverridden(const std::string& feature_name) const;
// Returns true if the state of |feature_name| has been overridden via
// |InitializeFromCommandLine()|. This includes features explicitly
// disabled/enabled with --disable-features and --enable-features, as well as
// any extra feature overrides that depend on command line switches.
bool IsFeatureOverriddenFromCommandLine(
const std::string& feature_name) const;
// Returns true if the state |feature_name| has been overridden by
// |InitializeFromCommandLine()| and the state matches |state|.
bool IsFeatureOverriddenFromCommandLine(const std::string& feature_name,
OverrideState state) const;
// Associates a field trial for reporting purposes corresponding to the
// command-line setting the feature state to |for_overridden_state|. The trial
// will be activated when the state of the feature is first queried. This
// should be called during registration, after InitializeFromCommandLine() has
// been called but before the instance is registered via SetInstance().
void AssociateReportingFieldTrial(const std::string& feature_name,
OverrideState for_overridden_state,
FieldTrial* field_trial);
// Registers a field trial to override the enabled state of the specified
// feature to |override_state|. Command-line overrides still take precedence
// over field trials, so this will have no effect if the feature is being
// overridden from the command-line. The associated field trial will be
// activated when the feature state for this feature is queried. This should
// be called during registration, after InitializeFromCommandLine() has been
// called but before the instance is registered via SetInstance().
void RegisterFieldTrialOverride(const std::string& feature_name,
OverrideState override_state,
FieldTrial* field_trial);
// Adds extra overrides (not associated with a field trial). Should be called
// before SetInstance().
// The ordering of calls with respect to InitializeFromCommandLine(),
// RegisterFieldTrialOverride(), etc. matters. The first call wins out,
// because the |overrides_| map uses insert(), which retains the first
// inserted entry and does not overwrite it on subsequent calls to insert().
void RegisterExtraFeatureOverrides(
const std::vector<FeatureOverrideInfo>& extra_overrides);
// Loops through feature overrides and serializes them all into |allocator|.
void AddFeaturesToAllocator(PersistentMemoryAllocator* allocator);
// Returns comma-separated lists of feature names (in the same format that is
// accepted by InitializeFromCommandLine()) corresponding to features that
// have been overridden - either through command-line or via FieldTrials. For
// those features that have an associated FieldTrial, the output entry will be
// of the format "FeatureName<TrialName" (|include_group_name|=false) or
// "FeatureName<TrialName.GroupName" (if |include_group_name|=true), where
// "TrialName" is the name of the FieldTrial and "GroupName" is the group
// name of the FieldTrial. Features that have overrides with
// OVERRIDE_USE_DEFAULT will be added to |enable_overrides| with a '*'
// character prefix. Must be called only after the instance has been
// initialized and registered.
void GetFeatureOverrides(std::string* enable_overrides,
std::string* disable_overrides,
bool include_group_names = false) const;
// Like GetFeatureOverrides(), but only returns overrides that were specified
// explicitly on the command-line, omitting the ones from field trials.
void GetCommandLineFeatureOverrides(std::string* enable_overrides,
std::string* disable_overrides) const;
// Returns the field trial associated with the given feature |name|. Used for
// getting the FieldTrial without requiring a struct Feature.
base::FieldTrial* GetAssociatedFieldTrialByFeatureName(
StringPiece name) const;
// DO NOT USE outside of internal field trial implementation code. Instead use
// GetAssociatedFieldTrialByFeatureName(), which performs some additional
// validation.
// Returns whether the given feature |name| is associated with a field trial.
// If the given feature |name| does not exist, return false. Unlike
// GetAssociatedFieldTrialByFeatureName(), this function must be called during
// |FeatureList| initialization; the returned value will report whether the
// provided |name| has been used so far.
bool HasAssociatedFieldTrialByFeatureName(StringPiece name) const;
// Get associated field trial for the given feature |name| only if override
// enables it.
FieldTrial* GetEnabledFieldTrialByFeatureName(StringPiece name) const;
// Construct an accessor allowing access to GetOverrideStateByFeatureName().
// This can only be called before the FeatureList is initialized, and is
// intended for very narrow use.
