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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 */
#ifndef SharedFontList_impl_h
#define SharedFontList_impl_h
#include "SharedFontList.h"
#include "base/shared_memory.h"
#include "gfxFontUtils.h"
#include "nsClassHashtable.h"
#include "nsTHashMap.h"
#include "nsXULAppAPI.h"
#include "mozilla/UniquePtr.h"
// This is split out from SharedFontList.h because that header is included
// quite widely (via gfxPlatformFontList.h, gfxTextRun.h, etc), and other code
// such as the generated DOM bindings code gets upset at (indirect) inclusion
// of <windows.h> via SharedMemoryBasic.h. So this header, which defines the
// actual shared-memory FontList class, is included only by the .cpp files that
// implement or directly interface with the font list, to avoid polluting other
// headers.
namespace mozilla {
namespace fontlist {
* Data used to initialize a font family alias (a "virtual" family that refers
* to some or all of the faces of another family, used when alternate family
* names are found in the font resource for localization or for styled
* subfamilies). AliasData records are collected incrementally while scanning
* the fonts, and then used to set up the Aliases list in the shared font list.
struct AliasData {
nsTArray<Pointer> mFaces;
nsCString mBaseFamily;
uint32_t mIndex = 0;
FontVisibility mVisibility = FontVisibility::Unknown;
bool mBundled = false;
bool mBadUnderline = false;
bool mForceClassic = false;
void InitFromFamily(const Family* aFamily, const nsCString& aBaseFamily) {
mBaseFamily = aBaseFamily;
mIndex = aFamily->Index();
mVisibility = aFamily->Visibility();
mBundled = aFamily->IsBundled();
mBadUnderline = aFamily->IsBadUnderlineFamily();
mForceClassic = aFamily->IsForceClassic();
* The Shared Font List is a collection of data that lives in shared memory
* so that all processes can use it, rather than maintaining their own copies,
* and provides the metadata needed for CSS font-matching (a list of all the
* available font families and their faces, style properties, etc, as well as
* character coverage).
* An important assumption is that all processes see the same collection of
* installed fonts; therefore it is valid for them all to share the same set
* of font metadata. The data is updated only by the parent process; content
* processes have read-only access to it.
* The total size of this data varies greatly depending on the user's installed
* fonts; and it is not known at startup because we load a lot of the font data
* on first use rather than preloading during initialization (because that's
* too expensive/slow).
* Therefore, the shared memory area needs to be able to grow during the
* session; we can't predict how much space will be needed, and we can't afford
* to pre-allocate such a huge block that it could never overflow. To handle
* this, we maintain a (generally short) array of blocks of shared memory,
* and then allocate our Family, Face, etc. objects within these. Because we
* only ever add data (never delete what we've already stored), we can use a
* simplified allocator that doesn't ever need to free blocks; the only time
* the memory is released during a session is the (rare) case where a font is
* installed or deleted while the browser is running, and in this case we just
* delete the entire shared font list and start afresh.
class FontList {
friend struct Pointer;
explicit FontList(uint32_t aGeneration);
* Initialize the master list of installed font families. This must be
* set during font-list creation, before the list is shared with any
* content processes. All installed font families known to the browser
* appear in this list, although some may be marked as "hidden" so that
* they are not exposed to the font-family property.
* The passed-in array may be modified (to eliminate duplicates of bundled
* fonts, or restrict the available list to a specified subset), so if the
* caller intends to make further use of it this should be kept in mind.
* Once initialized, the master family list is immutable; in the (rare)
* event that the system's collection of installed fonts changes, we discard
* the FontList and create a new one.
* In some cases, a font family may be known by multiple names (e.g.
* localizations in multiple languages, or there may be legacy family names
* that correspond to specific styled faces like "Arial Black"). Such names
* do not appear in this master list, but are referred to as aliases (see
* SetAliases below); the alias list need not be populated before the font
* list is shared to content processes and used.
* Only used in the parent process.
void SetFamilyNames(nsTArray<Family::InitData>& aFamilies);
* Aliases are Family records whose Face entries are already part of another
* family (either because the family has multiple localized names, or because
* the alias family is a legacy name like "Arial Narrow" that is a subset of
* the faces in the main "Arial" family). The table of aliases is initialized
* from a hash of alias family name -> array of Face records.
* Like the master family list, the list of family aliases is immutable once
* initialized.
