Prototype changes for multitree

This change contains a prototype implementation for multitree. Several
interfaces and modules are added.

1. Imported/Exported
Modules implementing Exportable interface can export artifacts to other
components. "imported_filegroup" modules can import generated artifacts from other exported modules.

2. Multitree metadata
It contains information about imported/exported modules in each
component, and can be generated via "m update-meta".

3. cc library stub
It's based on prototype stub libraries. It uses imported/exported
mechanism to expose a C API, with a map.txt file and header files.

Bug: 230448564
Test: m
Change-Id: Id7ff7618e2c630c5617a564d8b23b60a1cc9c8e8

android/config.go

@@ -1482,6 +1482,10 @@ func (c *config) MissingUsesLibraries() []string {
 	return c.productVariables.MissingUsesLibraries
 }
 
+func (c *config) TargetMultitreeUpdateMeta() bool {
+	return c.productVariables.MultitreeUpdateMeta
+}
+
 func (c *deviceConfig) DeviceArch() string {
 	return String(c.config.productVariables.DeviceArch)
 }

android/paths.go

@@ -1057,7 +1057,8 @@ func safePathForSource(ctx PathContext, pathComponents ...string) (SourcePath, e
 	}
 
 	// absolute path already checked by validateSafePath
-	if strings.HasPrefix(ret.String(), ctx.Config().soongOutDir) {
+	// special-case api surface gen files for now
+	if strings.HasPrefix(ret.String(), ctx.Config().soongOutDir) && !strings.Contains(ret.String(), ctx.Config().soongOutDir+"/.export") {
 		return ret, fmt.Errorf("source path %q is in output", ret.String())
 	}
 
@@ -1073,7 +1074,8 @@ func pathForSource(ctx PathContext, pathComponents ...string) (SourcePath, error
 	}
 
 	// absolute path already checked by validatePath
-	if strings.HasPrefix(ret.String(), ctx.Config().soongOutDir) {
+	// special-case for now
+	if strings.HasPrefix(ret.String(), ctx.Config().soongOutDir) && !strings.Contains(ret.String(), ctx.Config().soongOutDir+"/.export") {
 		return ret, fmt.Errorf("source path %q is in output", ret.String())
 	}
 

android/variable.go

@@ -351,6 +351,8 @@ type productVariables struct {
 	RecoverySnapshotDirsIncluded []string `json:",omitempty"`
 	HostFakeSnapshotEnabled      bool     `json:",omitempty"`
 
+	MultitreeUpdateMeta bool `json:",omitempty"`
+
 	BoardVendorSepolicyDirs           []string `json:",omitempty"`
 	BoardOdmSepolicyDirs              []string `json:",omitempty"`
 	BoardReqdMaskPolicy               []string `json:",omitempty"`

apex/Android.bp

@@ -14,6 +14,7 @@ bootstrap_go_package {
         "soong-cc",
         "soong-filesystem",
         "soong-java",
+        "soong-multitree",
         "soong-provenance",
         "soong-python",
         "soong-rust",

apex/apex.go

@@ -33,6 +33,7 @@ import (
 	prebuilt_etc "android/soong/etc"
 	"android/soong/filesystem"
 	"android/soong/java"
+	"android/soong/multitree"
 	"android/soong/python"
 	"android/soong/rust"
 	"android/soong/sh"
@@ -358,6 +359,7 @@ type apexBundle struct {
 	android.OverridableModuleBase
 	android.SdkBase
 	android.BazelModuleBase
+	multitree.ExportableModuleBase
 
 	// Properties
 	properties            apexBundleProperties
@@ -1359,6 +1361,21 @@ func (a *apexBundle) OutputFiles(tag string) (android.Paths, error) {
 	}
 }
 
