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+/* -*- 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 "nsTHashMap.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 is sanitized, as defined by the preference
+ // service.
+ uint8_t mIsSanitized : 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 IsSanitized() const { return mEntry->mIsSanitized; }
+ 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 mIsSanitized : 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());
+
+ return mDefaultEntries.WithEntryHandle(aDefaultValue, [&](auto&& entry) {
+ entry.OrInsertWith([&] { return Entry{index, false, aDefaultValue}; });
+
+ return ValueIdx{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 (const auto& data : mDefaultEntries.Values()) {
+ buffer[defaultsOffset + data.mIndex] = data.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;
+
+ nsTHashMap<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, uint32_t(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 mIsSanitized : 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