[res] Don't create extra asset provider when not needed

MultiAssetsProvider is useful to combine a ResourcesLoader with
a file-based provider, but most of the time it's used without
any loaders, and ends up with an EmptyAssetsProvider object that
just wastes cycles and RAM.

This CL optimizes that out, only creating the extra layer when
we have a real loader, and falling back to the only remaining
provider otherwise

Bug: 319137634
Test: build + boot + manual + atest libandroidfw_tests
Flag: EXEMPT optimization
Change-Id: Ibc5ac60adc5e008b70a62481a747758c89083ff9

2 files changed, 12 insertions, 7 deletions

diff --git a/libs/androidfw/AssetsProvider.cpp b/libs/androidfw/AssetsProvider.cpp
index 59c6aba7b8b0..11b12eb030a6 100644
--- a/libs/androidfw/AssetsProvider.cpp
+++ b/libs/androidfw/AssetsProvider.cpp
@@ -24,9 +24,8 @@
 #include <ziparchive/zip_archive.h>
 
 namespace android {
-namespace {
-constexpr const char* kEmptyDebugString = "<empty>";
-} // namespace
+
+static constexpr std::string_view kEmptyDebugString = "<empty>";
 
 std::unique_ptr<Asset> AssetsProvider::Open(const std::string& path, Asset::AccessMode mode,
                                             bool* file_exists) const {
@@ -356,8 +355,14 @@ MultiAssetsProvider::MultiAssetsProvider(std::unique_ptr<AssetsProvider>&& prima
 
 std::unique_ptr<AssetsProvider> MultiAssetsProvider::Create(
     std::unique_ptr<AssetsProvider>&& primary, std::unique_ptr<AssetsProvider>&& secondary) {
-  if (primary == nullptr || secondary == nullptr) {
-    return nullptr;
+  if (primary == nullptr && secondary == nullptr) {
+    return EmptyAssetsProvider::Create();
+  }
+  if (!primary) {
+    return secondary;
+  }
+  if (!secondary) {
+    return primary;
   }
   return std::unique_ptr<MultiAssetsProvider>(new MultiAssetsProvider(std::move(primary),
                                                                       std::move(secondary)));
@@ -425,7 +430,7 @@ const std::string& EmptyAssetsProvider::GetDebugName() const {
   if (path_.has_value()) {
     return *path_;
   }
-  const static std::string kEmpty = kEmptyDebugString;
+  constexpr static std::string kEmpty{kEmptyDebugString};
   return kEmpty;
 }
 
diff --git a/libs/androidfw/include/androidfw/AssetsProvider.h b/libs/androidfw/include/androidfw/AssetsProvider.h
index 46d7a57ab763..e3b3ae41f7f4 100644
--- a/libs/androidfw/include/androidfw/AssetsProvider.h
+++ b/libs/androidfw/include/androidfw/AssetsProvider.h
@@ -164,7 +164,7 @@ struct DirectoryAssetsProvider : public AssetsProvider {
 // `secondary` asset provider if the asset cannot be found in the `primary`.
 struct MultiAssetsProvider : public AssetsProvider {
   static std::unique_ptr<AssetsProvider> Create(std::unique_ptr<AssetsProvider>&& primary,
-                                                std::unique_ptr<AssetsProvider>&& secondary);
+                                                std::unique_ptr<AssetsProvider>&& secondary = {});
 
   bool ForEachFile(const std::string& root_path,
                    base::function_ref<void(StringPiece, FileType)> f) const override;