Fix genrule tool dependencies when a prebuilt tool is preferred.

Since the genrule tool dep mutator runs after the prebuilt mutators,
this adds a helper function android.PrebuiltGetPreferred to resolve the
source or prebuilt as appropriate.

Test: m SOONG_CONFIG_art_module_source_build=false droid
  in internal
Bug: 214292395
Change-Id: I1a208fd048b998f9f19ad1f45d8389decda2cb9e

android/prebuilt.go

@@ -309,6 +309,54 @@ func GetEmbeddedPrebuilt(module Module) *Prebuilt {
 	return nil
 }
 
+// PrebuiltGetPreferred returns the module that is preferred for the given
+// module. That is either the module itself or the prebuilt counterpart that has
+// taken its place. The given module must be a direct dependency of the current
+// context module, and it must be the source module if both source and prebuilt
+// exist.
+//
+// This function is for use on dependencies after PrebuiltPostDepsMutator has
+// run - any dependency that is registered before that will already reference
+// the right module. This function is only safe to call after all mutators that
+// may call CreateVariations, e.g. in GenerateAndroidBuildActions.
+func PrebuiltGetPreferred(ctx BaseModuleContext, module Module) Module {
+	if !module.IsReplacedByPrebuilt() {
+		return module
+	}
+	if IsModulePrebuilt(module) {
+		// If we're given a prebuilt then assume there's no source module around.
+		return module
+	}
+
+	sourceModDepFound := false
+	var prebuiltMod Module
+
+	ctx.WalkDeps(func(child, parent Module) bool {
+		if prebuiltMod != nil {
+			return false
+		}
+		if parent == ctx.Module() {
+			// First level: Only recurse if the module is found as a direct dependency.
+			sourceModDepFound = child == module
+			return sourceModDepFound
+		}
+		// Second level: Follow PrebuiltDepTag to the prebuilt.
+		if t := ctx.OtherModuleDependencyTag(child); t == PrebuiltDepTag {
+			prebuiltMod = child
+		}
+		return false
+	})
+
+	if prebuiltMod == nil {
+		if !sourceModDepFound {
+			panic(fmt.Errorf("Failed to find source module as a direct dependency: %s", module))
+		} else {
+			panic(fmt.Errorf("Failed to find prebuilt for source module: %s", module))
+		}
+	}
+	return prebuiltMod
+}
+
 func RegisterPrebuiltsPreArchMutators(ctx RegisterMutatorsContext) {
 	ctx.BottomUp("prebuilt_rename", PrebuiltRenameMutator).Parallel()
 }