// If you're tempted to use it, do so only in consultation with feature_list
std::unique_ptr<Accessor> ConstructAccessor();
// Returns whether the given |feature| is enabled. Must only be called after
// the singleton instance has been registered via SetInstance(). Additionally,
// a feature with a given name must only have a single corresponding Feature
// struct, which is checked in builds with DCHECKs enabled.
static bool IsEnabled(const Feature& feature);
// If the given |feature| is overridden, returns its enabled state; otherwise,
// returns an empty optional. Must only be called after the singleton instance
// has been registered via SetInstance(). Additionally, a feature with a given
// name must only have a single corresponding Feature struct, which is checked
// in builds with DCHECKs enabled.
static absl::optional<bool> GetStateIfOverridden(const Feature& feature);
// Returns the field trial associated with the given |feature|. Must only be
// called after the singleton instance has been registered via SetInstance().
static FieldTrial* GetFieldTrial(const Feature& feature);
// Splits a comma-separated string containing feature names into a vector. The
// resulting pieces point to parts of |input|.
static std::vector<base::StringPiece> SplitFeatureListString(
base::StringPiece input);
// Checks and parses the |enable_feature| (e.g.
// FeatureName<Study.Group:Param1/value1/) obtained by applying
// SplitFeatureListString() to the |enable_features| flag, and sets
// |feature_name| to be the feature's name, |study_name| and |group_name| to
// be the field trial name and its group name if the field trial is specified
// or field trial parameters are given, |params| to be the field trial
// parameters if exists.
static bool ParseEnableFeatureString(StringPiece enable_feature,
std::string* feature_name,
std::string* study_name,
std::string* group_name,
std::string* params);
// Initializes and sets an instance of FeatureList with feature overrides via
// command-line flags |enable_features| and |disable_features| if one has not
// already been set from command-line flags. Returns true if an instance did
// not previously exist. See InitializeFromCommandLine() for more details
// about |enable_features| and |disable_features| parameters.
static bool InitializeInstance(const std::string& enable_features,
const std::string& disable_features);
// Like the above, but also adds extra overrides. If a feature appears in
// |extra_overrides| and also |enable_features| or |disable_features|, the
// disable/enable will supersede the extra overrides.
static bool InitializeInstance(
const std::string& enable_features,
const std::string& disable_features,
const std::vector<FeatureOverrideInfo>& extra_overrides);
// Returns the singleton instance of FeatureList. Will return null until an
// instance is registered via SetInstance().
static FeatureList* GetInstance();
// Registers the given |instance| to be the singleton feature list for this
// process. This should only be called once and |instance| must not be null.
// Note: If you are considering using this for the purposes of testing, take
// a look at using base/test/scoped_feature_list.h instead.
static void SetInstance(std::unique_ptr<FeatureList> instance);
// Clears the previously-registered singleton instance for tests and returns
// the old instance.
// Note: Most tests should never call this directly. Instead consider using
// base::test::ScopedFeatureList.
static std::unique_ptr<FeatureList> ClearInstanceForTesting();
// Sets a given (initialized) |instance| to be the singleton feature list,
// for testing. Existing instance must be null. This is primarily intended
// to support base::test::ScopedFeatureList helper class.
static void RestoreInstanceForTesting(std::unique_ptr<FeatureList> instance);
// On some platforms, the base::FeatureList singleton might be duplicated to
// more than one module. If this function is called, then using base::Feature
// API will result in DCHECK if accessed from the same module as the callee.
// Has no effect if DCHECKs are not enabled.
static void ForbidUseForCurrentModule();
void SetCachingContextForTesting(uint16_t caching_context);
FRIEND_TEST_ALL_PREFIXES(FeatureListTest, CheckFeatureIdentity);
// Allow Accessor to access GetOverrideStateByFeatureName().
friend class Accessor;
struct OverrideEntry {
// The overridden enable (on/off) state of the feature.
OverrideState overridden_state;
// An optional associated field trial, which will be activated when the
// state of the feature is queried for the first time. Weak pointer to the
// FieldTrial object that is owned by the FieldTrialList singleton.
base::FieldTrial* field_trial;
// Specifies whether the feature's state is overridden by |field_trial|.
// If it's not, and |field_trial| is not null, it means it is simply an
// associated field trial for reporting purposes (and |overridden_state|
// came from the command-line).
bool overridden_by_field_trial;
// TODO(asvitkine): Expand this as more support is added.
// Constructs an OverrideEntry for the given |overridden_state|. If
// |field_trial| is not null, it implies that |overridden_state| comes from
// the trial, so |overridden_by_field_trial| will be set to true.
