Fall back to prebuilts if source modules are disabled

In lieu of having an environment variable to override prebuilts
preferences (for now), fall back to prebuilts whenever the source
module is disabled.  Complicated source modules (like LLVM) that
already have Go logic can then use an environment variariable to
disable their source module whenever they want to use prebuilts.

Computes UsePrebuilt once using information from both the source
and prebuilt modules, and stores the result in the prebuilt module.

Test: m -j libLLVM
Test: m -j libLLVM FORCE_BUILD_LLVM_COMPONENTS=true
Change-Id: Ib0819a03e9492e55f55de3402e05b3a043644a39

android/module.go

@@ -100,6 +100,7 @@ type Module interface {
 	Enabled() bool
 	Target() Target
 	InstallInData() bool
+	SkipInstall()
 }
 
 type nameProperties struct {

android/mutator.go

@@ -46,8 +46,8 @@ func registerMutators() {
 
 	ctx.BottomUp("deps", depsMutator).Parallel()
 
+	ctx.TopDown("prebuilt_select", PrebuiltSelectModuleMutator).Parallel()
 	ctx.BottomUp("prebuilt_replace", PrebuiltReplaceMutator).Parallel()
-	ctx.TopDown("prebuilt_disable", PrebuiltDisableMutator).Parallel()
 
 	register(postDeps)
 }

android/prebuilt.go

@@ -21,20 +21,6 @@ import "github.com/google/blueprint"
 
 var prebuiltDependencyTag blueprint.BaseDependencyTag
 
-func SourceModuleHasPrebuilt(ctx ModuleContext) OptionalPath {
-	var path Path
-	ctx.VisitDirectDeps(func(m blueprint.Module) {
-		if ctx.OtherModuleDependencyTag(m) == prebuiltDependencyTag {
-			p := m.(PrebuiltInterface).Prebuilt()
-			if p.usePrebuilt(ctx) {
-				path = p.Path(ctx)
-			}
-		}
-	})
-
-	return OptionalPathForPath(path)
-}
-
 type Prebuilt struct {
 	Properties struct {
 		Srcs []string `android:"arch_variant"`
@@ -43,6 +29,7 @@ type Prebuilt struct {
 		Prefer bool `android:"arch_variant"`
 
 		SourceExists bool `blueprint:"mutated"`
+		UsePrebuilt  bool `blueprint:"mutated"`
 	}
 	module Module
 }
@@ -68,11 +55,6 @@ func (p *Prebuilt) Path(ctx ModuleContext) Path {
 type PrebuiltInterface interface {
 	Module
 	Prebuilt() *Prebuilt
-	SkipInstall()
-}
-
-type PrebuiltSourceInterface interface {
-	SkipInstall()
 }
 
 // prebuiltMutator ensures that there is always a module with an undecorated name, and marks
@@ -90,6 +72,22 @@ func prebuiltMutator(ctx BottomUpMutatorContext) {
 	}
 }
 
+// PrebuiltSelectModuleMutator marks prebuilts that are overriding source modules, and disables
+// installing the source module.
+func PrebuiltSelectModuleMutator(ctx TopDownMutatorContext) {
+	if s, ok := ctx.Module().(Module); ok {
+		ctx.VisitDirectDeps(func(m blueprint.Module) {
+			if ctx.OtherModuleDependencyTag(m) == prebuiltDependencyTag {
+				p := m.(PrebuiltInterface).Prebuilt()
+				if p.usePrebuilt(ctx, s) {
+					p.Properties.UsePrebuilt = true
+					s.SkipInstall()
+				}
+			}
+		})
+	}
+}
+
 // PrebuiltReplaceMutator replaces dependencies on the source module with dependencies on the
 // prebuilt when both modules exist and the prebuilt should be used.  When the prebuilt should not
 // be used, disable installing it.
@@ -97,7 +95,7 @@ func PrebuiltReplaceMutator(ctx BottomUpMutatorContext) {
 	if m, ok := ctx.Module().(PrebuiltInterface); ok && m.Prebuilt() != nil {
 		p := m.Prebuilt()
 		name := m.base().BaseModuleName()
-		if p.usePrebuilt(ctx) {
+		if p.Properties.UsePrebuilt {
 			if p.Properties.SourceExists {
 				ctx.ReplaceDependencies(name)
 			}
@@ -107,21 +105,17 @@ func PrebuiltReplaceMutator(ctx BottomUpMutatorContext) {
 	}
 }
 
-// PrebuiltDisableMutator disables source modules that have prebuilts that should be used instead.
+// usePrebuilt returns true if a prebuilt should be used instead of the source module.  The prebuilt
-func PrebuiltDisableMutator(ctx TopDownMutatorContext) {
+// will be used if it is marked "prefer" or if the source module is disabled.
-	if s, ok := ctx.Module().(PrebuiltSourceInterface); ok {
+func (p *Prebuilt) usePrebuilt(ctx TopDownMutatorContext, source Module) bool {
-		ctx.VisitDirectDeps(func(m blueprint.Module) {
+	if len(p.Properties.Srcs) == 0 {
-			if ctx.OtherModuleDependencyTag(m) == prebuiltDependencyTag {
+		return false
-				p := m.(PrebuiltInterface).Prebuilt()
-				if p.usePrebuilt(ctx) {
-					s.SkipInstall()
-				}
-			}
-		})
 	}
-}
 
-func (p *Prebuilt) usePrebuilt(ctx BaseContext) bool {
+	// TODO: use p.Properties.Name and ctx.ModuleDir to override preference
-	// TODO: use p.Properties.Name and ctx.ModuleDir to override prefer
+	if p.Properties.Prefer {
-	return p.Properties.Prefer && len(p.Properties.Srcs) > 0
+		return true
+	}
+
+	return !source.Enabled()
 }