Create a AOSP Bazel overlay workspace with Soong

The Bazel overlay is a directory at out/soong/bazel_overlay that
replicates the layout of the AOSP Soong module tree, but as a Bazel
workspace. Each Soong module variant is represented as a BUILD target
created with the `soong_module` rule.

To create this overlay, run `m bazel_overlay`.

A `soong_module` target can depend on other `soong_module` targets.
These dependencies replicate each module's `directDeps` in the Blueprint
graph, just before `PrepareBuildActions`.

This enables users to use bazel query as a way to introspect the Soong
module graph. For example,

- Direct reverse dependencies of //bionic/libc:generated_android_ids in
//bionic/libc/...:

$ bazel query 'rdeps(//bionic/libc/...,
//bionic/libc:generated_android_ids, 1)'
//bionic/libc:libc_bionic_ndk--android_recovery_arm_armv7-a-neon_static
//bionic/libc:libc_bionic_ndk--android_ramdisk_arm_armv7-a-neon_static
//bionic/libc:libc_bionic_ndk--android_arm_armv7-a-neon_static_com.android.runtime
//bionic/libc:libc_bionic_ndk--android_arm_armv7-a-neon_static
//bionic/libc:generated_android_ids

- Why does com.android.runtime depend on lzma?

$ bazel query
'somepath(//bionic/apex:com.android.runtime--android_common_com.android.runtime_image,
//external/lzma/...)'
//bionic/apex:com.android.runtime--android_common_com.android.runtime_image
//bionic/libc/malloc_debug:libc_malloc_debug--android_arm_armv7-a-neon_shared_com.android.runtime
//system/core/libunwindstack:libunwindstack--android_arm_armv7-a-neon_shared_com.android.runtime
//external/lzma/C:liblzma--android_arm_armv7-a-neon_shared_com.android.runtime

- What does the dep graph of //bionic/libc:crtbegin_so look like?

$ bazel query
'deps(//bionic/libc:crtbegin_so--android_arm_armv7-a-neon)'
--output=graph > graph.in && dot -Tpng < graph.in > graph.png

https://photos.app.goo.gl/DfsdoFRNsRjGwTmy8

Test:  croot && m bazel_overlay && cd out/soong/bazel_overlay && bazel
query //... && bazel query 'rdeps(//bionic/libc/...,
//bionic/libc:generated_android_ids, 1)'

Signed-off-by: Jingwen Chen <jingwen@google.com>
Change-Id: I3bf40309bfb2d963bb8a688706385a57ee304c37#

cmd/soong_build/main.go

@@ -26,11 +26,13 @@ import (
 )
 
 var (
-	docFile string
+	docFile         string
+	bazelOverlayDir string
 )
 
 func init() {
 	flag.StringVar(&docFile, "soong_docs", "", "build documentation file to output")
+	flag.StringVar(&bazelOverlayDir, "bazel_overlay_dir", "", "path to the bazel overlay directory")
 }
 
 func newNameResolver(config android.Config) *android.NameResolver {
@@ -65,7 +67,7 @@ func main() {
 		os.Exit(1)
 	}
 
-	if docFile != "" {
+	if !shouldPrepareBuildActions() {
 		configuration.SetStopBefore(bootstrap.StopBeforePrepareBuildActions)
 	}
 
@@ -85,6 +87,13 @@ func main() {
 
 	bootstrap.Main(ctx.Context, configuration, extraNinjaDeps...)
 
+	if bazelOverlayDir != "" {
+		if err := createBazelOverlay(ctx, bazelOverlayDir); err != nil {
+			fmt.Fprintf(os.Stderr, "%s", err)
+			os.Exit(1)
+		}
+	}
+
 	if docFile != "" {
 		if err := writeDocs(ctx, docFile); err != nil {
 			fmt.Fprintf(os.Stderr, "%s", err)
@@ -94,7 +103,7 @@ func main() {
 
 	// TODO(ccross): make this a command line argument.  Requires plumbing through blueprint
 	//  to affect the command line of the primary builder.
-	if docFile == "" {
+	if shouldPrepareBuildActions() {
 		metricsFile := filepath.Join(bootstrap.BuildDir, "soong_build_metrics.pb")
 		err = android.WriteMetrics(configuration, metricsFile)
 		if err != nil {
@@ -103,3 +112,9 @@ func main() {
 		}
 	}
 }
+
+func shouldPrepareBuildActions() bool {
+	// If we're writing soong_docs or bazel_overlay, don't write build.ninja or
+	// collect metrics.
+	return docFile == "" && bazelOverlayDir == ""
+}