libtxt: extend Minikin to find a fallback font if its font collection can not match a given character (#4623)

Minikin layout uses a FontCollection containing a list of pre-selected fonts
for a particular font family.  This patch extends the FontCollection to invoke
a hook provided by libtxt if layout sees a character that can not be rendered
by any font in the collection.

* Change the Minikin lock to a recursive mutex.  This is required because the
  fallback font provider may create new fonts during a layout operation that
  already holds the lock.
* Implement a fallback font provider hook that queries Skia for fonts matching
  an unrecognized character.
* Maintain a cache of fallback fonts.  Prepopulate the cache with fonts
  covering some commonly used character classes.
* Add a last resort font list for cases where Skia's font manager can not
  find any font for a character (similar to Blink's FontCache::getLastResortFallbackFont)

third_party/txt/src/minikin/FontCollection.cpp

@@ -59,7 +59,7 @@ FontCollection::FontCollection(
 
 void FontCollection::init(
     const vector<std::shared_ptr<FontFamily>>& typefaces) {
-  std::lock_guard<std::mutex> _l(gMinikinLock);
+  std::lock_guard<std::recursive_mutex> _l(gMinikinLock);
   mId = sNextId++;
   vector<uint32_t> lastChar;
   size_t nTypefaces = typefaces.size();
@@ -321,6 +321,15 @@ const std::shared_ptr<FontFamily>& FontCollection::getFamilyForChar(
     }
   }
   if (bestFamilyIndex == -1) {
+    // libtxt: check if the fallback font provider can match this character
+    if (mFallbackFontProvider) {
+      const std::shared_ptr<FontFamily>& fallback =
+          mFallbackFontProvider->matchFallbackFont(ch);
+      if (fallback) {
+        return fallback;
+      }
+    }
+
     UErrorCode errorCode = U_ZERO_ERROR;
     const UNormalizer2* normalizer = unorm2_getNFDInstance(&errorCode);
     if (U_SUCCESS(errorCode)) {
@@ -380,7 +389,7 @@ bool FontCollection::hasVariationSelector(uint32_t baseCodepoint,
     return false;
   }
 
-  std::lock_guard<std::mutex> _l(gMinikinLock);
+  std::lock_guard<std::recursive_mutex> _l(gMinikinLock);
 
   // Currently mRanges can not be used here since it isn't aware of the
   // variation sequence.

third_party/txt/src/minikin/FontCollection.h

@@ -32,6 +32,15 @@ class FontCollection {
       const std::vector<std::shared_ptr<FontFamily>>& typefaces);
   explicit FontCollection(std::shared_ptr<FontFamily>&& typeface);
 
+  // libtxt extension: an interface for looking up fallback fonts for characters
+  // that do not match this collection's font families.
+  class FallbackFontProvider {
+   public:
+    virtual ~FallbackFontProvider() = default;
+    virtual const std::shared_ptr<FontFamily>& matchFallbackFont(
+        uint32_t ch) = 0;
+  };
+
   struct Run {
     FakedFont fakedFont;
     int start;
@@ -63,6 +72,10 @@ class FontCollection {
 
   uint32_t getId() const;
 
+  void set_fallback_font_provider(std::unique_ptr<FallbackFontProvider> ffp) {
+    mFallbackFontProvider = std::move(ffp);
+  }
+
  private:
   static const int kLogCharsPerPage = 8;
   static const int kPageMask = (1 << kLogCharsPerPage) - 1;
@@ -131,6 +144,9 @@ class FontCollection {
 
   // Set of supported axes in this collection.
   std::unordered_set<AxisTag> mSupportedAxes;
+
+  // libtxt extension: Fallback font provider.
+  std::unique_ptr<FallbackFontProvider> mFallbackFontProvider;
 };
 
 }  // namespace minikin

third_party/txt/src/minikin/FontFamily.cpp

@@ -56,7 +56,7 @@ android::hash_t FontStyle::hash() const {
 
 // static
 uint32_t FontStyle::registerLanguageList(const std::string& languages) {
-  std::lock_guard<std::mutex> _l(gMinikinLock);
+  std::lock_guard<std::recursive_mutex> _l(gMinikinLock);
   return FontLanguageListCache::getId(languages);
 }
 
