1 files changed, 146 insertions, 0 deletions
diff --git a/android/singleton_module.go b/android/singleton_module.go
new file mode 100644
index 000000000..235173809
--- /dev/null
+++ b/android/singleton_module.go
@@ -0,0 +1,146 @@
+// Copyright 2020 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 android
+
+import (
+	"fmt"
+	"sync"
+
+	"github.com/google/blueprint"
+)
+
+// A SingletonModule is halfway between a Singleton and a Module.  It has access to visiting
+// other modules via its GenerateSingletonBuildActions method, but must be defined in an Android.bp
+// file and can also be depended on like a module.  It must be used zero or one times in an
+// Android.bp file, and it can only have a single variant.
+//
+// The SingletonModule's GenerateAndroidBuildActions method will be called before any normal or
+// singleton module that depends on it, but its GenerateSingletonBuildActions method will be called
+// after all modules, in registration order with other singletons and singleton modules.
+// GenerateAndroidBuildActions and GenerateSingletonBuildActions will not be called if the
+// SingletonModule was not instantiated in an Android.bp file.
+//
+// Since the SingletonModule rules likely depend on the modules visited during
+// GenerateSingletonBuildActions, the GenerateAndroidBuildActions is unlikely to produce any
+// rules directly.  Instead, it will probably set some providers to paths that will later have rules
+// generated to produce them in GenerateSingletonBuildActions.
+//
+// The expected use case for a SingletonModule is a module that produces files that depend on all
+// modules in the tree and will be used by other modules.  For example it could produce a text
+// file that lists all modules that meet a certain criteria, and that text file could be an input
+// to another module.  Care must be taken that the ninja rules produced by the SingletonModule
+// don't produce a cycle by referencing output files of rules of modules that depend on the
+// SingletonModule.
+//
+// A SingletonModule must embed a SingletonModuleBase struct, and its factory method must be
+// registered with RegisterSingletonModuleType from an init() function.
+//
+// A SingletonModule can also implement SingletonMakeVarsProvider to export values to Make.
+type SingletonModule interface {
+	Module
+	GenerateSingletonBuildActions(SingletonContext)
+	singletonModuleBase() *SingletonModuleBase
+}
+
+// SingletonModuleBase must be embedded into implementers of the SingletonModule interface.
+type SingletonModuleBase struct {
+	ModuleBase
+
+	lock    sync.Mutex
+	bp      string
+	variant string
+}
+
+// GenerateBuildActions wraps the ModuleBase GenerateBuildActions method, verifying it was only
+// called once to prevent multiple variants of a SingletonModule.
+func (smb *SingletonModuleBase) GenerateBuildActions(ctx blueprint.ModuleContext) {
+	smb.lock.Lock()
+	if smb.variant != "" {
+		ctx.ModuleErrorf("GenerateAndroidBuildActions already called for variant %q, SingletonModules can only  have one variant", smb.variant)
+	}
+	smb.variant = ctx.ModuleSubDir()
+	smb.lock.Unlock()
+
+	smb.ModuleBase.GenerateBuildActions(ctx)
+}
+
+// InitAndroidSingletonModule must be called from the SingletonModule's factory function to
+// initialize SingletonModuleBase.
+func InitAndroidSingletonModule(sm SingletonModule) {
+	InitAndroidModule(sm)
+}
+
+// singletonModuleBase retrieves the embedded SingletonModuleBase from a SingletonModule.
+func (smb *SingletonModuleBase) singletonModuleBase() *SingletonModuleBase { return smb }
+
+// SingletonModuleFactory is a factory method that returns a SingletonModule.
+type SingletonModuleFactory func() SingletonModule
+
+// SingletonModuleFactoryAdaptor converts a SingletonModuleFactory into a SingletonFactory and a
+// ModuleFactory.
+func SingletonModuleFactoryAdaptor(name string, factory SingletonModuleFactory) (SingletonFactory, ModuleFactory) {
+	// The sm variable acts as a static holder of the only SingletonModule instance.  Calls to the
+	// returned SingletonFactory and ModuleFactory lambdas will always return the same sm value.
+	// The SingletonFactory is only expected to be called once, but the ModuleFactory may be
+	// called multiple times if the module is replaced with a clone of itself at the end of
+	// blueprint.ResolveDependencies.
+	var sm SingletonModule
+	s := func() Singleton {
+		sm = factory()
+		return &singletonModuleSingletonAdaptor{sm}
+	}
+	m := func() Module {
+		if sm == nil {
+			panic(fmt.Errorf("Singleton %q for SingletonModule was not instantiated", name))
+		}
+
+		// Check for multiple uses of a SingletonModule in a LoadHook.  Checking directly in the
+		// factory would incorrectly flag when the factory was called again when the module is
+		// replaced with a clone of itself at the end of blueprint.ResolveDependencies.
+		AddLoadHook(sm, func(ctx LoadHookContext) {
+			smb := sm.singletonModuleBase()
+			smb.lock.Lock()
+			defer smb.lock.Unlock()
+			if smb.bp != "" {
+				ctx.ModuleErrorf("Duplicate SingletonModule %q, previously used in %s", name, smb.bp)
+			}
+			smb.bp = ctx.BlueprintsFile()
+		})
+		return sm
+	}
+	return s, m
+}
+
+// singletonModuleSingletonAdaptor makes a SingletonModule into a Singleton by translating the
+// GenerateSingletonBuildActions method to Singleton.GenerateBuildActions.
+type singletonModuleSingletonAdaptor struct {
+	sm SingletonModule
+}
+
+// GenerateBuildActions calls the SingletonModule's GenerateSingletonBuildActions method, but only
+// if the module was defined in an Android.bp file.
+func (smsa *singletonModuleSingletonAdaptor) GenerateBuildActions(ctx SingletonContext) {
+	if smsa.sm.singletonModuleBase().bp != "" {
+		smsa.sm.GenerateSingletonBuildActions(ctx)
+	}
+}
+
+func (smsa *singletonModuleSingletonAdaptor) MakeVars(ctx MakeVarsContext) {
+	if smsa.sm.singletonModuleBase().bp != "" {
+		if makeVars, ok := smsa.sm.(SingletonMakeVarsProvider); ok {
+			makeVars.MakeVars(ctx)
+		}
+	}
+}