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diff --git a/gfx/thebes/SharedFontList.h b/gfx/thebes/SharedFontList.h
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+++ b/gfx/thebes/SharedFontList.h
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef SharedFontList_h
+#define SharedFontList_h
+
+#include "gfxFontEntry.h"
+#include <atomic>
+
+class gfxCharacterMap;
+struct gfxFontStyle;
+struct GlobalFontMatch;
+
+namespace mozilla {
+namespace fontlist {
+
+class FontList;  // See the separate SharedFontList-impl.h header
+
+/**
+ * A Pointer in the shared font list contains a packed index/offset pair,
+ * with a 12-bit index into the array of shared-memory blocks, and a 20-bit
+ * offset into the block.
+ * The maximum size of each block is therefore 2^20 bytes (1 MB) if sub-parts
+ * of the block are to be allocated; however, a larger block (up to 2^32 bytes)
+ * can be created and used as a single allocation if necessary.
+ */
+struct Pointer {
+ private:
+  friend class FontList;
+  static const uint32_t kIndexBits = 12u;
+  static const uint32_t kBlockShift = 20u;
+  static_assert(kIndexBits + kBlockShift == 32u, "bad Pointer bit count");
+
+  static const uint32_t kNullValue = 0xffffffffu;
+  static const uint32_t kOffsetMask = (1u << kBlockShift) - 1;
+
+ public:
+  static Pointer Null() { return Pointer(); }
+
+  Pointer() : mBlockAndOffset(kNullValue) {}
+
+  Pointer(uint32_t aBlock, uint32_t aOffset)
+      : mBlockAndOffset((aBlock << kBlockShift) | aOffset) {
+    MOZ_ASSERT(aBlock < (1u << kIndexBits) && aOffset < (1u << kBlockShift));
+  }
+
+  Pointer(const Pointer& aOther) {
+    mBlockAndOffset.store(aOther.mBlockAndOffset);
+  }
+
+  Pointer(Pointer&& aOther) { mBlockAndOffset.store(aOther.mBlockAndOffset); }
+
+  /**
+   * Check if a Pointer has the null value.
+   *
+   * NOTE!
+   * In a child process, it is possible that conversion to a "real" pointer
+   * using ToPtr() will fail even when IsNull() is false, so calling code
+   * that may run in child processes must be prepared to handle this.
+   */
+  bool IsNull() const { return mBlockAndOffset == kNullValue; }
+
+  uint32_t Block() const { return mBlockAndOffset >> kBlockShift; }
+
+  uint32_t Offset() const { return mBlockAndOffset & kOffsetMask; }
+
+  /**
+   * Convert a fontlist::Pointer to a standard C++ pointer. This requires the
+   * FontList, which will know where the shared memory block is mapped in
+   * the current process's address space.
+   *
+   * NOTE!
+   * In child processes this may fail and return nullptr, even if IsNull() is
+   * false, in cases where the font list is in the process of being rebuilt.
+   */
+  void* ToPtr(FontList* aFontList) const;
+
+  Pointer& operator=(const Pointer& aOther) {
+    mBlockAndOffset.store(aOther.mBlockAndOffset);
+    return *this;
+  }
+
+  Pointer& operator=(Pointer&& aOther) {
+    mBlockAndOffset.store(aOther.mBlockAndOffset);
+    return *this;
+  }
+
+  // We store the block index and the offset within the block as a single
+  // atomic 32-bit value so we can safely modify a Pointer without other
+  // processes seeing a broken (partially-updated) value.
+  std::atomic<uint32_t> mBlockAndOffset;
+};
+
+/**
+ * Family and face names are stored as String records, which hold a length
+ * (in utf-8 code units) and a Pointer to the string's UTF-8 characters.
