Adding upstream version 86.0.1.upstream/86.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 843 insertions, 0 deletions

diff --git a/modules/libpref/SharedPrefMap.h b/modules/libpref/SharedPrefMap.h
new file mode 100644
index 0000000000..e27f3a2599
--- /dev/null
+++ b/modules/libpref/SharedPrefMap.h
@@ -0,0 +1,843 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 dom_ipc_SharedPrefMap_h
+#define dom_ipc_SharedPrefMap_h
+
+#include "mozilla/AutoMemMap.h"
+#include "mozilla/HashFunctions.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/Result.h"
+#include "mozilla/dom/ipc/StringTable.h"
+#include "nsDataHashtable.h"
+
+namespace mozilla {
+
+// The approximate number of preferences expected to be in an ordinary
+// preferences database.
+//
+// This number is used to determine initial allocation sizes for data structures
+// when building the shared preference map, and should be slightly higher than
+// the expected number of preferences in an ordinary database to avoid
+// unnecessary reallocations/rehashes.
+constexpr size_t kExpectedPrefCount = 4000;
+
+class SharedPrefMapBuilder;
+
+// This class provides access to a compact, read-only copy of a preference
+// database, backed by a shared memory buffer which can be shared between
+// processes. All state data for the database is stored in the shared memory
+// region, so individual instances require no dynamic memory allocation.
+//
+// Further, all strings returned from this API are nsLiteralCStrings with
+// pointers into the shared memory region, which means that they can be copied
+// into new nsCString instances without additional allocations. For instance,
+// the following (where `pref` is a Pref object) will not cause any string
+// copies, memory allocations, or atomic refcount changes:
+//
+//   nsCString prefName(pref.NameString());
+//
+// whereas if we returned a nsDependentCString or a dynamically allocated
+// nsCString, it would.
+//
+// The set of entries is stored in sorted order by preference name, so look-ups
+// are done by binary search. This means that look-ups have O(log n) complexity,
+// rather than the O(1) complexity of a dynamic hashtable. Consumers should keep
+// this in mind when planning their accesses.
+//
+// Important: The mapped memory created by this class is persistent. Once an
+// instance has been initialized, the memory that it allocates can never be
+// freed before process shutdown. Do not use it for short-lived mappings.
+class SharedPrefMap {
+  using FileDescriptor = mozilla::ipc::FileDescriptor;
+
+  friend class SharedPrefMapBuilder;
+
+  // Describes a block of memory within the shared memory region.
+  struct DataBlock {
+    // The byte offset from the start of the shared memory region to the start
+    // of the block.
+    size_t mOffset;
+    // The size of the block, in bytes. This is typically used only for bounds
+    // checking in debug builds.
+    size_t mSize;
+  };
+
+  // Describes the contents of the shared memory region, which is laid-out as
+  // follows:
+  //
+  // - The Header struct
+  //
+  // - An array of Entry structs with mEntryCount elements, lexicographically
+  //   sorted by preference name.
+  //
+  // - A set of data blocks, with offsets and sizes described by the DataBlock
+  //   entries in the header, described below.
+  //
+  // Each entry stores its name string and values as indices into these blocks,
+  // as documented in the Entry struct, but with some important optimizations:
+  //
+  // - Boolean values are always stored inline. Both the default and user
+  //   values can be retrieved directly from the entry. Other types have only
+  //   one value index, and their values appear at the same indices in the
+  //   default and user value arrays.
+  //
+  //   Aside from reducing our memory footprint, this space-efficiency means
+  //   that we can fit more entries in the CPU cache at once, and reduces the
+  //   number of likely cache misses during lookups.
+  //
+  // - Key strings are stored in a separate string table from value strings. As
+  //   above, this makes it more likely that the strings we need will be
+  //   available in the CPU cache during lookups by not interleaving them with
+  //   extraneous data.
+  //
+  // - Default and user values are stored in separate arrays. Entries with user
+  //   values always appear before entries with default values in the value
+  //   arrays, and entries without user values do not have entries in the user
+  //   array at all. Since the same index is used for both arrays, this means
+  //   that entries with a default value but no user value do not allocate any
+  //   space to store their user value.
+  //
+  // - For preferences with no user value, the entries in the default value are
+  //   de-duplicated. All preferences with the same default value (and no user
+  //   value) point to the same index in the default value array.
