Add RBE environment variables config example and documentation.

{

    "env": {

        "USE_RBE": "1",

        "RBE_R8_EXEC_STRATEGY": "remote_local_fallback",

        "RBE_CXX_EXEC_STRATEGY": "remote_local_fallback",

        "RBE_D8_EXEC_STRATEGY": "remote_local_fallback",

        "RBE_JAVAC_EXEC_STRATEGY": "remote_local_fallback",

        "RBE_JAVAC": "1",

        "RBE_R8": "1",

        "RBE_D8": "1",

        "RBE_instance": "[replace with your RBE instance]",

        "RBE_service": "[replace with your RBE service endpoint]",

        "RBE_DIR": "prebuilts/remoteexecution-client/live",

        "RBE_use_application_default_credentials": "true",

        "RBE_log_dir": "/tmp",

        "RBE_output_dir": "/tmp",

        "RBE_proxy_log_dir": "/tmp"

    }

}

# Build Android Platform on Remote Build Execution

Soong is integrated with Google's Remote Build Execution(RBE) service, which

implements the

[Remote Executaion API](https://github.com/bazelbuild/remote-apis).

With RBE enabled, it can speed up the Android Platform builds by distributing

build actions through a worker pool sharing a central cache of build results.

## Configuration

To enable RBE, you need to set several environment variables before triggering

the build. You can set them through a

[environment variables config file](https://android.googlesource.com/platform/build/soong/+/master/README.md#environment-variables-config-file).

As an example, [build/soong/docs/rbe.json](rbe.json) is a config that enables

RBE in the build. Once the config file is created, you need to let Soong load

the config file by specifying `ANDROID_BUILD_ENVIRONMENT_CONFIG_DIR` environment

variable and `ANDROID_BUILD_ENVIRONMENT_CONFIG` environment variable. The

following command starts Soong with [build/soong/docs/rbe.json](rbe.json)

loaded:

```shell

ANDROID_BUILD_ENVIRONMENT_CONFIG=rbe \

ANDROID_BUILD_ENVIRONMENT_CONFIG_DIR=build/soong/doc \

  build/soong/soong_ui.bash

```

### Configuration Explanation

Below a brief explanation of each field in

[build/soong/docs/rbe.json](rbe.json):

##### USE\_RBE:

If set to 1, enable RBE for the build.

##### RBE\_CXX\_EXEC\_STRATEGY / RBE\_JAVAC\_EXEC\_STRATEGY / RBE\_R8\_EXEC\_STRATEGY / RBE\_D8\_EXEC\_STRATEGY:

Sets strategies for C++/javac/r8/d8 action types. Available options are

(**Note**: all options will update the remote cache if the right permissions to

update cache are given to the user.):

*   **local**: Only execute locally.

*   **remote**: Only execute remotely.

*   **remote_local_fallback**: Try executing remotely and fall back to local

    execution if failed.

*   **racing**: Race remote execution and local execution and use the earlier

    result.

##### RBE\_JAVAC / RBE\_R8 / RBE\_D8

If set to 1, enable javac/r8/d8 support. C++ compilation is enabled by default.

##### RBE\_service / RBE\_instance

The remote execution service endpoint and instance ID to target when calling

remote execution via gRPC to execute actions.

##### RBE\_DIR

Where to find remote client binaries (rewrapper, reproxy)

##### RBE\_use\_application\_default\_credentials

reclient uses

[application default credentials](https://cloud.google.com/sdk/gcloud/reference/auth/application-default/login)

for autentication, as generated by `gcloud auth application-default login`

##### RBE\_log\_dir/RBE\_proxy\_log\_dir/RBE\_output\_dir

Logs generated by rewrapper and reproxy will go here.