libtxt: use a fixed set of font manager roles instead of a list of font managers (flutter/engine#4756)

Prior to this, the engine was adding font managers to the list every time the
app restarts (e.g. after a suspend-and-resume cycle), causing a leak.

engine/src/flutter/lib/ui/text/font_collection.cc

@@ -25,7 +25,7 @@ FontCollection& FontCollection::ForProcess() {
 
 FontCollection::FontCollection()
     : collection_(std::make_shared<txt::FontCollection>()) {
-  collection_->PushBack(SkFontMgr::RefDefault());
+  collection_->SetDefaultFontManager(SkFontMgr::RefDefault());
 }
 
 FontCollection::~FontCollection() = default;
@@ -106,7 +106,7 @@ void FontCollection::RegisterFontsFromAssetProvider(
     }
   }
 
-  collection_->PushFront(
+  collection_->SetAssetFontManager(
       sk_make_sp<txt::AssetFontManager>(std::move(font_asset_data_provider)));
 }
 
@@ -120,7 +120,7 @@ void FontCollection::RegisterTestFonts() {
   asset_data_provider->RegisterTypeface(std::move(test_typeface),
                                         GetTestFontFamilyName());
 
-  collection_->PushFront(sk_make_sp<txt::TestFontManager>(
+  collection_->SetTestFontManager(sk_make_sp<txt::TestFontManager>(
       std::move(asset_data_provider), GetTestFontFamilyName()));
 
   collection_->DisableFontFallback();

engine/src/flutter/third_party/txt/benchmarks/utils.cc

@@ -44,7 +44,7 @@ void SetCommandLine(fxl::CommandLine cmd) {
 
 std::shared_ptr<FontCollection> GetTestFontCollection() {
   auto collection = std::make_shared<FontCollection>();
-  collection->PushBack(sk_make_sp<AssetFontManager>(
+  collection->SetAssetFontManager(sk_make_sp<AssetFontManager>(
       std::make_unique<DirectoryAssetDataProvider>(GetFontDir())));
   return collection;
 }

engine/src/flutter/third_party/txt/src/txt/font_collection.cc

@@ -60,23 +60,31 @@ FontCollection::FontCollection() : enable_font_fallback_(true) {}
 FontCollection::~FontCollection() = default;
 
 size_t FontCollection::GetFontManagersCount() const {
-  return skia_font_managers_.size();
+  return GetFontManagerOrder().size();
 }
 
-void FontCollection::PushFront(sk_sp<SkFontMgr> skia_font_manager) {
-  if (!skia_font_manager) {
-    return;
-  }
-  UpdateFallbackFonts(skia_font_manager);
-  skia_font_managers_.push_front(std::move(skia_font_manager));
+void FontCollection::SetDefaultFontManager(sk_sp<SkFontMgr> font_manager) {
+  default_font_manager_ = font_manager;
 }
 
-void FontCollection::PushBack(sk_sp<SkFontMgr> skia_font_manager) {
-  if (!skia_font_manager) {
-    return;
-  }
-  UpdateFallbackFonts(skia_font_manager);
-  skia_font_managers_.push_back(std::move(skia_font_manager));
+void FontCollection::SetAssetFontManager(sk_sp<SkFontMgr> font_manager) {
+  asset_font_manager_ = font_manager;
+}
+
+void FontCollection::SetTestFontManager(sk_sp<SkFontMgr> font_manager) {
+  test_font_manager_ = font_manager;
+}
+
+// Return the available font managers in the order they should be queried.
+std::vector<sk_sp<SkFontMgr>> FontCollection::GetFontManagerOrder() const {
+  std::vector<sk_sp<SkFontMgr>> order;
+  if (test_font_manager_)
+    order.push_back(test_font_manager_);
+  if (asset_font_manager_)
+    order.push_back(asset_font_manager_);
+  if (default_font_manager_)
+    order.push_back(default_font_manager_);
+  return order;
 }
 
 void FontCollection::DisableFontFallback() {
@@ -91,7 +99,7 @@ FontCollection::GetMinikinFontCollectionForFamily(const std::string& family) {
     return cached->second;
   }
 
-  for (sk_sp<SkFontMgr> manager : skia_font_managers_) {
+  for (sk_sp<SkFontMgr>& manager : GetFontManagerOrder()) {
     auto font_style_set = manager->matchFamily(family.c_str());
     if (font_style_set == nullptr || font_style_set->count() == 0) {
       continue;
@@ -157,7 +165,7 @@ FontCollection::GetMinikinFontCollectionForFamily(const std::string& family) {
 
 const std::shared_ptr<minikin::FontFamily>& FontCollection::MatchFallbackFont(
     uint32_t ch) {
-  for (const auto& manager : skia_font_managers_) {
+  for (const sk_sp<SkFontMgr>& manager : GetFontManagerOrder()) {
     sk_sp<SkTypeface> typeface(
         manager->matchFamilyStyleCharacter(0, SkFontStyle(), nullptr, 0, ch));
     if (!typeface)

engine/src/flutter/third_party/txt/src/txt/font_collection.h

@@ -39,9 +39,9 @@ class FontCollection : public std::enable_shared_from_this<FontCollection> {
 
   size_t GetFontManagersCount() const;
 
-  void PushFront(sk_sp<SkFontMgr> skia_font_manager);
-
-  void PushBack(sk_sp<SkFontMgr> skia_font_manager);
+  void SetDefaultFontManager(sk_sp<SkFontMgr> font_manager);
+  void SetAssetFontManager(sk_sp<SkFontMgr> font_manager);
+  void SetTestFontManager(sk_sp<SkFontMgr> font_manager);
 
   std::shared_ptr<minikin::FontCollection> GetMinikinFontCollectionForFamily(
       const std::string& family);
@@ -53,7 +53,9 @@ class FontCollection : public std::enable_shared_from_this<FontCollection> {
   void DisableFontFallback();
 
  private:
-  std::deque<sk_sp<SkFontMgr>> skia_font_managers_;
+  sk_sp<SkFontMgr> default_font_manager_;
+  sk_sp<SkFontMgr> asset_font_manager_;
+  sk_sp<SkFontMgr> test_font_manager_;
   std::unordered_map<std::string, std::shared_ptr<minikin::FontCollection>>
       font_collections_cache_;
   std::unordered_map<SkFontID, std::shared_ptr<minikin::FontFamily>>
@@ -61,6 +63,8 @@ class FontCollection : public std::enable_shared_from_this<FontCollection> {
   std::shared_ptr<minikin::FontFamily> null_family_;
   bool enable_font_fallback_;
 
+  std::vector<sk_sp<SkFontMgr>> GetFontManagerOrder() const;
+
   const std::shared_ptr<minikin::FontFamily>& GetFontFamilyForTypeface(
       const sk_sp<SkTypeface>& typeface);
 

engine/src/flutter/third_party/txt/tests/render_test.cc

@@ -30,7 +30,7 @@ namespace txt {
 
 RenderTest::RenderTest()
     : snapshots_(0), font_collection_(std::make_shared<FontCollection>()) {
-  font_collection_->PushBack(sk_make_sp<AssetFontManager>(
+  font_collection_->SetAssetFontManager(sk_make_sp<AssetFontManager>(
       std::make_unique<txt::DirectoryAssetDataProvider>(GetFontDir())));
 }