+var _ multitree.Exportable = (*apexBundle)(nil)
+
+func (a *apexBundle) Exportable() bool {
+	if a.properties.ApexType == flattenedApex {
+		return false
+	}
+	return true
+}
+
+func (a *apexBundle) TaggedOutputs() map[string]android.Paths {
+	ret := make(map[string]android.Paths)
+	ret["apex"] = android.Paths{a.outputFile}
+	return ret
+}
+
 var _ cc.Coverage = (*apexBundle)(nil)
 
 // Implements cc.Coverage
@@ -2372,6 +2389,7 @@ func newApexBundle() *apexBundle {
 	android.InitSdkAwareModule(module)
 	android.InitOverridableModule(module, &module.overridableProperties.Overrides)
 	android.InitBazelModule(module)
+	multitree.InitExportableModule(module)
 	return module
 }
 

cc/Android.bp

@@ -15,6 +15,7 @@ bootstrap_go_package {
         "soong-etc",
         "soong-fuzz",
         "soong-genrule",
+        "soong-multitree",
         "soong-snapshot",
         "soong-tradefed",
     ],
@@ -65,6 +66,7 @@ bootstrap_go_package {
         "library.go",
         "library_headers.go",
         "library_sdk_member.go",
+        "library_stub.go",
         "native_bridge_sdk_trait.go",
         "object.go",
         "test.go",
@@ -94,6 +96,7 @@ bootstrap_go_package {
         "gen_test.go",
         "genrule_test.go",
         "library_headers_test.go",
+        "library_stub_test.go",
         "library_test.go",
         "object_test.go",
         "prebuilt_test.go",

