Add SOONG_SDK_SNAPSHOT_PREFER support

By default the generated snapshot has prefer: false. Building it with
SOONG_SDK_SNAPSHOT_PREFER=true will force it to generate prefer: true.

Bug: 157884619
Test: m nothing
      m SOONG_SDK_SNAPSHOT_PREFER=true art-module-sdk
      - check the generated Android.bp file to make sure it contains
        prefer: true
Change-Id: Ied297b32d9bd4822a140fa99016f38e234c50f64

sdk/sdk_test.go

@@ -517,6 +517,45 @@ java_import {
     jars: ["java/myjavalib.jar"],
 }
 
+sdk_snapshot {
+    name: "mysdk@current",
+    visibility: ["//visibility:public"],
+    java_header_libs: ["mysdk_myjavalib@current"],
+}
+			`),
+		)
+	})
+
+	t.Run("SOONG_SDK_SNAPSHOT_PREFER=true", func(t *testing.T) {
+		result := android.GroupFixturePreparers(
+			preparer,
+			android.FixtureMergeEnv(map[string]string{
+				"SOONG_SDK_SNAPSHOT_PREFER": "true",
+			}),
+		).RunTest(t)
+
+		checkZipFile(t, result, "out/soong/.intermediates/mysdk/common_os/mysdk-current.zip")
+
+		CheckSnapshot(t, result, "mysdk", "",
+			checkAndroidBpContents(`
+// This is auto-generated. DO NOT EDIT.
+
+java_import {
+    name: "mysdk_myjavalib@current",
+    sdk_member_name: "myjavalib",
+    visibility: ["//visibility:public"],
+    apex_available: ["//apex_available:platform"],
+    jars: ["java/myjavalib.jar"],
+}
+
+java_import {
+    name: "myjavalib",
+    prefer: true,
+    visibility: ["//visibility:public"],
+    apex_available: ["//apex_available:platform"],
+    jars: ["java/myjavalib.jar"],
+}
+
 sdk_snapshot {
     name: "mysdk@current",
     visibility: ["//visibility:public"],

sdk/update.go

@@ -29,6 +29,14 @@ import (
 	"android/soong/android"
 )
 
+// Environment variables that affect the generated snapshot
+// ========================================================
+//
+// SOONG_SDK_SNAPSHOT_PREFER
+//     By default every unversioned module in the generated snapshot has prefer: false. Building it
+//     with SOONG_SDK_SNAPSHOT_PREFER=true will force them to use prefer: true.
+//
+
 var pctx = android.NewPackageContext("android/soong/sdk")
 
 var (
@@ -274,7 +282,14 @@ func (s *sdk) buildSnapshot(ctx android.ModuleContext, sdkVariants []*sdk) andro
 
 	// Create a transformer that will transform an unversioned module by replacing any references
 	// to internal members with a unique module name and setting prefer: false.
-	unversionedTransformer := unversionedTransformation{builder: builder}
+	unversionedTransformer := unversionedTransformation{
+		builder: builder,
+		// Set the prefer based on the environment variable. This is a temporary work around to allow a
+		// snapshot to be created that sets prefer: true.
+		// TODO(b/174997203): Remove once the ability to select the modules to prefer can be done
+		//  dynamically at build time not at snapshot generation time.
+		prefer: ctx.Config().IsEnvTrue("SOONG_SDK_SNAPSHOT_PREFER"),
+	}
 
 	for _, unversioned := range builder.prebuiltOrder {
 		// Prune any empty property sets.
@@ -614,6 +629,7 @@ func (t unversionedToVersionedTransformation) transformProperty(name string, val
 type unversionedTransformation struct {
 	identityTransformation
 	builder *snapshotBuilder
+	prefer  bool
 }
 
 func (t unversionedTransformation) transformModule(module *bpModule) *bpModule {
@@ -621,8 +637,11 @@ func (t unversionedTransformation) transformModule(module *bpModule) *bpModule {
 	name := module.getValue("name").(string)
 	module.setProperty("name", t.builder.unversionedSdkMemberName(name, true))
 
-	// Set prefer: false - this is not strictly required as that is the default.
-	module.insertAfter("name", "prefer", false)
+	// Set prefer. Setting this to false is not strictly required as that is the default but it does
+	// provide a convenient hook to post-process the generated Android.bp file, e.g. in tests to check
+	// the behavior when a prebuilt is preferred. It also makes it explicit what the default behavior
+	// is for the module.
+	module.insertAfter("name", "prefer", t.prefer)
 
 	return module
 }