@@ -115,7 +115,7 @@ FontFamily::FontFamily(uint32_t langId, int variant, std::vector<Font>&& fonts)
 bool FontFamily::analyzeStyle(const std::shared_ptr<MinikinFont>& typeface,
                               int* weight,
                               bool* italic) {
-  std::lock_guard<std::mutex> _l(gMinikinLock);
+  std::lock_guard<std::recursive_mutex> _l(gMinikinLock);
   const uint32_t os2Tag = MinikinFont::MakeTag('O', 'S', '/', '2');
   HbBlob os2Table(getFontTable(typeface.get(), os2Tag));
   if (os2Table.get() == nullptr)
@@ -175,7 +175,7 @@ bool FontFamily::isColorEmojiFamily() const {
 }
 
 void FontFamily::computeCoverage() {
-  std::lock_guard<std::mutex> _l(gMinikinLock);
+  std::lock_guard<std::recursive_mutex> _l(gMinikinLock);
   const FontStyle defaultStyle;
   const MinikinFont* typeface = getClosestMatch(defaultStyle).font;
   const uint32_t cmapTag = MinikinFont::MakeTag('c', 'm', 'a', 'p');
@@ -233,7 +233,7 @@ std::shared_ptr<FontFamily> FontFamily::createFamilyWithVariation(
   std::vector<Font> fonts;
   for (const Font& font : mFonts) {
     bool supportedVariations = false;
-    std::lock_guard<std::mutex> _l(gMinikinLock);
+    std::lock_guard<std::recursive_mutex> _l(gMinikinLock);
     std::unordered_set<AxisTag> supportedAxes = font.getSupportedAxesLocked();
     if (!supportedAxes.empty()) {
       for (const FontVariation& variation : variations) {

third_party/txt/src/minikin/Layout.cpp

@@ -604,7 +604,7 @@ void Layout::doLayout(const uint16_t* buf,
                       const FontStyle& style,
                       const MinikinPaint& paint,
                       const std::shared_ptr<FontCollection>& collection) {
-  std::lock_guard<std::mutex> _l(gMinikinLock);
+  std::lock_guard<std::recursive_mutex> _l(gMinikinLock);
 
   LayoutContext ctx;
   ctx.style = style;
@@ -628,7 +628,7 @@ float Layout::measureText(const uint16_t* buf,
                           const MinikinPaint& paint,
                           const std::shared_ptr<FontCollection>& collection,
                           float* advances) {
-  std::lock_guard<std::mutex> _l(gMinikinLock);
+  std::lock_guard<std::recursive_mutex> _l(gMinikinLock);
 
   LayoutContext ctx;
   ctx.style = style;
@@ -1197,7 +1197,7 @@ void Layout::getBounds(MinikinRect* bounds) const {
 }
 
 void Layout::purgeCaches() {
-  std::lock_guard<std::mutex> _l(gMinikinLock);
+  std::lock_guard<std::recursive_mutex> _l(gMinikinLock);
   LayoutCache& layoutCache = LayoutEngine::getInstance().layoutCache;
   layoutCache.clear();
   purgeHbFontCacheLocked();

third_party/txt/src/minikin/MinikinFont.cpp

@@ -21,7 +21,7 @@
 namespace minikin {
 
 MinikinFont::~MinikinFont() {
-  std::lock_guard<std::mutex> _l(gMinikinLock);
+  std::lock_guard<std::recursive_mutex> _l(gMinikinLock);
   purgeHbFontLocked(this);
 }
 

third_party/txt/src/minikin/MinikinInternal.cpp

@@ -24,7 +24,7 @@
 
 namespace minikin {
 
-std::mutex gMinikinLock;
+std::recursive_mutex gMinikinLock;
 
 void assertMinikinLocked() {
 #ifdef ENABLE_RACE_DETECTION

third_party/txt/src/minikin/MinikinInternal.h

@@ -31,7 +31,7 @@ namespace minikin {
 // Presently, that's implemented by through a global lock, and having
 // all external interfaces take that lock.
 
-extern std::mutex gMinikinLock;
+extern std::recursive_mutex gMinikinLock;
 
 // Aborts if gMinikinLock is not acquired. Do nothing on the release build.
 void assertMinikinLocked();

third_party/txt/src/txt/font_collection.cc

@@ -41,8 +41,29 @@ const std::vector<SkUnichar> fallback_characters{
     0x627,    // Arabic
 };
 
+// Font families that will be used as a last resort if no font manager provides
+// a font matching a particular character.
+const std::vector<std::string> last_resort_fonts{
+    "Arial",
+};
+
 }  // anonymous namespace
 
+class TxtFallbackFontProvider
+    : public minikin::FontCollection::FallbackFontProvider {
+ public:
+  TxtFallbackFontProvider(std::shared_ptr<FontCollection> font_collection)
+      : font_collection_(font_collection) {}
+
+  virtual const std::shared_ptr<minikin::FontFamily>& matchFallbackFont(
+      uint32_t ch) {
+    return font_collection_->MatchFallbackFont(ch);
+  }
+
+ private:
+  std::shared_ptr<FontCollection> font_collection_;
+};
+
 FontCollection::FontCollection() = default;
 
 FontCollection::~FontCollection() = default;
@@ -117,6 +138,8 @@ FontCollection::GetMinikinFontCollectionForFamily(const std::string& family) {
     // Create the minikin font collection.
     auto font_collection =
         std::make_shared<minikin::FontCollection>(std::move(minikin_families));
+    font_collection->set_fallback_font_provider(
+        std::make_unique<TxtFallbackFontProvider>(shared_from_this()));
 