cc/library_stub.go

@@ -0,0 +1,163 @@
+// Copyright 2021 Google Inc. All rights reserved.
+//
+// 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 cc
+
+import (
+	"android/soong/android"
+	"android/soong/multitree"
+)
+
+func init() {
+	RegisterLibraryStubBuildComponents(android.InitRegistrationContext)
+}
+
+func RegisterLibraryStubBuildComponents(ctx android.RegistrationContext) {
+	// cc_api_stub_library shares a lot of ndk_library, and this will be refactored later
+	ctx.RegisterModuleType("cc_api_stub_library", CcApiStubLibraryFactory)
+	ctx.RegisterModuleType("cc_api_contribution", CcApiContributionFactory)
+}
+
+func CcApiStubLibraryFactory() android.Module {
+	module, decorator := NewLibrary(android.DeviceSupported)
+	apiStubDecorator := &apiStubDecorator{
+		libraryDecorator: decorator,
+	}
+	apiStubDecorator.BuildOnlyShared()
+
+	module.compiler = apiStubDecorator
+	module.linker = apiStubDecorator
+	module.installer = nil
+	module.library = apiStubDecorator
+	module.Properties.HideFromMake = true // TODO: remove
+
+	android.InitAndroidArchModule(module, android.DeviceSupported, android.MultilibBoth)
+	module.AddProperties(&module.Properties,
+		&apiStubDecorator.properties,
+		&apiStubDecorator.MutatedProperties,
+		&apiStubDecorator.apiStubLibraryProperties)
+	return module
+}
+
+type apiStubLiraryProperties struct {
+	Imported_includes []string `android:"path"`
+}
+
+type apiStubDecorator struct {
+	*libraryDecorator
+	properties               libraryProperties
+	apiStubLibraryProperties apiStubLiraryProperties
+}
+
+func (compiler *apiStubDecorator) stubsVersions(ctx android.BaseMutatorContext) []string {
+	firstVersion := String(compiler.properties.First_version)
+	return ndkLibraryVersions(ctx, android.ApiLevelOrPanic(ctx, firstVersion))
+}
+
+func (decorator *apiStubDecorator) compile(ctx ModuleContext, flags Flags, deps PathDeps) Objects {
+	if decorator.stubsVersion() == "" {
+		decorator.setStubsVersion("current")
+	} // TODO: fix
+	symbolFile := String(decorator.properties.Symbol_file)
+	nativeAbiResult := parseNativeAbiDefinition(ctx, symbolFile,
+		android.ApiLevelOrPanic(ctx, decorator.stubsVersion()),
+		"")
+	return compileStubLibrary(ctx, flags, nativeAbiResult.stubSrc)
+}
+
+func (decorator *apiStubDecorator) link(ctx ModuleContext, flags Flags, deps PathDeps, objects Objects) android.Path {
+	decorator.reexportDirs(android.PathsForModuleSrc(ctx, decorator.apiStubLibraryProperties.Imported_includes)...)
+	return decorator.libraryDecorator.link(ctx, flags, deps, objects)
+}
+
+func init() {
+	pctx.HostBinToolVariable("gen_api_surface_build_files", "gen_api_surface_build_files")
+}
+
+type CcApiContribution struct {
+	android.ModuleBase
+	properties ccApiContributionProperties
+}
+
+type ccApiContributionProperties struct {
+	Symbol_file        *string `android:"path"`
+	First_version      *string
+	Export_include_dir *string
+}
+
+func CcApiContributionFactory() android.Module {
+	module := &CcApiContribution{}
+	module.AddProperties(&module.properties)
+	android.InitAndroidModule(module)
+	return module
+}
+
+// Do some simple validations
+// Majority of the build rules will be created in the ctx of the api surface this module contributes to
+func (contrib *CcApiContribution) GenerateAndroidBuildActions(ctx android.ModuleContext) {
+	if contrib.properties.Symbol_file == nil {
+		ctx.PropertyErrorf("symbol_file", "%v does not have symbol file", ctx.ModuleName())
+	}
+	if contrib.properties.First_version == nil {
+		ctx.PropertyErrorf("first_version", "%v does not have first_version for stub variants", ctx.ModuleName())
+	}
+}
+
+// Path is out/soong/.export/ but will be different in final multi-tree layout
+func outPathApiSurface(ctx android.ModuleContext, myModuleName string, pathComponent string) android.OutputPath {
+	return android.PathForOutput(ctx, ".export", ctx.ModuleName(), myModuleName, pathComponent)
+}
+
+func (contrib *CcApiContribution) CopyFilesWithTag(apiSurfaceContext android.ModuleContext) map[string]android.Paths {
+	// copy map.txt for now
+	// hardlinks cannot be created since nsjail creates a different mountpoint for out/
+	myDir := apiSurfaceContext.OtherModuleDir(contrib)
+	genMapTxt := outPathApiSurface(apiSurfaceContext, contrib.Name(), String(contrib.properties.Symbol_file))
+	apiSurfaceContext.Build(pctx, android.BuildParams{
+		Rule:        android.Cp,
+		Description: "import map.txt file",
+		Input:       android.PathForSource(apiSurfaceContext, myDir, String(contrib.properties.Symbol_file)),
+		Output:      genMapTxt,
+	})
+
+	outputs := make(map[string]android.Paths)
+	outputs["map"] = []android.Path{genMapTxt}
+
+	if contrib.properties.Export_include_dir != nil {
+		includeDir := android.PathForSource(apiSurfaceContext, myDir, String(contrib.properties.Export_include_dir))
+		outputs["export_include_dir"] = []android.Path{includeDir}
+	}
+	return outputs
+}
+
+var _ multitree.ApiContribution = (*CcApiContribution)(nil)
+
+/*
+func (contrib *CcApiContribution) GenerateBuildFiles(apiSurfaceContext android.ModuleContext) android.Paths {
+	genAndroidBp := outPathApiSurface(apiSurfaceContext, contrib.Name(), "Android.bp")
+
+	// generate Android.bp
+	apiSurfaceContext.Build(pctx, android.BuildParams{
+		Rule:        genApiSurfaceBuildFiles,
+		Description: "generate API surface build files",
+		Outputs:     []android.WritablePath{genAndroidBp},
+		Args: map[string]string{
+			"name":          contrib.Name() + "." + apiSurfaceContext.ModuleName(), //e.g. liblog.ndk
+			"symbol_file":   String(contrib.properties.Symbol_file),
+			"first_version": String(contrib.properties.First_version),
+		},
+	})
+	return []android.Path{genAndroidBp}
+}
+*/