+  //
+  //
+  // For example, a preference database containing:
+  //
+  // +---------+-------------------------------+-------------------------------+
+  // | Name    | Default Value | User Value    |                               |
+  // +---------+---------------+---------------+-------------------------------+
+  // | string1 | "meh"         | "hem"         |                               |
+  // | string2 |               | "b"           |                               |
+  // | string3 | "a"           |               |                               |
+  // | string4 | "foo"         |               |                               |
+  // | string5 | "foo"         |               |                               |
+  // | string6 | "meh"         |               |                               |
+  // +---------+---------------+---------------+-------------------------------+
+  // | bool1   | false         | true          |                               |
+  // | bool2   |               | false         |                               |
+  // | bool3   | true          |               |                               |
+  // +---------+---------------+---------------+-------------------------------+
+  // | int1    | 18            | 16            |                               |
+  // | int2    |               | 24            |                               |
+  // | int3    | 42            |               |                               |
+  // | int4    | 12            |               |                               |
+  // | int5    | 12            |               |                               |
+  // | int6    | 18            |               |                               |
+  // +---------+---------------+---------------+-------------------------------+
+  //
+  // Results in a database that looks like:
+  //
+  // +-------------------------------------------------------------------------+
+  // | Header:                                                                 |
+  // +-------------------------------------------------------------------------+
+  // |  mEntryCount = 15                                                       |
+  // |  ...                                                                    |
+  // +-------------------------------------------------------------------------+
+  //
+  // +-------------------------------------------------------------------------+
+  // | Key strings:                                                            |
+  // +--------+----------------------------------------------------------------+
+  // | Offset | Value                                                          |
+  // +--------+----------------------------------------------------------------+
+  // |      0 | string1\0                                                      |
+  // |      8 | string2\0                                                      |
+  // |     16 | string3\0                                                      |
+  // |     24 | string4\0                                                      |
+  // |     32 | string5\0                                                      |
+  // |     40 | string6\0                                                      |
+  // |     48 | bool1\0                                                        |
+  // |     54 | bool2\0                                                        |
+  // |     60 | bool3\0                                                        |
+  // |     66 | int1\0                                                         |
+  // |     71 | int2\0                                                         |
+  // |     76 | int3\0                                                         |
+  // |     81 | int4\0                                                         |
+  // |     86 | int6\0                                                         |
+  // |     91 | int6\0                                                         |
+  // +--------+----------------------------------------------------------------+
+  //
+  // +-------------------------------------------------------------------------+
+  // | Entries:                                                                |
+  // +---------------------+------+------------+------------+------------------+
+  // | Key[1]              | Type | HasDefault | HasUser    | Value            |
+  // +---------------------+------+------------+------------+------------------+
+  // | K["bool1", 48, 5]   | 3    | true       | true       | { false, true }  |
+  // | K["bool2", 54, 5]   | 3    | false      | true       | { 0,     false } |
+  // | K["bool3", 60, 5]   | 3    | true       | false      | { true,  0    }  |
+  // | K["int1", 66, 4]    | 2    | true       | true       | 0                |
+  // | K["int2", 71, 4]    | 2    | false      | true       | 1                |
+  // | K["int3", 76, 4]    | 2    | true       | false      | 2                |
+  // | K["int4", 81, 4]    | 2    | true       | false      | 3                |
+  // | K["int5", 86, 4]    | 2    | true       | false      | 3                |
+  // | K["int6", 91, 4]    | 2    | true       | false      | 4                |
+  // | K["string1",  0, 6] | 1    | true       | true       | 0                |
+  // | K["string2",  8, 6] | 1    | false      | true       | 1                |
+  // | K["string3", 16, 6] | 1    | true       | false      | 2                |
+  // | K["string4", 24, 6] | 1    | true       | false      | 3                |
+  // | K["string5", 32, 6] | 1    | true       | false      | 3                |
+  // | K["string6", 40, 6] | 1    | true       | false      | 4                |
+  // +---------------------+------+------------+------------+------------------+
+  // | [1]: Encoded as an offset into the key table and a length. Specified    |
+  // |      as K[string, offset, length] for clarity.                          |
+  // +-------------------------------------------------------------------------+
+  //
+  // +------------------------------------+------------------------------------+