* Only used in the parent process.
void SetAliases(nsClassHashtable<nsCStringHashKey, AliasData>& aAliasTable);
* Local names are PostScript or Full font names of individual faces, used
* to look up faces for @font-face { src: local(...) } rules. Some platforms
* (e.g. macOS) can look up local names directly using platform font APIs,
* in which case the local names table here is unused.
* The list of local names is immutable once initialized. Local font name
* lookups may occur before this list has been set up, in which case they
* will use the SearchForLocalFace method.
* Only used in the parent process.
void SetLocalNames(
nsTHashMap<nsCStringHashKey, LocalFaceRec::InitData>& aLocalNameTable);
* Look up a Family record by name, typically to satisfy the font-family
* property or a font family listed in preferences.
Family* FindFamily(const nsCString& aName, bool aPrimaryNameOnly = false);
* Look up an individual Face by PostScript or Full name, for @font-face
* rules using src:local(...). This requires the local names list to have
* been initialized.
LocalFaceRec* FindLocalFace(const nsCString& aName);
* Search families for a face with local name aName; should only be used if
* the mLocalFaces array has not yet been set up, as this will be a more
* expensive search than FindLocalFace.
void SearchForLocalFace(const nsACString& aName, Family** aFamily,
Face** aFace);
* Return the localized name for the given family in the current system
* locale (if multiple localizations are available).
nsCString LocalizedFamilyName(const Family* aFamily);
bool Initialized() { return mBlocks.Length() > 0 && NumFamilies() > 0; }
uint32_t NumFamilies() { return GetHeader().mFamilyCount; }
Family* Families() {
return static_cast<Family*>(GetHeader().mFamilies.ToPtr(this));
uint32_t NumAliases() { return GetHeader().mAliasCount; }
Family* AliasFamilies() {
return static_cast<Family*>(GetHeader().mAliases.ToPtr(this));
uint32_t NumLocalFaces() { return GetHeader().mLocalFaceCount; }
LocalFaceRec* LocalFaces() {
return static_cast<LocalFaceRec*>(GetHeader().mLocalFaces.ToPtr(this));
* Ask the font list to initialize the character map for a given face.
void LoadCharMapFor(Face& aFace, const Family* aFamily);
* Allocate shared-memory space for a record of aSize bytes. The returned
* pointer will be 32-bit aligned. (This method may trigger the allocation of
* a new shared memory block, if required.)
* Only used in the parent process.
Pointer Alloc(uint32_t aSize);
* Convert a native pointer to a shared-memory Pointer record that can be
* passed between processes.
Pointer ToSharedPointer(const void* aPtr);
uint32_t GetGeneration() { return GetHeader().mGeneration; }
* Header fields present in every shared-memory block. The mBlockSize field
* is not modified after initial block creation (before the block has been
* shared to any other process), and the mAllocated field is used only by
* the parent process, so neither of these needs to be std::atomic<>.
struct BlockHeader {
uint32_t mAllocated; // Space allocated from this block.
uint32_t mBlockSize; // Total size of this block.
* Header info that is stored at the beginning of the first shared-memory
* block for the font list.
* (Subsequent blocks have only the mBlockHeader.)
* The mGeneration and mFamilyCount fields are set by the parent process
* during font-list construction, before the list has been shared with any
* other process, and subsequently never change; therefore, we don't need
* to use std::atomic<> for these.
struct Header {
BlockHeader mBlockHeader;
uint32_t mGeneration; // Font-list generation ID
uint32_t mFamilyCount; // Number of font families in the list
std::atomic<uint32_t> mBlockCount; // Total number of blocks that exist
std::atomic<uint32_t> mAliasCount; // Number of family aliases
std::atomic<uint32_t> mLocalFaceCount; // Number of local face names
Pointer mFamilies; // Pointer to array of |mFamilyCount| families
Pointer mAliases; // Pointer to array of |mAliasCount| aliases
Pointer mLocalFaces; // Pointer to array of |mLocalFaceCount| face records
* Used by the parent process to pass a handle to a shared block to a
* specific child process. This is used when a child process requests
* an additional block that was not already passed to it (because the
* list has changed/grown since the child was first initialized).
