docs/origin_trials_integration.md

# Integrating a feature with the origin trials framework

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

[TOC]

## Code Changes

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].

### Runtime Enabled Features

First, you’ll need to configure [runtime\_enabled\_features.json5]. This is
explained in the file, but you use `origin_trial_feature_name` to associate your
runtime feature flag with a name for your origin trial.  The name can be the
same as your runtime feature flag, or different.  Eventually, this configured
name will be used in the origin trials developer console. You can have both
`status: experimental` and `origin_trial_feature_name` if you want your feature
to be enabled either by using the `--enable-experimental-web-platform-features`
flag **or** the origin trial.

You will have to change all callers of the no-argument overload of
`RuntimeEnabledFeatures::FooEnabled()` to the overload that takes a
`const FeatureContext*`. You can pass an `ExecutionContext` here
(e.g. using `ExecutionContext::From(ScriptState*)`).

You may have a feature that is not available on all platforms, or need to limit
the trial to specific platforms. Use `origin_trial_os: [list]` to specify which
platforms 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]).

#### Examples

Flag name and trial name are the same:
```
{
  name: "MyFeature",
  origin_trial_feature_name: "MyFeature",
  status: "experimental",
},
```
Flag name and trial name are different:
```
{
  name: "MyFeature",
  origin_trial_feature_name: "MyFeatureTrial",
  status: "experimental",
},
```
Trial limited to specific platform:
``` json
{
  name: "MyFeature",
  origin_trial_feature_name: "MyFeature",
  origin_trial_os: ["android"],
  status: "experimental",
},
```

### CSS Properties

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.


**Example:** [origin-trial-test-property] defines a test css property controlled via
runtime feature `OriginTrialsSampleAPI` and subsequently an origin trial named `Frobulate`.

### 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. A native C++ method that you can call in Blink code at runtime to expose your
    feature: `bool RuntimeEnabledFeatures::MyFeatureEnabled(ExecutionContext*)`
2. 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;
}
```

**NOTE:** 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.

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

**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.

### 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.
```c++
#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.
};
```

## 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.

## 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=`

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`


### 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].

[build_config.h]: /build/build_config.h
[chrome_origin_trial_policy.cc]: /chrome/common/origin_trials/chrome_origin_trial_policy.cc
[generate_token.py]: /tools/origin_trials/generate_token.py
[Developer Guide]: https://github.com/jpchase/OriginTrials/blob/gh-pages/developer-guide.md
[RuntimeEnabled]: /third_party/blink/renderer/bindings/IDLExtendedAttributes.md#RuntimeEnabled_i_m_a_c
[origin_trials/webexposed]: /third_party/blink/web_tests/http/tests/origin_trials/webexposed/
[runtime\_enabled\_features.json5]: /third_party/blink/renderer/platform/runtime_enabled_features.json5
[trial_token_unittest.cc]: /third_party/blink/common/origin_trials/trial_token_unittest.cc
[web\_feature.mojom]: /third_party/blink/public/mojom/use_counter/metrics/web_feature.mojom
[update\_use\_counter\_feature\_enum.py]: /tools/metrics/histograms/update_use_counter_feature_enum.py
[Measure]: /third_party/blink/renderer/bindings/IDLExtendedAttributes.md#Measure_i_m_a_c
[css\_properties.json5]: /third_party/blink/renderer/core/css/css_properties.json5
[origin-trial-test-property]: https://chromium.googlesource.com/chromium/src/+/ff2ab8b89745602c8300322c2a0158e210178c7e/third_party/blink/renderer/core/css/css_properties.json5#2635
[CSSStyleDeclaration]: /third_party/blink/renderer/core/css/css_style_declaration.idl
[Running an Origin Trial]: https://www.chromium.org/blink/origin-trials/running-an-origin-trial
[user subset exclusion]: https://docs.google.com/document/d/1xALH9W7rWmX0FpjudhDeS2TNTEOXuPn4Tlc9VmuPdHA/edit#heading=h.myaz1twlipw