Build-time system feature support

Overview

System features exposed from PackageManager are defined and aggregated as <feature> xml attributes across various partitions, and are currently queried at runtime through the framework. This directory contains tooling that supports build-time queries of select system features, enabling optimizations like code stripping and conditionally dependencies when so configured.

System Feature Codegen

As not all system features can be fully specified or defined at build time (e.g. updatable partitisions and apex modules can change/remove such features), we use a conditional, build flag approach that allows a given device to customize the subset of build-time defined system features that are immutable and cannot be updated.

Build Flags

System features that can be fixed at build-time are declared in a common location, build/release/flag_declarations/. These have the form RELEASE_SYSTEM_FEATURE_${X}, where ${X} corresponds to a feature defined in PackageManager, e.g., TELEVISION or WATCH.

Build flag values can then be defined per device (or form factor), where such values either indicate the existence/version of the system feature, or that the feature is unavailable, e.g., for TV, we could define these build flag values:

name: "RELEASE_SYSTEM_FEATURE_TELEVISION"
value: {
  string_value: "0"  # Feature version = 0
}

name: "RELEASE_SYSTEM_FEATURE_WATCH"
value: {
  string_value: "UNAVAILABLE"
}

See also SystemFeaturesGenerator for more details.

Runtime Queries

Each declared build flag system feature is routed into codegen, generating a getter API in the internal class, com.android.internal.pm.RoSystemFeatures:

class RoSystemFeatures {
    ...
    public static boolean hasFeatureX(Context context);
    ...
}

By default, these queries simply fall back to the usual PackageManager.hasSystemFeature(...) runtime queries. However, if a device defines these features via build flags, the generated code will add annotations indicating fixed value for this query, and adjust the generated code to return the value directly. This in turn enables build-time stripping and optimization.

NOTE: Any build-time defined system features will also be implicitly used to accelerate calls to PackageManager.hasSystemFeature(...) for the feature, avoiding binder calls when possible.

Lint

A new ErrorProne rule is introduced to assist with migration and maintenance of codegen APIs for build-time defined system features. This is defined in the systemfeatures-errorprone build rule, which can be added to any Java target's plugins list.

// TODO(b/203143243): Add plugin to key system targets after initial migration.

1. Add the plugin dependency to a given ${TARGET}:

java_library {
    name: "${TARGET}",
    plugins: ["systemfeatures-errorprone"],
}

2. Run locally:

RUN_ERROR_PRONE=true m ${TARGET}

3. (Optional) Update the target rule to generate in-place patch files:

java_library {
    name: "${TARGET}",
    plugins: ["systemfeatures-errorprone"],
    // DO NOT SUBMIT: GENERATE IN-PLACE PATCH FILES
    errorprone: {
        javacflags: [
            "-XepPatchChecks:RoSystemFeaturesChecker",
            "-XepPatchLocation:IN_PLACE",
        ],
    }
    ...
}

RUN_ERROR_PRONE=true m ${TARGET}

See also RoSystemFeaturesChecker for more details.

NOTE: Not all system feature queries or targets need or should be migrated. Only system features that are explicitly declared with build flags, and only targets that are built with the platform (i.e., not updatable), are candidates for this linting and migration, e.g., SystemUI, System Server, etc...

// TODO(b/203143243): Wrap the in-place lint updates with a simple script for convenience.