cc/library_stub_test.go

@@ -0,0 +1,108 @@
+// Copyright 2021 Google Inc. All rights reserved.
+//
+// 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 cc
+
+import (
+	_ "fmt"
+	_ "sort"
+
+	"testing"
+
+	"android/soong/android"
+	"android/soong/multitree"
+)
+
+func TestCcApiStubLibraryOutputFiles(t *testing.T) {
+	bp := `
+		cc_api_stub_library {
+			name: "foo",
+			symbol_file: "foo.map.txt",
+			first_version: "29",
+		}
+	`
+	result := prepareForCcTest.RunTestWithBp(t, bp)
+	outputs := result.ModuleForTests("foo", "android_arm64_armv8-a_shared").AllOutputs()
+	expected_file_suffixes := []string{".c", "stub.map", ".o", ".so"}
+	for _, expected_file_suffix := range expected_file_suffixes {
+		android.AssertBoolEquals(t, expected_file_suffix+" file not found in output", true, android.SuffixInList(outputs, expected_file_suffix))
+	}
+}
+
+func TestCcApiStubLibraryVariants(t *testing.T) {
+	bp := `
+		cc_api_stub_library {
+			name: "foo",
+			symbol_file: "foo.map.txt",
+			first_version: "29",
+		}
+	`
+	result := prepareForCcTest.RunTestWithBp(t, bp)
+	variants := result.ModuleVariantsForTests("foo")
+	expected_variants := []string{"29", "30", "S", "Tiramisu"} //TODO: make this test deterministic by using fixtures
+	for _, expected_variant := range expected_variants {
+		android.AssertBoolEquals(t, expected_variant+" variant not found in foo", true, android.SubstringInList(variants, expected_variant))
+	}
+}
+
+func TestCcLibraryUsesCcApiStubLibrary(t *testing.T) {
+	bp := `
+		cc_api_stub_library {
+			name: "foo",
+			symbol_file: "foo.map.txt",
+			first_version: "29",
+		}
+		cc_library {
+			name: "foo_user",
+			shared_libs: [
+				"foo#29",
+			],
+		}
+
+	`
+	prepareForCcTest.RunTestWithBp(t, bp)
+}
+
+func TestApiSurfaceOutputs(t *testing.T) {
+	bp := `
+		api_surface {
+			name: "mysdk",
+			contributions: [
+				"foo",
+			],
+		}
+
+		cc_api_contribution {
+			name: "foo",
+			symbol_file: "foo.map.txt",
+			first_version: "29",
+		}
+	`
+	result := android.GroupFixturePreparers(
+		prepareForCcTest,
+		multitree.PrepareForTestWithApiSurface,
+	).RunTestWithBp(t, bp)
+	mysdk := result.ModuleForTests("mysdk", "")
+
+	actual_surface_inputs := mysdk.Rule("phony").BuildParams.Inputs.Strings()
+	expected_file_suffixes := []string{"mysdk/foo/foo.map.txt"}
+	for _, expected_file_suffix := range expected_file_suffixes {
+		android.AssertBoolEquals(t, expected_file_suffix+" file not found in input", true, android.SuffixInList(actual_surface_inputs, expected_file_suffix))
+	}
+
+	// check args/inputs to rule
+	/*api_surface_gen_rule_args := result.ModuleForTests("mysdk", "").Rule("genApiSurfaceBuildFiles").Args
+	android.AssertStringEquals(t, "name", "foo.mysdk", api_surface_gen_rule_args["name"])
+	android.AssertStringEquals(t, "symbol_file", "foo.map.txt", api_surface_gen_rule_args["symbol_file"])*/
+}

cc/ndk_library.go

@@ -93,7 +93,7 @@ var (
 type libraryProperties struct {
 	// Relative path to the symbol map.
 	// An example file can be seen here: TODO(danalbert): Make an example.
-	Symbol_file *string
+	Symbol_file *string `android:"path"`
 
