Add bp2build mode to soong_build.

This CL adds a new CONVERT_TO_BAZEL env var, and a bp2build goal. It
reuses the queryview architecture, but registers no mutators before
converting to BUILD files. This gives us a blank slate of the module
graph to work with, and create a shadow/alternate pipeline of mutators
to converge the Bazel BUILD graph to be semantically equivalent with the
Android.bp graph (after apply its current set of mutators).

The command to do so is:

    $ CONVERT_TO_BAZEL=true m bp2build

To not clobber with queryview, the generated files are in
out/soong/bp2build.

Test: CONVERT_TO_BAZEL=true m bp2build && bazel query --config=bp2build //...
Test: m queryview && bazel query --config=queryview //..
Test: soong tests
Test: TH presubmit
Fixes: 174465461
Signed-off-by: Jingwen Chen <jingwen@google.com>
Change-Id: I4dd0ccc73abc345d70a50ca2803d6f400cd8c863
diff --git a/android/queryview.go b/android/queryview.go
index 970ae01..1b7e77d 100644
--- a/android/queryview.go
+++ b/android/queryview.go
@@ -26,15 +26,40 @@
 // for calling the soong_build primary builder in the main build.ninja file.
 func init() {
 	RegisterSingletonType("bazel_queryview", BazelQueryViewSingleton)
+	RegisterSingletonType("bazel_converter", BazelConverterSingleton)
 }
 
+// BazelQueryViewSingleton is the singleton responsible for registering the
+// soong_build build statement that will convert the Soong module graph after
+// applying *all* mutators, enabing the feature to query the final state of the
+// Soong graph. This mode is meant for querying the build graph state, and not meant
+// for generating BUILD files to be checked in.
 func BazelQueryViewSingleton() Singleton {
 	return &bazelQueryViewSingleton{}
 }
 
-type bazelQueryViewSingleton struct{}
+// BazelConverterSingleton is the singleton responsible for registering the soong_build
+// build statement that will convert the Soong module graph by applying an alternate
+// pipeline of mutators, with the goal of reaching semantic equivalence between the original
+// Blueprint and final BUILD files. Using this mode, the goal is to be able to
+// build with these BUILD files directly in the source tree.
+func BazelConverterSingleton() Singleton {
+	return &bazelConverterSingleton{}
+}
 
-func (c *bazelQueryViewSingleton) GenerateBuildActions(ctx SingletonContext) {
+type bazelQueryViewSingleton struct{}
+type bazelConverterSingleton struct{}
+
+func generateBuildActionsForBazelConversion(ctx SingletonContext, converterMode bool) {
+	name := "queryview"
+	additionalEnvVars := ""
+	descriptionTemplate := "[EXPERIMENTAL, PRE-PRODUCTION] Creating the Bazel QueryView workspace with %s at $outDir"
+	if converterMode {
+		name = "bp2build"
+		additionalEnvVars = "CONVERT_TO_BAZEL=true"
+		descriptionTemplate = "[EXPERIMENTAL, PRE-PRODUCTION] Converting all Android.bp to Bazel BUILD files with %s at $outDir"
+	}
+
 	// Create a build and rule statement, using the Bazel QueryView's WORKSPACE
 	// file as the output file marker.
 	var deps Paths
@@ -42,22 +67,23 @@
 	deps = append(deps, moduleListFilePath)
 	deps = append(deps, pathForBuildToolDep(ctx, ctx.Config().ProductVariablesFileName))
 
-	bazelQueryViewDirectory := PathForOutput(ctx, "queryview")
+	bazelQueryViewDirectory := PathForOutput(ctx, name)
 	bazelQueryViewWorkspaceFile := bazelQueryViewDirectory.Join(ctx, "WORKSPACE")
 	primaryBuilder := primaryBuilderPath(ctx)
 	bazelQueryView := ctx.Rule(pctx, "bazelQueryView",
 		blueprint.RuleParams{
 			Command: fmt.Sprintf(
 				"rm -rf ${outDir}/* && "+
-					"%s --bazel_queryview_dir ${outDir} %s && "+
+					"%s %s --bazel_queryview_dir ${outDir} %s && "+
 					"echo WORKSPACE: `cat %s` > ${outDir}/.queryview-depfile.d",
+				additionalEnvVars,
 				primaryBuilder.String(),
 				strings.Join(os.Args[1:], " "),
 				moduleListFilePath.String(), // Use the contents of Android.bp.list as the depfile.
 			),
 			CommandDeps: []string{primaryBuilder.String()},
 			Description: fmt.Sprintf(
-				"[EXPERIMENTAL, PRE-PRODUCTION] Creating the Bazel QueryView workspace with %s at $outDir",
+				descriptionTemplate,
 				primaryBuilder.Base()),
 			Deps:    blueprint.DepsGCC,
 			Depfile: "${outDir}/.queryview-depfile.d",
@@ -73,6 +99,14 @@
 		},
 	})
 
-	// Add a phony target for building the Bazel QueryView
-	ctx.Phony("queryview", bazelQueryViewWorkspaceFile)
+	// Add a phony target for generating the workspace
+	ctx.Phony(name, bazelQueryViewWorkspaceFile)
+}
+
+func (c *bazelQueryViewSingleton) GenerateBuildActions(ctx SingletonContext) {
+	generateBuildActionsForBazelConversion(ctx, false)
+}
+
+func (c *bazelConverterSingleton) GenerateBuildActions(ctx SingletonContext) {
+	generateBuildActionsForBazelConversion(ctx, true)
 }