Create an empty system_ext partition for aosp_cf_*

Cuttlefish uses erofs, which is now supported. With this CL, empty
system and system_ext filesystem soong modules will be created

This CL also makes ext4 the default fs. If
`BOARD_$PARTITION_IMAGE_FILE_SYSTEM_TYPE` is empty, ext4 will be used.

Bug: 372487849
Test: lunch aosp_cf_x86_64_phone-trunk_staging-userdebug && m nothing
Test: verified that soong modules are created for system and system_ext
Change-Id: Ib4eecb63e86a09a45853279c1afce5c26e5202d4
diff --git a/filesystem/filesystem.go b/filesystem/filesystem.go
index 87c6381..1ab07a2 100644
--- a/filesystem/filesystem.go
+++ b/filesystem/filesystem.go
@@ -198,6 +198,10 @@
 	unknown
 )
 
+func (fs fsType) IsUnknown() bool {
+	return fs == unknown
+}
+
 type FilesystemInfo struct {
 	// A text file containing the list of paths installed on the partition.
 	FileListFile android.Path
@@ -205,8 +209,7 @@
 
 var FilesystemProvider = blueprint.NewProvider[FilesystemInfo]()
 
-func (f *filesystem) fsType(ctx android.ModuleContext) fsType {
-	typeStr := proptools.StringDefault(f.properties.Type, "ext4")
+func GetFsTypeFromString(ctx android.EarlyModuleContext, typeStr string) fsType {
 	switch typeStr {
 	case "ext4":
 		return ext4Type
@@ -220,6 +223,11 @@
 		ctx.PropertyErrorf("type", "%q not supported", typeStr)
 		return unknown
 	}
+
+}
+
+func (f *filesystem) fsType(ctx android.ModuleContext) fsType {
+	return GetFsTypeFromString(ctx, proptools.StringDefault(f.properties.Type, "ext4"))
 }
 
 func (f *filesystem) installFileName() string {
diff --git a/fsgen/filesystem_creator.go b/fsgen/filesystem_creator.go
index 7db70e8..5ab1b9b 100644
--- a/fsgen/filesystem_creator.go
+++ b/fsgen/filesystem_creator.go
@@ -59,7 +59,7 @@
 }
 
 func (f *filesystemCreator) createInternalModules(ctx android.LoadHookContext) {
-	for _, partitionType := range []string{"system"} {
+	for _, partitionType := range []string{"system", "system_ext"} {
 		if f.createPartition(ctx, partitionType) {
 			f.properties.Generated_partition_types = append(f.properties.Generated_partition_types, partitionType)
 		} else {
@@ -88,11 +88,14 @@
 		Name: proptools.StringPtr(f.generatedModuleName(ctx.Config(), "device")),
 	}
 
-	// Currently, only the system partition module is created.
+	// Currently, only the system and system_ext partition module is created.
 	partitionProps := &filesystem.PartitionNameProperties{}
 	if android.InList("system", f.properties.Generated_partition_types) {
 		partitionProps.System_partition_name = proptools.StringPtr(f.generatedModuleNameForPartition(ctx.Config(), "system"))
 	}
+	if android.InList("system_ext", f.properties.Generated_partition_types) {
+		partitionProps.System_ext_partition_name = proptools.StringPtr(f.generatedModuleNameForPartition(ctx.Config(), "system_ext"))
+	}
 
 	ctx.CreateModule(filesystem.AndroidDeviceFactory, baseProps, partitionProps)
 }
@@ -128,11 +131,13 @@
 
 	fsProps.Partition_name = proptools.StringPtr(partitionType)
 	// BOARD_SYSTEMIMAGE_FILE_SYSTEM_TYPE
-	fsProps.Type = proptools.StringPtr(specificPartitionVars.BoardFileSystemType)
-	if *fsProps.Type != "ext4" {
-		// TODO(b/372522486): Support other FS types.
-		// Currently the android_filesystem module type only supports ext4:
-		// https://cs.android.com/android/platform/superproject/main/+/main:build/soong/filesystem/filesystem.go;l=416;drc=98047cfd07944b297a12d173453bc984806760d2
+	fsType := specificPartitionVars.BoardFileSystemType
+	if fsType == "" {
+		fsType = "ext4" //default
+	}
+	fsProps.Type = proptools.StringPtr(fsType)
+	if filesystem.GetFsTypeFromString(ctx, *fsProps.Type).IsUnknown() {
+		// Currently the android_filesystem module type only supports a handful of FS types like ext4, erofs
 		return false
 	}