Extract logic to gather deps added for <apex>:<module> pairs

Trades having to traverse a module's direct dependencies multiple times
for reusable code. While at the minute the amount of duplicated code is
small (checking a tag and appending to a list) follow up changes will
modify the gatherApexModulePairDepsWithTag module to improve error
reporting greatly increasing the benefit of deduping.

The cost of traversing a module's direct dependencies is very small as
there will be only a very few dependencies.

Bug: 177892522
Test: m nothing
Change-Id: I16389102abd8038e1bfa52b63f4153bdf92ff553

java/bootclasspath.go

@@ -73,6 +73,8 @@ func addDependencyOntoApexVariants(ctx android.BottomUpMutatorContext, propertyN
 // module. This adds dependencies onto the prebuilt and source modules with the specified name,
 // depending on which ones are available. Visiting must use isActiveModule to select the preferred
 // module when both source and prebuilt modules are available.
+//
+// Use gatherApexModulePairDepsWithTag to retrieve the dependencies.
 func addDependencyOntoApexModulePair(ctx android.BottomUpMutatorContext, apex string, name string, tag blueprint.DependencyTag) {
 	var variations []blueprint.Variation
 	if apex != "platform" && apex != "system_ext" {
@@ -118,6 +120,19 @@ func reportMissingVariationDependency(ctx android.BottomUpMutatorContext, variat
 	ctx.AddFarVariationDependencies(variations, nil, name)
 }
 
+// gatherApexModulePairDepsWithTag returns the list of dependencies with the supplied tag that was
+// added by addDependencyOntoApexModulePair.
+func gatherApexModulePairDepsWithTag(ctx android.BaseModuleContext, tag blueprint.DependencyTag) []android.Module {
+	var modules []android.Module
+	ctx.VisitDirectDepsIf(isActiveModule, func(module android.Module) {
+		t := ctx.OtherModuleDependencyTag(module)
+		if t == tag {
+			modules = append(modules, module)
+		}
+	})
+	return modules
+}
+
 // ApexVariantReference specifies a particular apex variant of a module.
 type ApexVariantReference struct {
 	// The name of the module apex variant, i.e. the apex containing the module variant.

java/platform_bootclasspath.go

@@ -152,14 +152,8 @@ func addDependenciesOntoBootImageModules(ctx android.BottomUpMutatorContext, mod
 func (b *platformBootclasspathModule) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 	b.classpathFragmentBase().generateAndroidBuildActions(ctx)
 
-	ctx.VisitDirectDepsIf(isActiveModule, func(module android.Module) {
+	b.configuredModules = gatherApexModulePairDepsWithTag(ctx, platformBootclasspathModuleDepTag)
-		tag := ctx.OtherModuleDependencyTag(module)
+	b.fragments = gatherApexModulePairDepsWithTag(ctx, bootclasspathFragmentDepTag)
-		if tag == platformBootclasspathModuleDepTag {
-			b.configuredModules = append(b.configuredModules, module)
-		} else if tag == bootclasspathFragmentDepTag {
-			b.fragments = append(b.fragments, module)
-		}
-	})
 
 	b.generateHiddenAPIBuildActions(ctx, b.configuredModules, b.fragments)