 	// The first API level a library was available. A library will be generated
 	// for every API level beginning with this one.

cc/testing.go

@@ -29,6 +29,7 @@ func RegisterRequiredBuildComponentsForTest(ctx android.RegistrationContext) {
 	RegisterBinaryBuildComponents(ctx)
 	RegisterLibraryBuildComponents(ctx)
 	RegisterLibraryHeadersBuildComponents(ctx)
+	RegisterLibraryStubBuildComponents(ctx)
 
 	ctx.RegisterModuleType("cc_benchmark", BenchmarkFactory)
 	ctx.RegisterModuleType("cc_object", ObjectFactory)

multitree/Android.bp

@@ -0,0 +1,19 @@
+package {
+    default_applicable_licenses: ["Android-Apache-2.0"],
+}
+
+bootstrap_go_package {
+    name: "soong-multitree",
+    pkgPath: "android/soong/multitree",
+    deps: [
+        "blueprint",
+        "soong-android",
+    ],
+    srcs: [
+        "api_surface.go",
+        "export.go",
+        "metadata.go",
+        "import.go",
+    ],
+    pluginFor: ["soong_build"],
+}

multitree/api_surface.go

@@ -0,0 +1,119 @@
+// Copyright 2021 Google Inc. All rights reserved.
+//
+// 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 multitree
+
+import (
+	"android/soong/android"
+	"fmt"
+
+	"github.com/google/blueprint"
+)
+
+var (
+	pctx = android.NewPackageContext("android/soong/multitree")
+)
+
+func init() {
+	RegisterApiSurfaceBuildComponents(android.InitRegistrationContext)
+}
+
+var PrepareForTestWithApiSurface = android.FixtureRegisterWithContext(RegisterApiSurfaceBuildComponents)
+
+func RegisterApiSurfaceBuildComponents(ctx android.RegistrationContext) {
+	ctx.RegisterModuleType("api_surface", ApiSurfaceFactory)
+}
+
+type ApiSurface struct {
+	android.ModuleBase
+	ExportableModuleBase
+	properties apiSurfaceProperties
+
+	allOutputs    android.Paths
+	taggedOutputs map[string]android.Paths
+}
+
+type apiSurfaceProperties struct {
+	Contributions []string
+}
+
+func ApiSurfaceFactory() android.Module {
+	module := &ApiSurface{}
+	module.AddProperties(&module.properties)
+	android.InitAndroidModule(module)
+	InitExportableModule(module)
+	return module
+}
+
+func (surface *ApiSurface) DepsMutator(ctx android.BottomUpMutatorContext) {
+	if surface.properties.Contributions != nil {
+		ctx.AddVariationDependencies(nil, nil, surface.properties.Contributions...)
+	}
+
+}
+func (surface *ApiSurface) GenerateAndroidBuildActions(ctx android.ModuleContext) {
+	contributionFiles := make(map[string]android.Paths)
+	var allOutputs android.Paths
+	ctx.WalkDeps(func(child, parent android.Module) bool {
+		if contribution, ok := child.(ApiContribution); ok {
+			copied := contribution.CopyFilesWithTag(ctx)
+			for tag, files := range copied {
+				contributionFiles[child.Name()+"#"+tag] = files
+			}
+			for _, paths := range copied {
+				allOutputs = append(allOutputs, paths...)
+			}
+			return false // no transitive dependencies
+		}
+		return false
+	})
+
+	// phony target
+	ctx.Build(pctx, android.BuildParams{
+		Rule:   blueprint.Phony,
+		Output: android.PathForPhony(ctx, ctx.ModuleName()),
+		Inputs: allOutputs,
+	})
+
+	surface.allOutputs = allOutputs
+	surface.taggedOutputs = contributionFiles
+}
+
+func (surface *ApiSurface) OutputFiles(tag string) (android.Paths, error) {
+	if tag != "" {
+		return nil, fmt.Errorf("unknown tag: %q", tag)
+	}
+	return surface.allOutputs, nil
+}
+
+func (surface *ApiSurface) TaggedOutputs() map[string]android.Paths {
+	return surface.taggedOutputs
+}
+
+func (surface *ApiSurface) Exportable() bool {
+	return true
+}
+
+var _ android.OutputFileProducer = (*ApiSurface)(nil)
+var _ Exportable = (*ApiSurface)(nil)
+
+type ApiContribution interface {
+	// copy files necessaryt to construct an API surface
+	// For C, it will be map.txt and .h files
+	// For Java, it will be api.txt
+	CopyFilesWithTag(ctx android.ModuleContext) map[string]android.Paths // output paths
+
+	// Generate Android.bp in out/ to use the exported .txt files
+	// GenerateBuildFiles(ctx ModuleContext) Paths //output paths
+}

