362 lines
17 KiB
C
362 lines
17 KiB
C
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// Copyright 2015 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_FEATURE_LIST_H_
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#define BASE_FEATURE_LIST_H_
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#include <functional>
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#include <map>
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#include <memory>
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#include <string>
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#include <vector>
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#include "base/base_export.h"
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#include "base/gtest_prod_util.h"
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#include "base/macros.h"
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#include "base/metrics/persistent_memory_allocator.h"
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#include "base/strings/string_piece.h"
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#include "base/synchronization/lock.h"
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namespace base {
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class FieldTrial;
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class FieldTrialList;
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// Specifies whether a given feature is enabled or disabled by default.
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// NOTE: The actual runtime state may be different, due to a field trial or a
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// command line switch.
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enum FeatureState {
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FEATURE_DISABLED_BY_DEFAULT,
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FEATURE_ENABLED_BY_DEFAULT,
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};
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// The Feature struct is used to define the default state for a feature. See
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// comment below for more details. There must only ever be one struct instance
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// for a given feature name - generally defined as a constant global variable or
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// file static. It should never be used as a constexpr as it breaks
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// pointer-based identity lookup.
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struct BASE_EXPORT Feature {
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// The name of the feature. This should be unique to each feature and is used
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// for enabling/disabling features via command line flags and experiments.
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// It is strongly recommended to use CamelCase style for feature names, e.g.
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// "MyGreatFeature".
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const char* const name;
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// The default state (i.e. enabled or disabled) for this feature.
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// NOTE: The actual runtime state may be different, due to a field trial or a
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// command line switch.
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const FeatureState default_state;
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};
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#if defined(DCHECK_IS_CONFIGURABLE)
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// DCHECKs have been built-in, and are configurable at run-time to be fatal, or
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// not, via a DcheckIsFatal feature. We define the Feature here since it is
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// checked in FeatureList::SetInstance(). See https://crbug.com/596231.
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extern BASE_EXPORT const Feature kDCheckIsFatalFeature;
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#endif // defined(DCHECK_IS_CONFIGURABLE)
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// The FeatureList class is used to determine whether a given feature is on or
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// off. It provides an authoritative answer, taking into account command-line
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// overrides and experimental control.
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//
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// The basic use case is for any feature that can be toggled (e.g. through
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// command-line or an experiment) to have a defined Feature struct, e.g.:
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//
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// const base::Feature kMyGreatFeature {
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// "MyGreatFeature", base::FEATURE_ENABLED_BY_DEFAULT
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// };
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//
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// Then, client code that wishes to query the state of the feature would check:
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//
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// if (base::FeatureList::IsEnabled(kMyGreatFeature)) {
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// // Feature code goes here.
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// }
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//
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// Behind the scenes, the above call would take into account any command-line
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// flags to enable or disable the feature, any experiments that may control it
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// and finally its default state (in that order of priority), to determine
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// whether the feature is on.
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//
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// Features can be explicitly forced on or off by specifying a list of comma-
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// separated feature names via the following command-line flags:
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//
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// --enable-features=Feature5,Feature7
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// --disable-features=Feature1,Feature2,Feature3
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//
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// To enable/disable features in a test, do NOT append --enable-features or
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// --disable-features to the command-line directly. Instead, use
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// ScopedFeatureList. See base/test/scoped_feature_list.h for details.
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//
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// After initialization (which should be done single-threaded), the FeatureList
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// API is thread safe.
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//
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// Note: This class is a singleton, but does not use base/memory/singleton.h in
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// order to have control over its initialization sequence. Specifically, the
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// intended use is to create an instance of this class and fully initialize it,
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// before setting it as the singleton for a process, via SetInstance().
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class BASE_EXPORT FeatureList {
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public:
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FeatureList();
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~FeatureList();
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// Used by common test fixture classes to prevent abuse of ScopedFeatureList
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// after multiple threads have started.
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class BASE_EXPORT ScopedDisallowOverrides {
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public:
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explicit ScopedDisallowOverrides(const char* reason);
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~ScopedDisallowOverrides();
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private:
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#if DCHECK_IS_ON()
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const char* const previous_reason_;
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#endif
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DISALLOW_COPY_AND_ASSIGN(ScopedDisallowOverrides);
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};
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// Specifies whether a feature override enables or disables the feature.
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enum OverrideState {
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OVERRIDE_USE_DEFAULT,
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OVERRIDE_DISABLE_FEATURE,
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OVERRIDE_ENABLE_FEATURE,
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};
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// Describes a feature override. The first member is a Feature that will be
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// overridden with the state given by the second member.
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using FeatureOverrideInfo =
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std::pair<const std::reference_wrapper<const Feature>, OverrideState>;
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// Initializes feature overrides via command-line flags |enable_features| and
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// |disable_features|, each of which is a comma-separated list of features to
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// enable or disable, respectively. If a feature appears on both lists, then
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// it will be disabled. If a list entry has the format "FeatureName<TrialName"
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// then this initialization will also associate the feature state override
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// with the named field trial, if it exists. If a feature name is prefixed
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// with the '*' character, it will be created with OVERRIDE_USE_DEFAULT -
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// which is useful for associating with a trial while using the default state.
