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Integrating a feature with the Origin Trials framework

To expose your feature via the Origin Trials framework, there are a few code changes required.

*** note WARNING: This is only available for features implemented in Blink.

[TOC]

Code Changes

*** promo NOTE: You can land these code changes before requesting to run an origin trial. These code changes make it possible to control a feature via an origin trial, but don't require an origin trial to be approved. For more on the process, see Running an Origin Trial.

Step 1: Add Runtime Enabled Feature in Blink for Origin Trial

First, you’ll need to configure runtime_enabled_features.json5. If you don't have a Blink's Runtime Enabled Feature flag yet, you will need to add an entry in this file.

The following fields of an entry are relevant:

  • name: The name of your runtime enabled feature, e.g. "MyFeature".
  • origin_trial_feature_name: The name of your runtime enabled feature in the origin trial. This can be the same as your runtime feature flag (i.e. name field), or different. Eventually, this configured name will be used in the origin trials developer console.
  • origin_trial_os: Specifies a [list] of platforms where they will allow the trial to be enabled. The list values are case-insensitive, but must match one of the defined OS_<platform> macros (see build_config.h).
  • origin_trial_allows_third_party: Must be enabled to allow third-party tokens to work correctly. Set to true, if (and only if) you intend to support third-party matching.
  • base_feature: Generates a base::Feature in the blink::features namespace if the value is not "none". It helps to control the Origin Trial remotely. See also Generate a base::Feature instance from a Blink Feature.

Not specific to Origin Trial:

  • status: Controls when the runtime enabled feature is enabled in Blink. See also the Status table.
  • base_feature_status: Controls when the base::Feature defined by base_feature is enabled.

More details are explained in the json5 file and in the above linked doc.

If the runtime enabled feature flag is used in C++, you will have to change all callers of the no-argument overload of RuntimeEnabledFeatures::MyFeatureEnabled() to the overload that takes a const FeatureContext*. You can pass an ExecutionContext here, e.g. using ExecutionContext::From(ScriptState*).

Examples

RuntimeEnabledFeature flag name, trial name and base::Feature are all the same:

{
  name: "MyFeature",  // Generates `RuntimeEnabledFeatures::MyFeatureEnabled()`
  origin_trial_feature_name: "MyFeature",
  status: "experimental",
  // No need to specify base_feature.
},

RuntimeEnabledFeature flag name, trial name, and base::Feature name are different:

{
  name: "MyFeature",
  origin_trial_feature_name: "MyFeatureTrial",
  base_feature: "MyBaseFeature",  // Generates blink::features::kMyBaseFeature
  status: "experimental",
},

Trial limited to specific platform:

{
  name: "MyFeature",
  origin_trial_feature_name: "MyFeature",
  origin_trial_os: ["android"],
  status: "experimental",
},

WebView considerations

Because WebView is built as part of the "android" os target, it is not possible to exclude a trial from WebView if it is enabled on Android.

If the feature under trial can be enabled on WebView alongside other Android platforms, this is preferred.

In situations where this is not feasible, the recommended solution is to explicitly disable the origin trial in AwMainDelegate::BasicStartupComplete() in aw_main_delegate.cc by appending the embedder_support::kOriginTrialDisabledFeatures switch with the disabled trial names as values.

See https://crrev.com/c/3733267 for an example of how this can be done.

Step 2: Gating Access

Once configured, there are two mechanisms to gate access to your feature behind an origin trial. You can use either mechanism, or both, as appropriate to your feature implementation.

1) In C++

A native C++ method that you can call in Blink code at runtime to expose your feature:

bool RuntimeEnabledFeatures::MyFeatureEnabled(ExecutionContext*)

*** note WARNING: Your feature implementation must not persist the result of the enabled check. Your code should simply call RuntimeEnabledFeatures::MyFeatureEnabled(ExecutionContext*) as often as necessary to gate access to your feature.

2-1) In Web IDL

An IDL attribute [RuntimeEnabled] that you can use to automatically generate code to expose and hide JavaScript methods/attributes/objects.

[RuntimeEnabled=MyFeature]
partial interface Navigator {
     readonly attribute MyFeatureManager myFeature;
}

2-2) CSS Properties

*** promo NOTE: For CSS properties, you do not need to edit the IDL files, as the exposure on the CSSStyleDeclaration is handled at runtime.