multitree/export.go

@@ -0,0 +1,67 @@
+// Copyright 2022 Google Inc. All rights reserved.
+//
+// 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 multitree
+
+import (
+	"android/soong/android"
+
+	"github.com/google/blueprint/proptools"
+)
+
+type moduleExportProperty struct {
+	// True if the module is exported to the other components in a multi-tree.
+	// Any components in the multi-tree can import this module to use.
+	Export *bool
+}
+
+type ExportableModuleBase struct {
+	properties moduleExportProperty
+}
+
+type Exportable interface {
+	// Properties for the exporable module.
+	exportableModuleProps() *moduleExportProperty
+
+	// Check if this module can be exported.
+	// If this returns false, the module will not be exported regardless of the 'export' value.
+	Exportable() bool
+
+	// Returns 'true' if this module has 'export: true'
+	// This module will not be exported if it returns 'false' to 'Exportable()' interface even if
+	// it has 'export: true'.
+	IsExported() bool
+
+	// Map from tags to outputs.
+	// Each module can tag their outputs for convenience.
+	TaggedOutputs() map[string]android.Paths
+}
+
+type ExportableModule interface {
+	android.Module
+	android.OutputFileProducer
+	Exportable
+}
+
+func InitExportableModule(module ExportableModule) {
+	module.AddProperties(module.exportableModuleProps())
+}
+
+func (m *ExportableModuleBase) exportableModuleProps() *moduleExportProperty {
+	return &m.properties
+}
+
+func (m *ExportableModuleBase) IsExported() bool {
+	return proptools.Bool(m.properties.Export)
+}