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// Must only be invoked during the initialization phase (before
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// FinalizeInitialization() has been called).
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void InitializeFromCommandLine(const std::string& enable_features,
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const std::string& disable_features);
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// Initializes feature overrides through the field trial allocator, which
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// we're using to store the feature names, their override state, and the name
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// of the associated field trial.
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void InitializeFromSharedMemory(PersistentMemoryAllocator* allocator);
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// Returns true if the state of |feature_name| has been overridden via
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// |InitializeFromCommandLine()|. This includes features explicitly
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// disabled/enabled with --disable-features and --enable-features, as well as
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// any extra feature overrides that depend on command line switches.
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bool IsFeatureOverriddenFromCommandLine(const std::string& feature_name,
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OverrideState state) const;
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// Associates a field trial for reporting purposes corresponding to the
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// command-line setting the feature state to |for_overridden_state|. The trial
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// will be activated when the state of the feature is first queried. This
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// should be called during registration, after InitializeFromCommandLine() has
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// been called but before the instance is registered via SetInstance().
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void AssociateReportingFieldTrial(const std::string& feature_name,
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OverrideState for_overridden_state,
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FieldTrial* field_trial);
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// Registers a field trial to override the enabled state of the specified
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// feature to |override_state|. Command-line overrides still take precedence
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// over field trials, so this will have no effect if the feature is being
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// overridden from the command-line. The associated field trial will be
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// activated when the feature state for this feature is queried. This should
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// be called during registration, after InitializeFromCommandLine() has been
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// called but before the instance is registered via SetInstance().
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void RegisterFieldTrialOverride(const std::string& feature_name,
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OverrideState override_state,
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FieldTrial* field_trial);
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// Adds extra overrides (not associated with a field trial). Should be called
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// before SetInstance().
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// The ordering of calls with respect to InitializeFromCommandLine(),
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// RegisterFieldTrialOverride(), etc. matters. The first call wins out,
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// because the |overrides_| map uses insert(), which retains the first
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// inserted entry and does not overwrite it on subsequent calls to insert().
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void RegisterExtraFeatureOverrides(
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const std::vector<FeatureOverrideInfo>& extra_overrides);
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// Loops through feature overrides and serializes them all into |allocator|.
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void AddFeaturesToAllocator(PersistentMemoryAllocator* allocator);
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// Returns comma-separated lists of feature names (in the same format that is
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// accepted by InitializeFromCommandLine()) corresponding to features that
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// have been overridden - either through command-line or via FieldTrials. For
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// those features that have an associated FieldTrial, the output entry will be
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// of the format "FeatureName<TrialName", where "TrialName" is the name of the
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// FieldTrial. Features that have overrides with OVERRIDE_USE_DEFAULT will be
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// added to |enable_overrides| with a '*' character prefix. Must be called
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// only after the instance has been initialized and registered.
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void GetFeatureOverrides(std::string* enable_overrides,
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std::string* disable_overrides);
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// Like GetFeatureOverrides(), but only returns overrides that were specified
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// explicitly on the command-line, omitting the ones from field trials.
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void GetCommandLineFeatureOverrides(std::string* enable_overrides,
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std::string* disable_overrides);
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// Returns whether the given |feature| is enabled. Must only be called after
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// the singleton instance has been registered via SetInstance(). Additionally,
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// a feature with a given name must only have a single corresponding Feature
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// struct, which is checked in builds with DCHECKs enabled.
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static bool IsEnabled(const Feature& feature);
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// Returns the field trial associated with the given |feature|. Must only be
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// called after the singleton instance has been registered via SetInstance().
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static FieldTrial* GetFieldTrial(const Feature& feature);
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// Splits a comma-separated string containing feature names into a vector. The
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// resulting pieces point to parts of |input|.
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static std::vector<base::StringPiece> SplitFeatureListString(
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base::StringPiece input);
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// Initializes and sets an instance of FeatureList with feature overrides via
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// command-line flags |enable_features| and |disable_features| if one has not
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// already been set from command-line flags. Returns true if an instance did
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// not previously exist. See InitializeFromCommandLine() for more details
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// about |enable_features| and |disable_features| parameters.
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static bool InitializeInstance(const std::string& enable_features,
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const std::string& disable_features);
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// Like the above, but also adds extra overrides. If a feature appears in
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// |extra_overrides| and also |enable_features| or |disable_features|, the
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// disable/enable will supersede the extra overrides.
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static bool InitializeInstance(
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const std::string& enable_features,
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const std::string& disable_features,
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const std::vector<FeatureOverrideInfo>& extra_overrides);
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// Returns the singleton instance of FeatureList. Will return null until an
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// instance is registered via SetInstance().