+ */
+struct String {
+  String() : mPointer(Pointer::Null()), mLength(0) {}
+
+  String(FontList* aList, const nsACString& aString)
+      : mPointer(Pointer::Null()) {
+    Assign(aString, aList);
+  }
+
+  const nsCString AsString(FontList* aList) const {
+    MOZ_ASSERT(!mPointer.IsNull());
+    // It's tempting to use AssignLiteral here so that we get an nsCString that
+    // simply wraps the character data in the shmem block without needing to
+    // allocate or copy. But that's unsafe because in the event of font-list
+    // reinitalization, that shared memory will be unmapped; then any copy of
+    // the nsCString that may still be around will crash if accessed.
+    return nsCString(static_cast<const char*>(mPointer.ToPtr(aList)), mLength);
+  }
+
+  void Assign(const nsACString& aString, FontList* aList);
+
+  const char* BeginReading(FontList* aList) const {
+    MOZ_ASSERT(!mPointer.IsNull());
+    auto str = static_cast<const char*>(mPointer.ToPtr(aList));
+    return str ? str : "";
+  }
+
+  uint32_t Length() const { return mLength; }
+
+  /**
+   * Return whether the String has been set to a value.
+   *
+   * NOTE!
+   * In a child process, accessing the value could fail even if IsNull()
+   * returned false. In this case, the nsCString constructor used by AsString()
+   * will be passed a null pointer, and return an empty string despite the
+   * non-zero Length() recorded here.
+   */
+  bool IsNull() const { return mPointer.IsNull(); }
+
+ private:
+  Pointer mPointer;
+  uint32_t mLength;
+};
+
+/**
+ * A Face record represents an individual font resource; it has the style
+ * properties needed for font matching, as well as a pointer to a character
+ * map that records the supported character set. This may be Null if we have
+ * not yet loaded the data.
+ * The mDescriptor and mIndex fields provide the information needed to
+ * instantiate a (platform-specific) font reference that can be used with
+ * platform font APIs; their content depends on the requirements of the
+ * platform APIs (e.g. font PostScript name, file pathname, serialized
+ * fontconfig pattern, etc).
+ */
+struct Face {
+  // Data required to initialize a Face
+  struct InitData {
+    nsCString mDescriptor;  // descriptor that can be used to instantiate a
+                            // platform font reference
+    uint16_t mIndex;        // an index used with descriptor (on some platforms)
+    bool mFixedPitch;       // is the face fixed-pitch (monospaced)?
+    mozilla::WeightRange mWeight;      // CSS font-weight value
+    mozilla::StretchRange mStretch;    // CSS font-stretch value
+    mozilla::SlantStyleRange mStyle;   // CSS font-style value
+    RefPtr<gfxCharacterMap> mCharMap;  // character map, or null if not loaded
+  };
+
+  // Note that mCharacterMap is not set from the InitData by this constructor;
+  // the caller must use SetCharacterMap to handle that separately if required.
+  Face(FontList* aList, const InitData& aData)
+      : mDescriptor(aList, aData.mDescriptor),
+        mIndex(aData.mIndex),
+        mFixedPitch(aData.mFixedPitch),
+        mWeight(aData.mWeight),
+        mStretch(aData.mStretch),
+        mStyle(aData.mStyle),
+        mCharacterMap(Pointer::Null()) {}
+
+  bool HasValidDescriptor() const {
+    return !mDescriptor.IsNull() && mIndex != uint16_t(-1);
+  }
+
+  void SetCharacterMap(FontList* aList, gfxCharacterMap* aCharMap);
+
+  String mDescriptor;
+  uint16_t mIndex;
+  bool mFixedPitch;
+  mozilla::WeightRange mWeight;
+  mozilla::StretchRange mStretch;
+  mozilla::SlantStyleRange mStyle;
+  Pointer mCharacterMap;
+};
+
+/**
+ * A Family record represents an available (installed) font family; it has
+ * a name (for display purposes) and a key (lowercased, for case-insensitive
+ * lookups), as well as a pointer to an array of Faces. Depending on the
+ * platform, the array of faces may be lazily initialized the first time we
+ * want to use the family.
