android/deapexer.go

// Copyright (C) 2021 The Android Open Source Project
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package android

import (
	"github.com/google/blueprint"
)

// Provides support for interacting with the `deapexer` module to which a `prebuilt_apex` module
// will delegate the work to export files from a prebuilt '.apex` file.
//
// The actual processing that is done is quite convoluted but it is all about combining information
// from multiple different sources in order to allow a prebuilt module to use a file extracted from
// an apex file. As follows:
//
// 1. A prebuilt module, e.g. prebuilt_bootclasspath_fragment or java_import needs to use a file
//    from a prebuilt_apex/apex_set. It knows the path of the file within the apex but does not know
//    where the apex file is or what apex to use.
//
// 2. The connection between the prebuilt module and the prebuilt_apex/apex_set is created through
//    use of an exported_... property on the latter. That causes four things to occur:
//    a. A `deapexer` mopdule is created by the prebuilt_apex/apex_set to extract files from the
//       apex file.
//    b. A dependency is added from the prebuilt_apex/apex_set modules onto the prebuilt modules
//       listed in those properties.
//    c. An APEX variant is created for each of those prebuilt modules.
//    d. A dependency is added from the prebuilt modules to the `deapexer` module.
//
// 3. The prebuilt_apex/apex_set modules do not know which files are available in the apex file.
//    That information could be specified on the prebuilt_apex/apex_set modules but without
//    automated generation of those modules it would be expensive to maintain. So, instead they
//    obtain that information from the prebuilt modules. They do not know what files are actually in
//    the apex file either but they know what files they need from it. So, the
//    prebuilt_apex/apex_set modules obtain the files that should be in the apex file from those
//    modules and then pass those onto the `deapexer` module.
//
// 4. The `deapexer` module's ninja rule extracts all the files from the apex file into an output
//    directory and checks that all the expected files are there. The expected files are declared as
//    the outputs of the ninja rule so they are available to other modules.
//
// 5. The prebuilt modules then retrieve the paths to the files that they needed from the `deapexer`
//    module.
//
// The files that are passed to `deapexer` and those that are passed back have a unique identifier
// that links them together. e.g. If the `deapexer` is passed something like this:
//     core-libart{.dexjar} -> javalib/core-libart.jar
// it will return something like this:
//     core-libart{.dexjar} -> out/soong/.....deapexer.../javalib/core-libart.jar
//
// The reason why the `deapexer` module is separate from the prebuilt_apex/apex_set is to avoid
// cycles. e.g.
//   prebuilt_apex "com.android.art" depends upon java_import "core-libart":
//       This is so it can create an APEX variant of the latter and obtain information about the
//       files that it needs from the apex file.
//   java_import "core-libart" depends upon `deapexer` module:
//       This is so it can retrieve the paths to the files it needs.

// The information exported by the `deapexer` module, access it using `DeapxerInfoProvider`.
type DeapexerInfo struct {
	// map from the name of an exported file from a prebuilt_apex to the path to that file. The
	// exported file name is of the form <module>{<tag>}.
	//
	// See Prebuilt.ApexInfoMutator for more information.
	exports map[string]Path
}

// PrebuiltExportPath provides the path, or nil if not available, of a file exported from the
// prebuilt_apex that created this ApexInfo.
//
// The exported file is identified by the module name and the tag:
// * The module name is the name of the module that contributed the file when the .apex file
//   referenced by the prebuilt_apex was built. It must be specified in one of the exported_...
//   properties on the prebuilt_apex module.
// * The tag identifies the type of file and is dependent on the module type.
//
// See apex/deapexer.go for more information.
func (i DeapexerInfo) PrebuiltExportPath(name, tag string) Path {
	path := i.exports[name+"{"+tag+"}"]
	return path
}

// Provider that can be used from within the `GenerateAndroidBuildActions` of a module that depends
// on a `deapexer` module to retrieve its `DeapexerInfo`.
var DeapexerProvider = blueprint.NewProvider(DeapexerInfo{})

// NewDeapexerInfo creates and initializes a DeapexerInfo that is suitable
// for use with a prebuilt_apex module.
//
// See apex/deapexer.go for more information.
func NewDeapexerInfo(exports map[string]Path) DeapexerInfo {
	return DeapexerInfo{
		exports: exports,
	}
}

type deapexerTagStruct struct {
	blueprint.BaseDependencyTag
}

// Mark this tag so dependencies that use it are excluded from APEX contents.
func (t deapexerTagStruct) ExcludeFromApexContents() {}

var _ ExcludeFromApexContentsTag = DeapexerTag

// A tag that is used for dependencies on the `deapexer` module.
var DeapexerTag = deapexerTagStruct{}

// RequiredFilesFromPrebuiltApex must be implemented by modules that require files to be exported
// from a prebuilt_apex/apex_set.
type RequiredFilesFromPrebuiltApex interface {
	// RequiredFilesFromPrebuiltApex returns a map from the key (module name plus tag) to the required
	// path of the file within the prebuilt .apex file.
	//
	// For each key/file pair this will cause the file to be extracted out of the prebuilt .apex file,
	// and the path to the extracted file will be stored in the DeapexerInfo using that key, The path
	// can then be retrieved using the PrebuiltExportPath(name, tag) method.
	RequiredFilesFromPrebuiltApex(ctx BaseModuleContext) map[string]string
}