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static FeatureList* GetInstance();
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// Registers the given |instance| to be the singleton feature list for this
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// process. This should only be called once and |instance| must not be null.
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// Note: If you are considering using this for the purposes of testing, take
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// a look at using base/test/scoped_feature_list.h instead.
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static void SetInstance(std::unique_ptr<FeatureList> instance);
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// Clears the previously-registered singleton instance for tests and returns
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// the old instance.
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// Note: Most tests should never call this directly. Instead consider using
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// base::test::ScopedFeatureList.
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static std::unique_ptr<FeatureList> ClearInstanceForTesting();
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// Sets a given (initialized) |instance| to be the singleton feature list,
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// for testing. Existing instance must be null. This is primarily intended
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// to support base::test::ScopedFeatureList helper class.
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static void RestoreInstanceForTesting(std::unique_ptr<FeatureList> instance);
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private:
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FRIEND_TEST_ALL_PREFIXES(FeatureListTest, CheckFeatureIdentity);
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FRIEND_TEST_ALL_PREFIXES(FeatureListTest,
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StoreAndRetrieveFeaturesFromSharedMemory);
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FRIEND_TEST_ALL_PREFIXES(FeatureListTest,
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StoreAndRetrieveAssociatedFeaturesFromSharedMemory);
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struct OverrideEntry {
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// The overridden enable (on/off) state of the feature.
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const OverrideState overridden_state;
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// An optional associated field trial, which will be activated when the
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// state of the feature is queried for the first time. Weak pointer to the
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// FieldTrial object that is owned by the FieldTrialList singleton.
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base::FieldTrial* field_trial;
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// Specifies whether the feature's state is overridden by |field_trial|.
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// If it's not, and |field_trial| is not null, it means it is simply an
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// associated field trial for reporting purposes (and |overridden_state|
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// came from the command-line).
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const bool overridden_by_field_trial;
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// TODO(asvitkine): Expand this as more support is added.
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// Constructs an OverrideEntry for the given |overridden_state|. If
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// |field_trial| is not null, it implies that |overridden_state| comes from
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// the trial, so |overridden_by_field_trial| will be set to true.
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OverrideEntry(OverrideState overridden_state, FieldTrial* field_trial);
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};
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// Finalizes the initialization state of the FeatureList, so that no further
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// overrides can be registered. This is called by SetInstance() on the
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// singleton feature list that is being registered.
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void FinalizeInitialization();
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// Returns whether the given |feature| is enabled. This is invoked by the
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// public FeatureList::IsEnabled() static function on the global singleton.
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// Requires the FeatureList to have already been fully initialized.
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bool IsFeatureEnabled(const Feature& feature);
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// Returns the field trial associated with the given |feature|. This is
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// invoked by the public FeatureList::GetFieldTrial() static function on the
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// global singleton. Requires the FeatureList to have already been fully
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// initialized.
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base::FieldTrial* GetAssociatedFieldTrial(const Feature& feature);
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// For each feature name in comma-separated list of strings |feature_list|,
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// registers an override with the specified |overridden_state|. Also, will
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// associate an optional named field trial if the entry is of the format
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// "FeatureName<TrialName".
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void RegisterOverridesFromCommandLine(const std::string& feature_list,
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OverrideState overridden_state);
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// Registers an override for feature |feature_name|. The override specifies
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// whether the feature should be on or off (via |overridden_state|), which
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// will take precedence over the feature's default state. If |field_trial| is
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// not null, registers the specified field trial object to be associated with
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// the feature, which will activate the field trial when the feature state is
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// queried. If an override is already registered for the given feature, it
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// will not be changed.
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void RegisterOverride(StringPiece feature_name,
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OverrideState overridden_state,
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FieldTrial* field_trial);
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// Implementation of GetFeatureOverrides() with a parameter that specifies
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// whether only command-line enabled overrides should be emitted. See that
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// function's comments for more details.
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void GetFeatureOverridesImpl(std::string* enable_overrides,
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std::string* disable_overrides,
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bool command_line_only);
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// Verifies that there's only a single definition of a Feature struct for a
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// given feature name. Keeps track of the first seen Feature struct for each
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// feature. Returns false when called on a Feature struct with a different
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// address than the first one it saw for that feature name. Used only from
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// DCHECKs and tests.
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bool CheckFeatureIdentity(const Feature& feature);
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// Map from feature name to an OverrideEntry struct for the feature, if it
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// exists.
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std::map<std::string, OverrideEntry, std::less<>> overrides_;
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|
// 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.
|
||
|
|
Lock feature_identity_tracker_lock_;
|
||
|
|
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.
|
||
|
|
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;
|
||
|
|
|
||
|
|
DISALLOW_COPY_AND_ASSIGN(FeatureList);
|
||
|
|
};
|
||
|
|
|
||
|
|
} // namespace base
|
||
|
|
|
||
|
|
#endif // BASE_FEATURE_LIST_H_
|