+  // | User integer values                | Default integer values             |
+  // +-------+----------------------------+-------+----------------------------+
+  // | Index | Contents                   | Index | Contents                   |
+  // +-------+----------------------------+-------+----------------------------+
+  // |     0 | 16                         |     0 | 18                         |
+  // |     1 | 24                         |     1 |                            |
+  // |       |                            |     2 | 42                         |
+  // |       |                            |     3 | 12                         |
+  // |       |                            |     4 | 18                         |
+  // +-------+----------------------------+-------+----------------------------+
+  // | * Note: Tables are laid out sequentially in memory, but displayed       |
+  // |         here side-by-side for clarity.                                  |
+  // +-------------------------------------------------------------------------+
+  //
+  // +------------------------------------+------------------------------------+
+  // | User string values                 | Default string values              |
+  // +-------+----------------------------+-------+----------------------------+
+  // | Index | Contents[1]                | Index | Contents[1]                |
+  // +-------+----------------------------+-------+----------------------------+
+  // |     0 | V["hem", 0, 3]             |     0 | V["meh", 4, 3]             |
+  // |     1 | V["b", 8, 1]               |     1 |                            |
+  // |       |                            |     2 | V["a", 10, 1]              |
+  // |       |                            |     3 | V["foo", 12, 3]            |
+  // |       |                            |     4 | V["meh", 4, 3]             |
+  // |-------+----------------------------+-------+----------------------------+
+  // | [1]: Encoded as an offset into the value table and a length. Specified  |
+  // |      as V[string, offset, length] for clarity.                          |
+  // +-------------------------------------------------------------------------+
+  // | * Note: Tables are laid out sequentially in memory, but displayed       |
+  // |         here side-by-side for clarity.                                  |
+  // +-------------------------------------------------------------------------+
+  //
+  // +-------------------------------------------------------------------------+
+  // | Value strings:                                                          |
+  // +--------+----------------------------------------------------------------+
+  // | Offset | Value                                                          |
+  // +--------+----------------------------------------------------------------+
+  // |      0 | hem\0                                                          |
+  // |      4 | meh\0                                                          |
+  // |      8 | b\0                                                            |
+  // |     10 | a\0                                                            |
+  // |     12 | foo\0                                                          |
+  // +--------+----------------------------------------------------------------+
+  struct Header {
+    // The number of entries in this map.
+    uint32_t mEntryCount;
+
+    // The StringTable data block for preference name strings, which act as keys
+    // in the map.
+    DataBlock mKeyStrings;
+
+    // The int32_t arrays of user and default int preference values. Entries in
+    // the map store their values as indices into these arrays.
+    DataBlock mUserIntValues;
+    DataBlock mDefaultIntValues;
+
+    // The StringTableEntry arrays of user and default string preference values.
+    //
+    // Strings are stored as StringTableEntry structs with character offsets
+    // into the mValueStrings string table and their corresponding lengths.
+    //
+    // Entries in the map, likewise, store their string values as indices into
+    // these arrays.
+    DataBlock mUserStringValues;
+    DataBlock mDefaultStringValues;
+
+    // The StringTable data block for string preference values, referenced by
+    // the above two data blocks.
+    DataBlock mValueStrings;
+  };
+
+  using StringTableEntry = mozilla::dom::ipc::StringTableEntry;
+
+  // Represents a preference value, as either a pair of boolean values, or an
+  // index into one of the above value arrays.
+  union Value {
+    Value(bool aDefaultValue, bool aUserValue)
+        : mDefaultBool(aDefaultValue), mUserBool(aUserValue) {}
+
+    MOZ_IMPLICIT Value(uint16_t aIndex) : mIndex(aIndex) {}
+
+    // The index of this entry in the value arrays.
+    //
+    // User and default preference values have the same indices in their
+    // respective arrays. However, entries without a user value are not
+    // guaranteed to have space allocated for them in the user value array, and
+    // likewise for preferences without default values in the default value
+    // array. This means that callers must only access value entries for entries
+    // which claim to have a value of that type.
+    uint16_t mIndex;
+    struct {
+      bool mDefaultBool;
+      bool mUserBool;
+    };
+  };
+
+  // Represents a preference entry in the map, containing its name, type info,
+  // flags, and a reference to its value.
+  struct Entry {
+    // A pointer to the preference name in the KeyTable string table.
+    StringTableEntry mKey;
+
+    // The preference's value, either as a pair of booleans, or an index into
+    // the value arrays. Please see the documentation for the Value struct
+    // above.
+    Value mValue;
+
+    // The preference's type, as a PrefType enum value. This must *never* be
+    // PrefType::None for values in a shared array.
+    uint8_t mType : 2;
+    // True if the preference has a default value. Callers must not attempt to
+    // access the entry's default value if this is false.