OverrideEntry(OverrideState overridden_state, FieldTrial* field_trial);
// Returns the override for the field trial associated with the given feature
// |name| or null if the feature is not found.
const base::FeatureList::OverrideEntry* GetOverrideEntryByFeatureName(
StringPiece name) const;
// Finalizes the initialization state of the FeatureList, so that no further
// overrides can be registered. This is called by SetInstance() on the
// singleton feature list that is being registered.
void FinalizeInitialization();
// Returns whether the given |feature| is enabled. This is invoked by the
// public FeatureList::IsEnabled() static function on the global singleton.
// Requires the FeatureList to have already been fully initialized.
bool IsFeatureEnabled(const Feature& feature) const;
// Returns whether the given |feature| is enabled. This is invoked by the
// public FeatureList::GetStateIfOverridden() static function on the global
// singleton. Requires the FeatureList to have already been fully initialized.
absl::optional<bool> IsFeatureEnabledIfOverridden(
const Feature& feature) const;
// Returns the override state of a given |feature|. If the feature was not
// overridden, returns OVERRIDE_USE_DEFAULT. Performs any necessary callbacks
// for when the feature state has been observed, e.g. activating field trials.
OverrideState GetOverrideState(const Feature& feature) const;
// Same as GetOverrideState(), but without a default value.
OverrideState GetOverrideStateByFeatureName(StringPiece feature_name) const;
// Returns the field trial associated with the given |feature|. This is
// invoked by the public FeatureList::GetFieldTrial() static function on the
// global singleton. Requires the FeatureList to have already been fully
// initialized.
base::FieldTrial* GetAssociatedFieldTrial(const Feature& feature) const;
// For each feature name in comma-separated list of strings |feature_list|,
// registers an override with the specified |overridden_state|. Also, will
// associate an optional named field trial if the entry is of the format
// "FeatureName<TrialName".
void RegisterOverridesFromCommandLine(const std::string& feature_list,
OverrideState overridden_state);
// Registers an override for feature |feature_name|. The override specifies
// whether the feature should be on or off (via |overridden_state|), which
// will take precedence over the feature's default state. If |field_trial| is
// not null, registers the specified field trial object to be associated with
// the feature, which will activate the field trial when the feature state is
// queried. If an override is already registered for the given feature, it
// will not be changed.
void RegisterOverride(StringPiece feature_name,
OverrideState overridden_state,
FieldTrial* field_trial);
// Implementation of GetFeatureOverrides() with a parameter that specifies
// whether only command-line enabled overrides should be emitted. See that
// function's comments for more details.
void GetFeatureOverridesImpl(std::string* enable_overrides,
std::string* disable_overrides,
bool command_line_only,
bool include_group_name = false) const;
// Verifies that there's only a single definition of a Feature struct for a
// given feature name. Keeps track of the first seen Feature struct for each
// feature. Returns false when called on a Feature struct with a different
// address than the first one it saw for that feature name. Used only from
// DCHECKs and tests. This is const because it's called from const getters and
// doesn't modify externally visible state.
bool CheckFeatureIdentity(const Feature& feature) const;
// Map from feature name to an OverrideEntry struct for the feature, if it
// exists.
base::flat_map<std::string, OverrideEntry> overrides_;
// Locked map that keeps track of seen features, to ensure a single feature is
// only defined once. This verification is only done in builds with DCHECKs
// enabled. This is mutable as it's not externally visible and needs to be
// usable from const getters.
mutable Lock feature_identity_tracker_lock_;
mutable std::map<std::string, const Feature*> feature_identity_tracker_
// Tracks the associated FieldTrialList for DCHECKs. This is used to catch
// the scenario where multiple FieldTrialList are used with the same
// FeatureList - which can lead to overrides pointing to invalid FieldTrial
// objects.
raw_ptr<base::FieldTrialList> field_trial_list_ = nullptr;
// Whether this object has been fully initialized. This gets set to true as a
// result of FinalizeInitialization().
bool initialized_ = false;
// Whether this object has been initialized from command line.
bool initialized_from_command_line_ = false;
// Used when querying `base::Feature` state to determine if the cached value
// in the `Feature` object is populated and valid. See the comment on
// `base::Feature::cached_value` for more details.
uint16_t caching_context_ = 1;
} // namespace base