+ */
+struct Family {
+  static constexpr uint32_t kNoIndex = uint32_t(-1);
+
+  // Data required to initialize a Family
+  struct InitData {
+    InitData(const nsACString& aKey,      // lookup key (lowercased)
+             const nsACString& aName,     // display name
+             uint32_t aIndex = kNoIndex,  // [win] system collection index
+             FontVisibility aVisibility = FontVisibility::Unknown,
+             bool aBundled = false,       // [win] font was bundled with the app
+                                          // rather than system-installed
+             bool aBadUnderline = false,  // underline-position in font is bad
+             bool aForceClassic = false,  // [win] use "GDI classic" rendering
+             bool aAltLocale = false      // font is alternate localized family
+             )
+        : mKey(aKey),
+          mName(aName),
+          mIndex(aIndex),
+          mVisibility(aVisibility),
+          mBundled(aBundled),
+          mBadUnderline(aBadUnderline),
+          mForceClassic(aForceClassic),
+          mAltLocale(aAltLocale) {}
+    bool operator<(const InitData& aRHS) const { return mKey < aRHS.mKey; }
+    bool operator==(const InitData& aRHS) const {
+      return mKey == aRHS.mKey && mName == aRHS.mName &&
+             mVisibility == aRHS.mVisibility && mBundled == aRHS.mBundled &&
+             mBadUnderline == aRHS.mBadUnderline;
+    }
+    const nsCString mKey;
+    const nsCString mName;
+    uint32_t mIndex;
+    FontVisibility mVisibility;
+    bool mBundled;
+    bool mBadUnderline;
+    bool mForceClassic;
+    bool mAltLocale;
+  };
+
+  /**
+   * Font families are considered "simple" if they contain only 4 faces with
+   * style attributes corresponding to Regular, Bold, Italic, and BoldItalic
+   * respectively, or a subset of these (e.g. only Regular and Bold). In this
+   * case, the faces are stored at predefined positions in the mFaces array,
+   * and a simplified (faster) style-matching algorithm can be used.
+   */
+  enum {
+    // Indexes into mFaces for families where mIsSimple is true
+    kRegularFaceIndex = 0,
+    kBoldFaceIndex = 1,
+    kItalicFaceIndex = 2,
+    kBoldItalicFaceIndex = 3,
+    // mask values for selecting face with bold and/or italic attributes
+    kBoldMask = 0x01,
+    kItalicMask = 0x02
+  };
+
+  Family(FontList* aList, const InitData& aData);
+
+  void AddFaces(FontList* aList, const nsTArray<Face::InitData>& aFaces);
+
+  void SetFacePtrs(FontList* aList, nsTArray<Pointer>& aFaces);
+
+  const String& Key() const { return mKey; }
+
+  const String& DisplayName() const { return mName; }
+
+  uint32_t Index() const { return mIndex; }
+  bool IsBundled() const { return mIsBundled; }
+
+  uint32_t NumFaces() const {
+    MOZ_ASSERT(IsInitialized());
+    return mFaceCount;
+  }
+
+  Pointer* Faces(FontList* aList) const {
+    MOZ_ASSERT(IsInitialized());
+    return static_cast<Pointer*>(mFaces.ToPtr(aList));
+  }
+
+  FontVisibility Visibility() const { return mVisibility; }
+  bool IsHidden() const { return Visibility() == FontVisibility::Hidden; }
+
+  bool IsBadUnderlineFamily() const { return mIsBadUnderlineFamily; }
+  bool IsForceClassic() const { return mIsForceClassic; }
+  bool IsSimple() const { return mIsSimple; }
+  bool IsAltLocaleFamily() const { return mIsAltLocale; }
+
+  // IsInitialized indicates whether the family has been populated with faces,
+  // and is therefore ready to use.
+  // It is possible that character maps have not yet been loaded.
+  bool IsInitialized() const { return !mFaces.IsNull(); }
+
+  // IsFullyInitialized indicates that not only faces but also character maps
+  // have been set up, so the family can be searched without the possibility
+  // that IPC messaging will be triggered.