+    uint8_t mHasDefaultValue : 1;
+    // True if the preference has a user value. Callers must not attempt to
+    // access the entry's user value if this is false.
+    uint8_t mHasUserValue : 1;
+    // True if the preference is sticky, as defined by the preference service.
+    uint8_t mIsSticky : 1;
+    // True if the preference is locked, as defined by the preference service.
+    uint8_t mIsLocked : 1;
+    // True if the preference should be skipped while iterating over the
+    // SharedPrefMap. This is used to internally store Once StaticPrefs.
+    // This property is not visible to users the way sticky and locked are.
+    uint8_t mIsSkippedByIteration : 1;
+  };
+
+ public:
+  NS_INLINE_DECL_REFCOUNTING(SharedPrefMap)
+
+  // A temporary wrapper class for accessing entries in the array. Instances of
+  // this class are valid as long as SharedPrefMap instance is alive, but
+  // generally should not be stored long term, or allocated on the heap.
+  //
+  // The class is implemented as two pointers, one to the SharedPrefMap
+  // instance, and one to the Entry that corresponds to the preference, and is
+  // meant to be cheaply returned by value from preference lookups and
+  // iterators. All property accessors lazily fetch the appropriate values from
+  // the shared memory region.
+  class MOZ_STACK_CLASS Pref final {
+   public:
+    const char* Name() const { return mMap->KeyTable().GetBare(mEntry->mKey); }
+
+    nsCString NameString() const { return mMap->KeyTable().Get(mEntry->mKey); }
+
+    PrefType Type() const {
+      MOZ_ASSERT(PrefType(mEntry->mType) != PrefType::None);
+      return PrefType(mEntry->mType);
+    }
+
+    bool HasDefaultValue() const { return mEntry->mHasDefaultValue; }
+    bool HasUserValue() const { return mEntry->mHasUserValue; }
+    bool IsLocked() const { return mEntry->mIsLocked; }
+    bool IsSticky() const { return mEntry->mIsSticky; }
+    bool IsSkippedByIteration() const { return mEntry->mIsSkippedByIteration; }
+
+    bool GetBoolValue(PrefValueKind aKind = PrefValueKind::User) const {
+      MOZ_ASSERT(Type() == PrefType::Bool);
+      MOZ_ASSERT(aKind == PrefValueKind::Default ? HasDefaultValue()
+                                                 : HasUserValue());
+
+      return aKind == PrefValueKind::Default ? mEntry->mValue.mDefaultBool
+                                             : mEntry->mValue.mUserBool;
+    }
+
+    int32_t GetIntValue(PrefValueKind aKind = PrefValueKind::User) const {
+      MOZ_ASSERT(Type() == PrefType::Int);
+      MOZ_ASSERT(aKind == PrefValueKind::Default ? HasDefaultValue()
+                                                 : HasUserValue());
+
+      return aKind == PrefValueKind::Default
+                 ? mMap->DefaultIntValues()[mEntry->mValue.mIndex]
+                 : mMap->UserIntValues()[mEntry->mValue.mIndex];
+    }
+
+   private:
+    const StringTableEntry& GetStringEntry(PrefValueKind aKind) const {
+      MOZ_ASSERT(Type() == PrefType::String);
+      MOZ_ASSERT(aKind == PrefValueKind::Default ? HasDefaultValue()
+                                                 : HasUserValue());
+
+      return aKind == PrefValueKind::Default
+                 ? mMap->DefaultStringValues()[mEntry->mValue.mIndex]
+                 : mMap->UserStringValues()[mEntry->mValue.mIndex];
+    }
+
+   public:
+    nsCString GetStringValue(PrefValueKind aKind = PrefValueKind::User) const {
+      return mMap->ValueTable().Get(GetStringEntry(aKind));
+    }
+
+    const char* GetBareStringValue(
+        PrefValueKind aKind = PrefValueKind::User) const {
+      return mMap->ValueTable().GetBare(GetStringEntry(aKind));
+    }
+
+    // Returns the entry's index in the map, as understood by GetKeyAt() and
+    // GetValueAt().
+    size_t Index() const { return mEntry - mMap->Entries().get(); }
+
+    bool operator==(const Pref& aPref) const { return mEntry == aPref.mEntry; }
+    bool operator!=(const Pref& aPref) const { return !(*this == aPref); }
+
+    // This is odd, but necessary in order for the C++ range iterator protocol
+    // to work here.