     // Cache the font collection for future queries.
     font_collections_cache_[family] = font_collection;
@@ -133,29 +156,62 @@ FontCollection::GetMinikinFontCollectionForFamily(const std::string& family) {
   return nullptr;
 }
 
+const std::shared_ptr<minikin::FontFamily>& FontCollection::MatchFallbackFont(
+    uint32_t ch) {
+  for (const auto& manager : skia_font_managers_) {
+    sk_sp<SkTypeface> typeface(
+        manager->matchFamilyStyleCharacter(0, SkFontStyle(), nullptr, 0, ch));
+    if (!typeface)
+      continue;
+
+    return GetFontFamilyForTypeface(typeface);
+  }
+
+  return null_family_;
+}
+
+const std::shared_ptr<minikin::FontFamily>&
+FontCollection::GetFontFamilyForTypeface(const sk_sp<SkTypeface>& typeface) {
+  SkFontID typeface_id = typeface->uniqueID();
+  auto fallback_it = fallback_fonts_.find(typeface_id);
+  if (fallback_it != fallback_fonts_.end()) {
+    return fallback_it->second;
+  }
+
+  std::vector<minikin::Font> minikin_fonts;
+  minikin_fonts.emplace_back(std::make_shared<FontSkia>(typeface),
+                             minikin::FontStyle());
+  auto insert_it = fallback_fonts_.insert(std::make_pair(
+      typeface_id,
+      std::make_shared<minikin::FontFamily>(std::move(minikin_fonts))));
+
+  // Clear the cache to force creation of new font collections that will include
+  // this fallback font.
+  font_collections_cache_.clear();
+
+  return insert_it.first->second;
+}
+
 void FontCollection::UpdateFallbackFonts(sk_sp<SkFontMgr> manager) {
-  char language_tag[ULOC_FULLNAME_CAPACITY];
-  UErrorCode uerr;
-  uloc_toLanguageTag(icu::Locale::getDefault().getName(), language_tag,
-                     ULOC_FULLNAME_CAPACITY, FALSE, &uerr);
-  if (U_FAILURE(uerr))
-    return;
-  const char* bcp47[] = {language_tag};
-
+  // Prepopulate the fallback font cache with fonts matching some widely
+  // used character classes.
   for (SkUnichar fallback_char : fallback_characters) {
-    if (fallback_fonts_.count(fallback_char))
-      continue;
+    sk_sp<SkTypeface> typeface(manager->matchFamilyStyleCharacter(
+        0, SkFontStyle(), nullptr, 0, fallback_char));
+    if (typeface) {
+      // Create a Minikin font family for this typeface if one does not already
+      // exist.
+      GetFontFamilyForTypeface(typeface);
+    }
+  }
 
-    sk_sp<SkTypeface> skia_typeface(manager->matchFamilyStyleCharacter(
-        0, SkFontStyle(), bcp47, 1, fallback_char));
-    if (!skia_typeface)
-      continue;
-
-    std::vector<minikin::Font> minikin_fonts;
-    minikin_fonts.emplace_back(std::make_shared<FontSkia>(skia_typeface),
-                               minikin::FontStyle());
-    fallback_fonts_[fallback_char] =
-        std::make_shared<minikin::FontFamily>(std::move(minikin_fonts));
+  // Add additional font families to be used if nothing else matches.
+  for (const std::string& family : last_resort_fonts) {
+    sk_sp<SkTypeface> typeface(
+        manager->matchFamilyStyle(family.c_str(), SkFontStyle()));
+    if (typeface) {
+      GetFontFamilyForTypeface(typeface);
+    }
   }
 }
 

third_party/txt/src/txt/font_collection.h

@@ -31,7 +31,7 @@
 
 namespace txt {
 
-class FontCollection {
+class FontCollection : public std::enable_shared_from_this<FontCollection> {
  public:
   FontCollection();
 
@@ -46,12 +46,18 @@ class FontCollection {
   std::shared_ptr<minikin::FontCollection> GetMinikinFontCollectionForFamily(
       const std::string& family);
 
+  const std::shared_ptr<minikin::FontFamily>& MatchFallbackFont(uint32_t ch);
+
  private:
   std::deque<sk_sp<SkFontMgr>> skia_font_managers_;
   std::unordered_map<std::string, std::shared_ptr<minikin::FontCollection>>
       font_collections_cache_;
-  std::unordered_map<SkUnichar, std::shared_ptr<minikin::FontFamily>>
+  std::unordered_map<SkFontID, std::shared_ptr<minikin::FontFamily>>
       fallback_fonts_;
+  std::shared_ptr<minikin::FontFamily> null_family_;
+
+  const std::shared_ptr<minikin::FontFamily>& GetFontFamilyForTypeface(
+      const sk_sp<SkTypeface>& typeface);
 
   void UpdateFallbackFonts(sk_sp<SkFontMgr> manager);