You can also run experiment for new CSS properties with origin trial. After you have configured your feature in runtime_enabled_features.json5 as above, head to css_properties.json5. As explained in the file, you use runtime_flag to associate the CSS property with the feature you just defined. This will automatically link the CSS property to the origin trial defined in the runtime feature. It will be available in both JavaScript (Element.style) and CSS (including @supports) when the trial is enabled.

*** promo EXAMPLE: origin-trial-test-property defines a test css property controlled via runtime feature OriginTrialsSampleAPI and subsequently an origin trial named Frobulate.

*** note ISSUE: In the rare cases where the origin trial token is added via script after the css style declaration, the css property will be enabled and is fully functional, however it will not appear on the CSSStyleDeclaration interface, i.e. not accessible in Element.style. This issue is tracked in crbug/1041993.

Step 3: Mapping Runtime Enabled Feature to base::Feature (optional)

Given the following example:

{
  name: "MyFeature",
  origin_trial_feature_name: "MyFeature",
  base_feature: "MyFeature",
  status: "experimental",
},
[RuntimeEnabled=MyFeature]
interface MyFeatureAPI {
  readonly attribute bool dummy;
}
// third_party/blink/.../my_feature_api.cc
bool MyFeatureAPI::ConnectToBrowser() {
  if (base::FeatureList::IsEnabled(blink::features::kMyFeature) {
    // Do something
  }
  return false;
}

The above example shows a new feature relies on a base::Feature generated from the base_feature definition in json file, e.g. blink::features::kMyFeature, in addition to the runtime enabled feature flag MyFeature. However, their values are not associated.

In addition, due to the limitation, the runtime enabled feature flag is not available in the browser process by default:

if you need to know in the browser process whether a feature should be enabled, then you will have to either have the renderer inform it at runtime, or else just assume that it's always enabled, and gate access to the feature from the renderer.

*** note TLDR: Turning on MyFeature doesn't automatically turning on blink::features::kMyFeature, and vice versa.

To mitigate the issue, there are several options:

Option 1: Fully Enabling base::Feature, e.g. kMyFeature

And letting Origin Trial decides when your feature (via runtime enabled feature flag blink::features::MyFeature) is available, as suggested in the above quote. The base::Feature can be enabled via a remote Finch config, or by updating the default value in C++.

However, after the Origin Trial ends, it will be impossible to ramp up the feature by Finch if the part controlled by MyFeature cannot be enabled independently. For example, if you have a new Web API MyFeatureAPI, enabling MyFeature will just make the IDL available to everyone without the Blink/browser implementation.

*** note Example Bug: https://crbug.com/1360678.

Option 2: Setting Up a Custom Mapping

  1. Make MyFeature depend on blink::features::kMyFeature so that the feature is not enabled if features::kMyFeatures is not enabled. In third_party/blink/renderer/core/origin_trials/origin_trial_context.cc:

    bool OriginTrialContext::CanEnableTrialFromName(const StringView& trial_name) {
  ...
  if (trial_name == "MyFeature") {
    return base::FeatureList::IsEnabled(blink::features::kMyFeatures);
  }
}

  2. Add custom relationship for MyFeature and blink::features::kMyFeature to handle your use case.

    Read Determine how your feature is initialized: Depends on the status of a base::Feature first. If the mappings described there don't meet your use case, refer to the following examples.

    In content/child/runtime_features.cc:

    void SetCustomizedRuntimeFeaturesFromCombinedArgs(
    const base::CommandLine& command_line) {
  // Example 1: https://bit.ly/configuring-trust-tokens
  // Example 2: https://crrev.com/c/3878922/14/content/child/runtime_features.cc
}

Step 4: Web Feature Counting

Once the feature is created, in order to run the origin trial you need to track how often users use your feature. You can do it in two ways.