+    Pref& operator*() { return *this; }
+
+    // Updates this wrapper to point to the next visible entry in the map. This
+    // should not be attempted unless Index() is less than the map's Count().
+    Pref& operator++() {
+      do {
+        mEntry++;
+      } while (mEntry->mIsSkippedByIteration && Index() < mMap->Count());
+      return *this;
+    }
+
+    Pref(const Pref& aPref) = default;
+
+   protected:
+    friend class SharedPrefMap;
+
+    Pref(const SharedPrefMap* aPrefMap, const Entry* aEntry)
+        : mMap(aPrefMap), mEntry(aEntry) {}
+
+   private:
+    const SharedPrefMap* const mMap;
+    const Entry* mEntry;
+  };
+
+  // Note: These constructors are infallible, because the preference database is
+  // critical to platform functionality, and we cannot operate without it.
+  SharedPrefMap(const FileDescriptor&, size_t);
+  explicit SharedPrefMap(SharedPrefMapBuilder&&);
+
+  // Searches for the given preference in the map, and returns true if it
+  // exists.
+  bool Has(const char* aKey) const;
+
+  bool Has(const nsCString& aKey) const { return Has(aKey.get()); }
+
+  // Searches for the given preference in the map, and if it exists, returns
+  // a Some<Pref> containing its details.
+  Maybe<const Pref> Get(const char* aKey) const;
+
+  Maybe<const Pref> Get(const nsCString& aKey) const { return Get(aKey.get()); }
+
+ private:
+  // Searches for an entry for the given key. If found, returns true, and
+  // places its index in the entry array in aIndex.
+  bool Find(const char* aKey, size_t* aIndex) const;
+
+ public:
+  // Returns the number of entries in the map.
+  uint32_t Count() const { return EntryCount(); }
+
+  // Returns the string entry at the given index. Keys are guaranteed to be
+  // sorted lexicographically.
+  //
+  // The given index *must* be less than the value returned by Count().
+  //
+  // The returned value is a literal string which references the mapped memory
+  // region.
+  nsCString GetKeyAt(uint32_t aIndex) const {
+    MOZ_ASSERT(aIndex < Count());
+    return KeyTable().Get(Entries()[aIndex].mKey);
+  }
+
+  // Returns the value for the entry at the given index.
+  //
+  // The given index *must* be less than the value returned by Count().
+  //
+  // The returned value is valid for the lifetime of this map instance.
+  const Pref GetValueAt(uint32_t aIndex) const {
+    MOZ_ASSERT(aIndex < Count());
+    return UncheckedGetValueAt(aIndex);
+  }
+
+ private:
+  // Returns a wrapper with a pointer to an entry without checking its bounds.
+  // This should only be used by range iterators, to check their end positions.
+  //
+  // Note: In debug builds, the RangePtr returned by entries will still assert
+  // that aIndex is no more than 1 past the last element in the array, since it
+  // also takes into account the ranged iteration use case.
+  Pref UncheckedGetValueAt(uint32_t aIndex) const {
+    return {this, (Entries() + aIndex).get()};
+  }
+
+ public:
+  // C++ range iterator protocol. begin() and end() return references to the
+  // first (non-skippable) and last entries in the array. The begin wrapper
+  // can be incremented until it matches the last element in the array, at which
+  // point it becomes invalid and the iteration is over.
+  Pref begin() const {
+    for (uint32_t aIndex = 0; aIndex < Count(); aIndex++) {
+      Pref pref = UncheckedGetValueAt(aIndex);
+      if (!pref.IsSkippedByIteration()) {
+        return pref;
+      }
+    }
+    return end();
+  }
+  Pref end() const { return UncheckedGetValueAt(Count()); }
+
+  // A cosmetic helper for range iteration. Returns a reference value from a
+  // pointer to this instance so that its .begin() and .end() methods can be
+  // accessed in a ranged for loop. `map->Iter()` is equivalent to `*map`, but
+  // makes its purpose slightly clearer.
+  const SharedPrefMap& Iter() const { return *this; }
+
+  // Returns a copy of the read-only file descriptor which backs the shared
+  // memory region for this map. The file descriptor may be passed between
+  // processes, and used to construct new instances of SharedPrefMap with
+  // the same data as this instance.
+  FileDescriptor CloneFileDescriptor() const;
+
+  // Returns the size of the mapped memory region. This size must be passed to
+  // the constructor when mapping the shared region in another process.
