gecko-dev/dom/ipc/SharedStringMap.h

/* -*- 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_SharedStringMap_h
#define dom_ipc_SharedStringMap_h

#include "mozilla/AutoMemMap.h"
#include "mozilla/Result.h"
#include "mozilla/dom/ipc/StringTable.h"
#include "nsDataHashtable.h"

namespace mozilla {
namespace dom {
namespace ipc {

class SharedStringMapBuilder;

/**
 * This class provides a simple, read-only key-value string store, with all
 * data packed into a single segment of memory, which can be shared between
 * processes.
 *
 * Look-ups are performed by binary search of a static table in the mapped
 * memory region, and all returned strings are literals which reference the
 * mapped data. No copies are performed on instantiation or look-up.
 *
 * 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 SharedStringMap {
  using FileDescriptor = mozilla::ipc::FileDescriptor;

 public:
  /**
   * The header at the beginning of the shared memory region describing its
   * layout. The layout of the shared memory is as follows:
   *
   * - Header:
   *   A Header struct describing the contents of the rest of the memory region.
   *
   * - Optional alignment padding for Header[].
   *
   * - Entry[header.mEntryCount]:
   *   An array of Entry structs, one for each entry in the map. Entries are
   *   lexocographically sorted by key.
   *
   * - StringTable<nsCString>:
   *   A region of flat, null-terminated C strings. Entry key strings are
   *   encoded as character offsets into this region.
   *
   * - Optional alignment padding for char16_t[]
   *
   * - StringTable<nsString>:
   *   A region of flat, null-terminated UTF-16 strings. Entry value strings are
   *   encoded as character (*not* byte) offsets into this region.
   */
  struct Header {
    // The number of entries in this map.
    uint32_t mEntryCount;

    // The raw byte offset of the beginning of the key string table, from the
    // start of the shared memory region, and its size in bytes.
    size_t mKeyStringsOffset;
    size_t mKeyStringsSize;

    // The raw byte offset of the beginning of the value string table, from the
    // start of the shared memory region, and its size in bytes (*not*
    // characters).
    size_t mValueStringsOffset;
    size_t mValueStringsSize;
  };

  /**
   * Describes a value in the string map, as offsets into the key and value
   * string tables.
   */
  struct Entry {
    // The offset and size of the entry's UTF-8 key in the key string table.
    StringTableEntry mKey;
    // The offset and size of the entry's UTF-16 value in the value string
    // table.
    StringTableEntry mValue;
  };

  NS_INLINE_DECL_REFCOUNTING(SharedStringMap)

  // Note: These constructors are infallible on the premise that this class
  // is used primarily in cases where it is critical to platform
  // functionality.
  explicit SharedStringMap(const FileDescriptor&, size_t);
  explicit SharedStringMap(SharedStringMapBuilder&&);

  /**
   * Searches for the given value in the map, and returns true if it exists.
   */
  bool Has(const nsCString& aKey);

  /**
   * Searches for the given value in the map, and, if it exists, returns true
   * and places its value in aValue.
   *
   * The returned value is a literal string which references the mapped memory
   * region.
   */
  bool Get(const nsCString& aKey, nsAString& aValue);

 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 nsCString& aKey, size_t* aIndex);

 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 lexographically.
   *
   * 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 string value for the entry at the given index.
   *
   * 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.
   */
  nsString GetValueAt(uint32_t aIndex) const {
    MOZ_ASSERT(aIndex < Count());
    return ValueTable().Get(Entries()[aIndex].mValue);
  }

  /**
   * 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 SharedStringMap with
   * the same data as this instance.
   */
  FileDescriptor CloneFileDescriptor() const;

  size_t MapSize() const { return mMap.size(); }

 protected:
  ~SharedStringMap() = default;

 private:
  // 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; }

  StringTable<nsCString> KeyTable() const {
    auto& header = GetHeader();
    return {{&mMap.get<uint8_t>()[header.mKeyStringsOffset],
             header.mKeyStringsSize}};
  }

  StringTable<nsString> ValueTable() const {
    auto& header = GetHeader();
    return {{&mMap.get<uint8_t>()[header.mValueStringsOffset],
             header.mValueStringsSize}};
  }

  loader::AutoMemMap mMap;
};

/**
 * A helper class which builds the contiguous look-up table used by
 * SharedStringMap. Each key-value pair in the final map is added to the
 * builder, before it is finalized and transformed into a snapshot.
 */
class MOZ_RAII SharedStringMapBuilder {
 public:
  SharedStringMapBuilder() = default;

  /**
   * Adds a key-value pair to the map.
   */
  void Add(const nsCString& aKey, const nsString& aValue);

  /**
   * 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.
   */
  Result<Ok, nsresult> Finalize(loader::AutoMemMap& aMap);

 private:
  using Entry = SharedStringMap::Entry;

  StringTableBuilder<nsCStringHashKey, nsCString> mKeyTable;
  StringTableBuilder<nsStringHashKey, nsString> mValueTable;

  nsDataHashtable<nsCStringHashKey, Entry> mEntries;
};

}  // namespace ipc
}  // namespace dom
}  // namespace mozilla

#endif  // dom_ipc_SharedStringMap_h