multitree/import.go

@@ -0,0 +1,96 @@
+// Copyright 2022 Google Inc. All rights reserved.
+//
+// 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 multitree
+
+import (
+	"android/soong/android"
+)
+
+var (
+	nameSuffix = ".imported"
+)
+
+type MultitreeImportedModuleInterface interface {
+	GetMultitreeImportedModuleName() string
+}
+
+func init() {
+	android.RegisterModuleType("imported_filegroup", importedFileGroupFactory)
+
+	android.PreArchMutators(RegisterMultitreePreArchMutators)
+}
+
+type importedFileGroupProperties struct {
+	// Imported modules from the other components in a multi-tree
+	Imported []string
+}
+
+type importedFileGroup struct {
+	android.ModuleBase
+
+	properties importedFileGroupProperties
+	srcs       android.Paths
+}
+
+func (ifg *importedFileGroup) Name() string {
+	return ifg.BaseModuleName() + nameSuffix
+}
+
+func importedFileGroupFactory() android.Module {
+	module := &importedFileGroup{}
+	module.AddProperties(&module.properties)
+
+	android.InitAndroidModule(module)
+	return module
+}
+
+var _ MultitreeImportedModuleInterface = (*importedFileGroup)(nil)
+
+func (ifg *importedFileGroup) GetMultitreeImportedModuleName() string {
+	// The base module name of the imported filegroup is used as the imported module name
+	return ifg.BaseModuleName()
+}
+
+var _ android.SourceFileProducer = (*importedFileGroup)(nil)
+
+func (ifg *importedFileGroup) Srcs() android.Paths {
+	return ifg.srcs
+}
+
+func (ifg *importedFileGroup) GenerateAndroidBuildActions(ctx android.ModuleContext) {
+	// srcs from this module must not be used. Adding a dot path to avoid the empty
+	// source failure. Still soong returns error when a module wants to build against
+	// this source, which is intended.
+	ifg.srcs = android.PathsForModuleSrc(ctx, []string{"."})
+}
+
+func RegisterMultitreePreArchMutators(ctx android.RegisterMutatorsContext) {
+	ctx.BottomUp("multitree_imported_rename", MultitreeImportedRenameMutator).Parallel()
+}
+
+func MultitreeImportedRenameMutator(ctx android.BottomUpMutatorContext) {
+	if m, ok := ctx.Module().(MultitreeImportedModuleInterface); ok {
+		name := m.GetMultitreeImportedModuleName()
+		if !ctx.OtherModuleExists(name) {
+			// Provide an empty filegroup not to break the build while updating the metadata.
+			// In other cases, soong will report an error to guide users to run 'm update-meta'
+			// first.
+			if !ctx.Config().TargetMultitreeUpdateMeta() {
+				ctx.ModuleErrorf("\"%s\" filegroup must be imported.\nRun 'm update-meta' first to import the filegroup.", name)
+			}
+			ctx.Rename(name)
+		}
+	}
+}

multitree/metadata.go

@@ -0,0 +1,74 @@
+// Copyright 2022 Google Inc. All rights reserved.
+//
+// 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 multitree
+
+import (
+	"android/soong/android"
+	"encoding/json"
+)
+
+func init() {
+	android.RegisterSingletonType("update-meta", UpdateMetaSingleton)
+}
+
+func UpdateMetaSingleton() android.Singleton {
+	return &updateMetaSingleton{}
+}
+
+type jsonImported struct {
+	FileGroups map[string][]string `json:",omitempty"`
+}
+
+type metadataJsonFlags struct {
+	Imported jsonImported        `json:",omitempty"`
+	Exported map[string][]string `json:",omitempty"`
+}
+
+type updateMetaSingleton struct {
+	importedModules       []string
+	generatedMetadataFile android.OutputPath
+}
+
+func (s *updateMetaSingleton) GenerateBuildActions(ctx android.SingletonContext) {
+	metadata := metadataJsonFlags{
+		Imported: jsonImported{
+			FileGroups: make(map[string][]string),
+		},
+		Exported: make(map[string][]string),
+	}
+	ctx.VisitAllModules(func(module android.Module) {
+		if ifg, ok := module.(*importedFileGroup); ok {
+			metadata.Imported.FileGroups[ifg.BaseModuleName()] = ifg.properties.Imported
+		}
+		if e, ok := module.(ExportableModule); ok {
+			if e.IsExported() && e.Exportable() {
+				for tag, files := range e.TaggedOutputs() {
+					// TODO(b/219846705): refactor this to a dictionary
+					metadata.Exported[e.Name()+":"+tag] = append(metadata.Exported[e.Name()+":"+tag], files.Strings()...)
+				}
+			}
+		}
+	})
+	jsonStr, err := json.Marshal(metadata)
+	if err != nil {
+		ctx.Errorf(err.Error())
+	}
+	s.generatedMetadataFile = android.PathForOutput(ctx, "multitree", "metadata.json")
+	android.WriteFileRule(ctx, s.generatedMetadataFile, string(jsonStr))
+}
+
+func (s *updateMetaSingleton) MakeVars(ctx android.MakeVarsContext) {
+	ctx.Strict("MULTITREE_METADATA", s.generatedMetadataFile.String())
+}