+  size_t MapSize() const { return mMap.size(); }
+
+ protected:
+  ~SharedPrefMap() = default;
+
+ private:
+  template <typename T>
+  using StringTable = mozilla::dom::ipc::StringTable<T>;
+
+  // Type-safe getters for values in the shared memory region:
+  const Header& GetHeader() const { return mMap.get<Header>()[0]; }
+
+  RangedPtr<const Entry> Entries() const {
+    return {reinterpret_cast<const Entry*>(&GetHeader() + 1), EntryCount()};
+  }
+
+  uint32_t EntryCount() const { return GetHeader().mEntryCount; }
+
+  template <typename T>
+  RangedPtr<const T> GetBlock(const DataBlock& aBlock) const {
+    return RangedPtr<uint8_t>(&mMap.get<uint8_t>()[aBlock.mOffset],
+                              aBlock.mSize)
+        .ReinterpretCast<const T>();
+  }
+
+  RangedPtr<const int32_t> DefaultIntValues() const {
+    return GetBlock<int32_t>(GetHeader().mDefaultIntValues);
+  }
+  RangedPtr<const int32_t> UserIntValues() const {
+    return GetBlock<int32_t>(GetHeader().mUserIntValues);
+  }
+
+  RangedPtr<const StringTableEntry> DefaultStringValues() const {
+    return GetBlock<StringTableEntry>(GetHeader().mDefaultStringValues);
+  }
+  RangedPtr<const StringTableEntry> UserStringValues() const {
+    return GetBlock<StringTableEntry>(GetHeader().mUserStringValues);
+  }
+
+  StringTable<nsCString> KeyTable() const {
+    auto& block = GetHeader().mKeyStrings;
+    return {{&mMap.get<uint8_t>()[block.mOffset], block.mSize}};
+  }
+
+  StringTable<nsCString> ValueTable() const {
+    auto& block = GetHeader().mValueStrings;
+    return {{&mMap.get<uint8_t>()[block.mOffset], block.mSize}};
+  }
+
+  loader::AutoMemMap mMap;
+};
+
+// A helper class which builds the contiguous look-up table used by
+// SharedPrefMap. Each preference in the final map is added to the builder,
+// before it is finalized and transformed into a read-only snapshot.
+class MOZ_RAII SharedPrefMapBuilder {
+ public:
+  SharedPrefMapBuilder() = default;
+
+  // The set of flags for the preference, as documented in SharedPrefMap::Entry.
+  struct Flags {
+    uint8_t mHasDefaultValue : 1;
+    uint8_t mHasUserValue : 1;
+    uint8_t mIsSticky : 1;
+    uint8_t mIsLocked : 1;
+    uint8_t mIsSkippedByIteration : 1;
+  };
+
+  void Add(const nsCString& aKey, const Flags& aFlags, bool aDefaultValue,
+           bool aUserValue);
+
+  void Add(const nsCString& aKey, const Flags& aFlags, int32_t aDefaultValue,
+           int32_t aUserValue);
+
+  void Add(const nsCString& aKey, const Flags& aFlags,
+           const nsCString& aDefaultValue, const nsCString& aUserValue);
+
+  // Finalizes the binary representation of the map, writes it to a shared
+  // memory region, and then initializes the given AutoMemMap with a reference
+  // to the read-only copy of it.
+  //
+  // This should generally not be used directly by callers. The
+  // SharedPrefMapBuilder instance should instead be passed to the SharedPrefMap
+  // constructor as a move reference.
+  Result<Ok, nsresult> Finalize(loader::AutoMemMap& aMap);
+
+ private:
+  using StringTableEntry = mozilla::dom::ipc::StringTableEntry;
+  template <typename T, typename U>
+  using StringTableBuilder = mozilla::dom::ipc::StringTableBuilder<T, U>;
+
+  // An opaque descriptor of the index of a preference entry in a value array,
+  // which can be converted numeric index after the ValueTableBuilder is
+  // finalized.
+  struct ValueIdx {
+    // The relative index of the entry, based on its class. Entries for
+    // preferences with user values appear at the value arrays. Entries with
+    // only default values begin after the last entry with a user value.
+    uint16_t mIndex;
+    bool mHasUserValue;
+  };
+
+  // A helper class for building default and user value arrays for preferences.
+  //
+  // As described in the SharedPrefMap class, this helper optimizes the way that
+  // it builds its value arrays, in that:
+  //
+  // - It stores value entries for all preferences with user values before
+  //   entries for preferences with only default values, and allocates no
+  //   entries for preferences with only default values in the user value array.