+  bool IsFullyInitialized() const {
+    return IsInitialized() && !mCharacterMap.IsNull();
+  }
+
+  void FindAllFacesForStyle(FontList* aList, const gfxFontStyle& aStyle,
+                            nsTArray<Face*>& aFaceList,
+                            bool aIgnoreSizeTolerance = false) const;
+
+  Face* FindFaceForStyle(FontList* aList, const gfxFontStyle& aStyle,
+                         bool aIgnoreSizeTolerance = false) const;
+
+  void SearchAllFontsForChar(FontList* aList, GlobalFontMatch* aMatchData);
+
+  void SetupFamilyCharMap(FontList* aList);
+
+ private:
+  std::atomic<uint32_t> mFaceCount;
+  String mKey;
+  String mName;
+  Pointer mCharacterMap;  // If non-null, union of character coverage of all
+                          // faces in the family
+  Pointer mFaces;         // Pointer to array of |mFaceCount| face pointers
+  uint32_t mIndex;        // [win] Top bit set indicates app-bundled font family
+  FontVisibility mVisibility;
+  bool mIsSimple;  // family allows simplified style matching: mFaces contains
+                   // exactly 4 entries [Regular, Bold, Italic, BoldItalic].
+  bool mIsBundled : 1;
+  bool mIsBadUnderlineFamily : 1;
+  bool mIsForceClassic : 1;
+  bool mIsAltLocale : 1;
+};
+
+/**
+ * For platforms where we build an index of local font face names (PS-name
+ * and fullname of the font) to support @font-face{src:local(...)}, we map
+ * each face name to an index into the family list, and an index into the
+ * family's list of faces.
+ */
+struct LocalFaceRec {
+  /**
+   * The InitData struct needs to record the family name rather than index,
+   * as we may be collecting these records at the same time as building the
+   * family list, so we don't yet know the final family index.
+   * Likewise, in some cases we don't know the final face index because the
+   * faces may be re-sorted to fit into predefined positions in a "simple"
+   * family (if we're reading names before the family has been fully set up).
+   * In that case, we'll store uint32_t(-1) as mFaceIndex, and record the
+   * string descriptor instead.
+   * When actually recorded in the FontList's mLocalFaces array, the family
+   * will be stored as a simple index into the mFamilies array, and the face
+   * as an index into the family's mFaces.
+   */
+  struct InitData {
+    nsCString mFamilyName;
+    nsCString mFaceDescriptor;
+    uint32_t mFaceIndex = uint32_t(-1);
+    InitData(const nsACString& aFamily, const nsACString& aFace)
+        : mFamilyName(aFamily), mFaceDescriptor(aFace) {}
+    InitData(const nsACString& aFamily, uint32_t aFaceIndex)
+        : mFamilyName(aFamily), mFaceIndex(aFaceIndex) {}
+    InitData() = default;
+  };
+  String mKey;
+  uint32_t mFamilyIndex;  // Index into the font list's Families array
+  uint32_t mFaceIndex;    // Index into the family's Faces array
+};
+
+}  // namespace fontlist
+}  // namespace mozilla
+
+#include "ipc/IPCMessageUtils.h"
+
+namespace IPC {
+
+template <>
+struct ParamTraits<mozilla::fontlist::Pointer> {
+  typedef mozilla::fontlist::Pointer paramType;
+  static void Write(Message* aMsg, const paramType& aParam) {
+    uint32_t v = aParam.mBlockAndOffset;
+    WriteParam(aMsg, v);
+  }
+  static bool Read(const Message* aMsg, PickleIterator* aIter,
+                   paramType* aResult) {
+    uint32_t v;
+    if (ReadParam(aMsg, aIter, &v)) {
+      aResult->mBlockAndOffset.store(v);
+      return true;
+    }
+    return false;
+  }
+};
+
+}  // namespace IPC
+
+#undef ERROR  // This is defined via Windows.h, but conflicts with some bindings
+              // code when this gets included in the same compilation unit.
+
+#endif /* SharedFontList_h */