Add support installing to root of filesystem image

If the partition of a spec is "root", that spec will be installed to
root instead. Normally that spec will be from prebuilt_root module with
install_in_root property.

Bug: 351258461
Test: USE_SOONG_DEFINED_SYSTEM_IMAGE=true m && cvd start
Change-Id: Iaaa9c2fb8a81fe0ba4710c641e1b65c5b71ad4a4
diff --git a/filesystem/filesystem.go b/filesystem/filesystem.go
index 5add954..5c7ef43 100644
--- a/filesystem/filesystem.go
+++ b/filesystem/filesystem.go
@@ -312,6 +312,25 @@
 	}
 }
 
+func (f *filesystem) copyPackagingSpecs(ctx android.ModuleContext, builder *android.RuleBuilder, specs map[string]android.PackagingSpec, rootDir, rebasedDir android.WritablePath) []string {
+	rootDirSpecs := make(map[string]android.PackagingSpec)
+	rebasedDirSpecs := make(map[string]android.PackagingSpec)
+
+	for rel, spec := range specs {
+		if spec.Partition() == "root" {
+			rootDirSpecs[rel] = spec
+		} else {
+			rebasedDirSpecs[rel] = spec
+		}
+	}
+
+	dirsToSpecs := make(map[android.WritablePath]map[string]android.PackagingSpec)
+	dirsToSpecs[rootDir] = rootDirSpecs
+	dirsToSpecs[rebasedDir] = rebasedDirSpecs
+
+	return f.CopySpecsToDirs(ctx, builder, dirsToSpecs)
+}
+
 func (f *filesystem) buildImageUsingBuildImage(ctx android.ModuleContext) android.OutputPath {
 	rootDir := android.PathForModuleOut(ctx, "root").OutputPath
 	rebasedDir := rootDir
@@ -322,7 +341,7 @@
 	// Wipe the root dir to get rid of leftover files from prior builds
 	builder.Command().Textf("rm -rf %s && mkdir -p %s", rootDir, rootDir)
 	specs := f.gatherFilteredPackagingSpecs(ctx)
-	f.entries = f.CopySpecsToDir(ctx, builder, specs, rebasedDir)
+	f.entries = f.copyPackagingSpecs(ctx, builder, specs, rootDir, rebasedDir)
 
 	f.buildNonDepsFiles(ctx, builder, rootDir)
 	f.addMakeBuiltFiles(ctx, builder, rootDir)
@@ -465,7 +484,7 @@
 	// Wipe the root dir to get rid of leftover files from prior builds
 	builder.Command().Textf("rm -rf %s && mkdir -p %s", rootDir, rootDir)
 	specs := f.gatherFilteredPackagingSpecs(ctx)
-	f.entries = f.CopySpecsToDir(ctx, builder, specs, rebasedDir)
+	f.entries = f.copyPackagingSpecs(ctx, builder, specs, rootDir, rebasedDir)
 
 	f.buildNonDepsFiles(ctx, builder, rootDir)
 	f.buildFsverityMetadataFiles(ctx, builder, specs, rootDir, rebasedDir)
diff --git a/filesystem/system_image.go b/filesystem/system_image.go
index 15cacfb..69d922d 100644
--- a/filesystem/system_image.go
+++ b/filesystem/system_image.go
@@ -94,9 +94,10 @@
 	return output
 }
 
-// Filter the result of GatherPackagingSpecs to discard items targeting outside "system" partition.
-// Note that "apex" module installs its contents to "apex"(fake partition) as well
+// Filter the result of GatherPackagingSpecs to discard items targeting outside "system" / "root"
+// partition.  Note that "apex" module installs its contents to "apex"(fake partition) as well
 // for symbol lookup by imitating "activated" paths.
 func (s *systemImage) filterPackagingSpec(ps android.PackagingSpec) bool {
-	return s.filesystem.filterInstallablePackagingSpec(ps) && ps.Partition() == "system"
+	return s.filesystem.filterInstallablePackagingSpec(ps) &&
+		(ps.Partition() == "system" || ps.Partition() == "root")
 }