Increment counter in your C++ code

  1. Add your feature counter to end of web_feature.mojom:

    enum WebFeature {
  // ...
  kLastFeatureBeforeYours = 1235,
  // Here, increment the last feature count before yours by 1.
  kMyFeature = 1236,

  kNumberOfFeatures,  // This enum value must be last.
};

  2. Run update_use_counter_feature_enum.py to update the UMA mapping.

  3. Increment your feature counter in C++ code.

    #include "third_party/blink/renderer/platform/instrumentation/use_counter.h"

// ...

  if (RuntimeEnabledFeatures::MyFeatureEnabled(context)) {
    UseCounter::Count(context, WebFeature::kMyFeature);
  }

Update counter with [Measure] IDL attribute

  1. Add [Measure] IDL attribute

    partial interface Navigator {
  [RuntimeEnabled=MyFeature, Measure]
  readonly attribute MyFeatureManager myFeature;

  2. The code to increment your feature counter will be generated in V8 automatically. But it requires you to follow [Measure] IDL attribute naming convention when you will add your feature counter to web_feature.mojom.

    enum WebFeature {
  // ...
  kLastFeatureBeforeYours = 1235,
  // Here, increment the last feature count before yours by 1.
  kV8Navigator_MyFeature_AttributeGetter = 1236,

  kNumberOfFeatures,  // This enum value must be last.
};

Step 5: Add Web Tests

When using the [RuntimeEnabled] IDL attribute, you should add web tests to verify that the V8 bindings code is working as expected. Depending on how your feature is exposed, you'll want tests for the exposed interfaces, as well as tests for script-added tokens. For examples, refer to the existing tests in origin_trials/webexposed.

Limitations

What you can't do, because of the nature of these origin trials, is know at either browser or renderer startup time whether your feature is going to be used in the current page/context. This means that if you require lots of expensive processing to begin (say you index the user's hard drive, or scan an entire city for interesting weather patterns,) that you will have to either do it on browser startup for all users, just in case it's used, or do it on first access. (If you go with first access, then only people trying the experiment will notice the delay, and hopefully only the first time they use it.). We are investigating providing a method like OriginTrials::myFeatureShouldInitialize() that will hint if you should do startup initialization. For example, this could include checks for trials that have been revoked (or throttled) due to usage, if the entire origin trials framework has been disabled, etc. The method would be conservative and assume initialization is required, but it could avoid expensive startup in some known scenarios.

Similarly, if you need to know in the browser process whether a feature should be enabled, then you will have to either have the renderer inform it at runtime, or else just assume that it's always enabled, and gate access to the feature from the renderer.

Manual Testing

To test an origin trial feature during development, follow these steps:

  1. Use generate_token.py to generate a token signed with the test private key. You can generate signed tokens for any origin that you need to help you test, including localhost or 127.0.0.1. Example:

    tools/origin_trials/generate_token.py http://localhost:8000 MyFeature

    There are additional flags to generate third-party tokens, set the expiry date, and control other options. See the command help for details (--help). For example, to generate a third-party token, with user subset exclusion:

    tools/origin_trials/generate_token.py --is-third-party --usage-restriction=subset http://localhost:8000 MyFeature

  2. Copy the token from the end of the output and use it in a <meta> tag or an Origin-Trial header as described in the Developer Guide.

  3. Run Chrome with the test public key by passing: --origin-trial-public-key=dRCs+TocuKkocNKa0AtZ4awrt9XKH2SQCI6o4FY6BNA=

You can also run Chrome with both the test public key and the default public key along side by passing: --origin-trial-public-key=dRCs+TocuKkocNKa0AtZ4awrt9XKH2SQCI6o4FY6BNA=,fMS4mpO6buLQ/QMd+zJmxzty/VQ6B1EUZqoCU04zoRU=

*** promo TIP: See this doc to apply commandline switches for Chrome for Android and this doc to apply commandline switches for Android WebView.

The --origin-trial-public-key switch is not needed with content_shell, as it uses the test public key by default.

The test private key is stored in the repo at tools/origin_trials/eftest.key. It's also used by Origin Trials unit tests and web tests.

If you cannot set command-line switches (e.g., on Chrome OS), you can also directly modify chrome_origin_trial_policy.cc.

To see additional information about origin trial token parsing (including reasons for failures, or token names for successful tokens), you can add these switches:

--vmodule=trial_token=2,origin_trial_context=1

If you are building with is_debug=false, then you will also need to add dcheck_always_on=true to your build options, and add this to the command line:

--enable-logging=stderr

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