+  //   Since most preferences have only default values, this dramatically
+  //   reduces the space required for value storage.
+  //
+  // - For preferences with only default values, it de-duplicates value entries,
+  //   and returns the same indices for all preferences with the same value.
+  //
+  // One important complication of this approach is that it means we cannot know
+  // the final index of any entry with only a default value until all entries
+  // have been added to the builder, since it depends on the final number of
+  // user entries in the output.
+  //
+  // To deal with this, when entries are added, we return an opaque ValueIndex
+  // struct, from which we can calculate the final index after the map has been
+  // finalized.
+  template <typename HashKey, typename ValueType_>
+  class ValueTableBuilder {
+   public:
+    using ValueType = ValueType_;
+
+    // Adds an entry for a preference with only a default value to the array,
+    // and returns an opaque descriptor for its index.
+    ValueIdx Add(const ValueType& aDefaultValue) {
+      auto index = uint16_t(mDefaultEntries.Count());
+
+      auto entry = mDefaultEntries.LookupForAdd(aDefaultValue).OrInsert([&]() {
+        return Entry{index, false, aDefaultValue};
+      });
+
+      return {entry.mIndex, false};
+    }
+
+    // Adds an entry for a preference with a user value to the array. Regardless
+    // of whether the preference has a default value, space must be allocated
+    // for it. For preferences with no default value, the actual value which
+    // appears in the array at its value index is ignored.
+    ValueIdx Add(const ValueType& aDefaultValue, const ValueType& aUserValue) {
+      auto index = uint16_t(mUserEntries.Length());
+
+      mUserEntries.AppendElement(Entry{index, true, aDefaultValue, aUserValue});
+
+      return {index, true};
+    }
+
+    // Returns the final index for an entry based on its opaque index
+    // descriptor. This must only be called after the caller has finished adding
+    // entries to the builder.
+    uint16_t GetIndex(const ValueIdx& aIndex) const {
+      uint16_t base = aIndex.mHasUserValue ? 0 : UserCount();
+      return base + aIndex.mIndex;
+    }
+
+    // Writes out the array of default values at the block beginning at the
+    // given pointer. The block must be at least as large as the value returned
+    // by DefaultSize().
+    void WriteDefaultValues(const RangedPtr<uint8_t>& aBuffer) const {
+      auto buffer = aBuffer.ReinterpretCast<ValueType>();
+
+      for (const auto& entry : mUserEntries) {
+        buffer[entry.mIndex] = entry.mDefaultValue;
+      }
+
+      size_t defaultsOffset = UserCount();
+      for (auto iter = mDefaultEntries.ConstIter(); !iter.Done(); iter.Next()) {
+        const auto& entry = iter.Data();
+        buffer[defaultsOffset + entry.mIndex] = entry.mDefaultValue;
+      }
+    }
+
+    // Writes out the array of user values at the block beginning at the
+    // given pointer. The block must be at least as large as the value returned
+    // by UserSize().
+    void WriteUserValues(const RangedPtr<uint8_t>& aBuffer) const {
+      auto buffer = aBuffer.ReinterpretCast<ValueType>();
+
+      for (const auto& entry : mUserEntries) {
+        buffer[entry.mIndex] = entry.mUserValue;
+      }
+    }
+
+    // These return the number of entries in the default and user value arrays,
+    // respectively.
+    uint32_t DefaultCount() const {
+      return UserCount() + mDefaultEntries.Count();
+    }
+    uint32_t UserCount() const { return mUserEntries.Length(); }
+
+    // These return the byte sizes of the default and user value arrays,
+    // respectively.
+    uint32_t DefaultSize() const { return DefaultCount() * sizeof(ValueType); }
+    uint32_t UserSize() const { return UserCount() * sizeof(ValueType); }
+
+    void Clear() {
+      mUserEntries.Clear();
+      mDefaultEntries.Clear();
+    }
+
+    static constexpr size_t Alignment() { return alignof(ValueType); }
+
+   private:
+    struct Entry {
+      uint16_t mIndex;
+      bool mHasUserValue;
+      ValueType mDefaultValue;
+      ValueType mUserValue{};
+    };
+
+    AutoTArray<Entry, 256> mUserEntries;
+
+    nsDataHashtable<HashKey, Entry> mDefaultEntries;
+  };
+
+  // A special-purpose string table builder for keys which are already
+  // guaranteed to be unique. Duplicate values will not be detected or
+  // de-duplicated.