void ShareShmBlockToProcess(uint32_t aIndex, base::ProcessId aPid,
base::SharedMemoryHandle* aOut) {
MOZ_RELEASE_ASSERT(mReadOnlyShmems.Length() == mBlocks.Length());
if (aIndex >= mReadOnlyShmems.Length()) {
// Block index out of range
*aOut = base::SharedMemory::NULLHandle();
if (!mReadOnlyShmems[aIndex]->ShareToProcess(aPid, aOut)) {
MOZ_CRASH("failed to share block");
* Collect an array of handles to all the shmem blocks, ready to be
* shared to the given process. This is used at child process startup
* to pass the complete list at once.
void ShareBlocksToProcess(nsTArray<base::SharedMemoryHandle>* aBlocks,
base::ProcessId aPid);
base::SharedMemoryHandle ShareBlockToProcess(uint32_t aIndex,
base::ProcessId aPid);
void ShmBlockAdded(uint32_t aGeneration, uint32_t aIndex,
base::SharedMemoryHandle aHandle);
* Support for memory reporter.
size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
size_t AllocatedShmemSize() const;
* Using a larger block size will speed up allocation, at the cost of more
* wasted space in the shared memory (on average).
// Android devices usually have a much smaller number of fonts than desktop
// systems, and memory is more constrained, so use a smaller default block
// size.
static constexpr uint32_t SHM_BLOCK_SIZE = 64 * 1024;
#elif XP_LINUX
// On Linux, font face descriptors are rather large (serialized FcPatterns),
// so use a larger block size for efficiency.
static constexpr uint32_t SHM_BLOCK_SIZE = 1024 * 1024;
// Default block size for Windows and macOS.
static constexpr uint32_t SHM_BLOCK_SIZE = 256 * 1024;
static_assert(SHM_BLOCK_SIZE <= (1 << Pointer::kBlockShift),
"SHM_BLOCK_SIZE too large");
struct ShmBlock {
// Takes ownership of aShmem. Note that in a child process, aShmem will be
// mapped as read-only.
explicit ShmBlock(mozilla::UniquePtr<base::SharedMemory>&& aShmem)
: mShmem(std::move(aShmem)) {}
// Get pointer to the mapped memory.
void* Memory() const { return mShmem->memory(); }
// Because only the parent process does allocation, this doesn't need to
// be an atomic.
// Further allocations may happen within the block after it has been shared
// to content processes, but as their access is read-only and they cannot
// do allocations themselves, they never look at this field; only the parent
// reads or updates it.
uint32_t& Allocated() const {
return static_cast<BlockHeader*>(Memory())->mAllocated;
// This is stored by the parent process during block creation and never
// changes, so does not need to be atomic.
// Note that some blocks may be larger than SHM_BLOCK_SIZE, if needed for
// individual large allocations.
uint32_t& BlockSize() const {
return static_cast<BlockHeader*>(Memory())->mBlockSize;
mozilla::UniquePtr<base::SharedMemory> mShmem;
Header& GetHeader() {
// It's invalid to try and access this before the first block exists.
MOZ_ASSERT(mBlocks.Length() > 0);
return *static_cast<Header*>(Pointer(0, 0).ToPtr(this));
* Create a new shared memory block and append to the FontList's list
* of blocks.
* Only used in the parent process.
bool AppendShmBlock(uint32_t aSizeNeeded);
* Used by child processes to ensure all the blocks are registered.
* Returns false on failure.
[[nodiscard]] bool UpdateShmBlocks();
* This makes a *sync* IPC call to get a shared block from the parent.
* As such, it may block for a while if the parent is busy; fortunately,
* we'll generally only call this a handful of times in the course of an
* entire session. If the requested block does not yet exist (because the
* child is wanting to allocate an object, and there wasn't room in any
* existing block), the parent will create a new shared block and return it.
* This may (in rare cases) return null, if the parent has recreated the
* font list and we actually need to reinitialize.
ShmBlock* GetBlockFromParent(uint32_t aIndex);
void DetachShmBlocks();
* Array of pointers to the shared-memory block records.
* NOTE: if mBlocks.Length() < GetHeader().mBlockCount, then the parent has
* added a block (or blocks) to the list, and we need to update!
nsTArray<mozilla::UniquePtr<ShmBlock>> mBlocks;
* Auxiliary array, used only in the parent process; holds read-only copies
* of the shmem blocks; these are what will be shared to child processes.
nsTArray<mozilla::UniquePtr<base::SharedMemory>> mReadOnlyShmems;
} // namespace fontlist
} // namespace mozilla
#endif /* SharedFontList_impl_h */