+  template <typename CharType>
+  class UniqueStringTableBuilder {
+   public:
+    using ElemType = CharType;
+
+    explicit UniqueStringTableBuilder(size_t aCapacity) : mEntries(aCapacity) {}
+
+    StringTableEntry Add(const nsTString<CharType>& aKey) {
+      auto entry =
+          mEntries.AppendElement(Entry{mSize, aKey.Length(), aKey.get()});
+
+      mSize += entry->mLength + 1;
+
+      return {entry->mOffset, entry->mLength};
+    }
+
+    void Write(const RangedPtr<uint8_t>& aBuffer) {
+      auto buffer = aBuffer.ReinterpretCast<ElemType>();
+
+      for (auto& entry : mEntries) {
+        memcpy(&buffer[entry.mOffset], entry.mValue,
+               sizeof(ElemType) * (entry.mLength + 1));
+      }
+    }
+
+    uint32_t Count() const { return mEntries.Length(); }
+
+    uint32_t Size() const { return mSize * sizeof(ElemType); }
+
+    void Clear() { mEntries.Clear(); }
+
+    static constexpr size_t Alignment() { return alignof(ElemType); }
+
+   private:
+    struct Entry {
+      uint32_t mOffset;
+      uint32_t mLength;
+      const CharType* mValue;
+    };
+
+    nsTArray<Entry> mEntries;
+    uint32_t mSize = 0;
+  };
+
+  // A preference value entry, roughly corresponding to the
+  // SharedPrefMap::Value struct, but with a temporary place-holder value rather
+  // than a final value index.
+  union Value {
+    Value(bool aDefaultValue, bool aUserValue)
+        : mDefaultBool(aDefaultValue), mUserBool(aUserValue) {}
+
+    MOZ_IMPLICIT Value(const ValueIdx& aIndex) : mIndex(aIndex) {}
+
+    // For Bool preferences, their default and user bool values.
+    struct {
+      bool mDefaultBool;
+      bool mUserBool;
+    };
+    // For Int and String preferences, an opaque descriptor for their entries in
+    // their value arrays. This must be passed to the appropriate
+    // ValueTableBuilder to obtain the final index when the entry is serialized.
+    ValueIdx mIndex;
+  };
+
+  // A preference entry, to be converted to a SharedPrefMap::Entry struct during
+  // serialization.
+  struct Entry {
+    // The entry's preference name, as passed to Add(). The caller is
+    // responsible for keeping this pointer alive until the builder is
+    // finalized.
+    const char* mKeyString;
+    // The entry in mKeyTable corresponding to mKeyString.
+    StringTableEntry mKey;
+    Value mValue;
+
+    uint8_t mType : 2;
+    uint8_t mHasDefaultValue : 1;
+    uint8_t mHasUserValue : 1;
+    uint8_t mIsSticky : 1;
+    uint8_t mIsLocked : 1;
+    uint8_t mIsSkippedByIteration : 1;
+  };
+
+  // Converts a builder Value struct to a SharedPrefMap::Value struct for
+  // serialization. This must not be called before callers have finished adding
+  // entries to the value array builders.
+  SharedPrefMap::Value GetValue(const Entry& aEntry) const {
+    switch (PrefType(aEntry.mType)) {
+      case PrefType::Bool:
+        return {aEntry.mValue.mDefaultBool, aEntry.mValue.mUserBool};
+      case PrefType::Int:
+        return {mIntValueTable.GetIndex(aEntry.mValue.mIndex)};
+      case PrefType::String:
+        return {mStringValueTable.GetIndex(aEntry.mValue.mIndex)};
+      default:
+        MOZ_ASSERT_UNREACHABLE("Invalid pref type");
+        return {false, false};
+    }
+  }
+
+  UniqueStringTableBuilder<char> mKeyTable{kExpectedPrefCount};
+  StringTableBuilder<nsCStringHashKey, nsCString> mValueStringTable;
+
+  ValueTableBuilder<nsUint32HashKey, uint32_t> mIntValueTable;
+  ValueTableBuilder<nsGenericHashKey<StringTableEntry>, StringTableEntry>
+      mStringValueTable;
+
+  nsTArray<Entry> mEntries{kExpectedPrefCount};
+};
+
+}  // namespace mozilla
+
+#endif  // dom_ipc_SharedPrefMap_h