parser/html/nsHtml5AtomTable.h
author Andrea Marchesini <amarchesini@mozilla.com>
Wed, 09 Jan 2019 12:16:04 +0100
changeset 510141 a11f752f828ceca6aed6cda69fc336f4624434fc
parent 505383 6f3709b3878117466168c40affa7bca0b60cf75b
permissions -rw-r--r--
Bug 1513298 - Fingerprinting url-classifier feature, r=dimi

/* 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 nsHtml5AtomTable_h
#define nsHtml5AtomTable_h

#include "nsHashKeys.h"
#include "nsTHashtable.h"
#include "nsAtom.h"
#include "nsISerialEventTarget.h"

#define RECENTLY_USED_PARSER_ATOMS_SIZE 31

/**
 * nsHtml5AtomTable provides non-locking lookup and creation of atoms for
 * nsHtml5Parser or nsHtml5StreamParser.
 *
 * The hashtable holds dynamically allocated atoms that are private to an
 * instance of nsHtml5Parser or nsHtml5StreamParser. (Static atoms are used on
 * interned nsHtml5ElementNames and interned nsHtml5AttributeNames. Also, when
 * the doctype name is 'html', that identifier needs to be represented as a
 * static atom.)
 *
 * Each instance of nsHtml5Parser has a single instance of nsHtml5AtomTable,
 * and each instance of nsHtml5StreamParser has a single instance of
 * nsHtml5AtomTable. Dynamic atoms obtained from an nsHtml5AtomTable are valid
 * for == comparison with each other or with atoms declared in nsHtml5Atoms
 * within the nsHtml5Tokenizer and the nsHtml5TreeBuilder instances owned by
 * the same nsHtml5Parser/nsHtml5StreamParser instance that owns the
 * nsHtml5AtomTable instance.
 *
 * Dynamic atoms (atoms whose IsStatic() returns false) obtained from
 * nsHtml5AtomTable must be re-obtained from another atom table when there's a
 * need to migrate atoms from an nsHtml5Parser to its nsHtml5StreamParser
 * (re-obtain from the other nsHtml5AtomTable), from an nsHtml5Parser to its
 * owner nsHtml5Parser (re-obtain from the other nsHtml5AtomTable) or from the
 * parser to the DOM (re-obtain from the application-wide atom table). To
 * re-obtain an atom from another atom table, obtain a string from the atom
 * using ToString(nsAString&) and look up an atom in the other table using that
 * string.
 *
 * An instance of nsHtml5AtomTable that belongs to an nsHtml5Parser is only
 * accessed from the main thread. An instance of nsHtml5AtomTable that belongs
 * to an nsHtml5StreamParser is accessed both from the main thread and from the
 * thread that executes the runnables of the nsHtml5StreamParser instance.
 * However, the threads never access the nsHtml5AtomTable instance concurrently
 * in the nsHtml5StreamParser case.
 *
 * Methods on the atoms obtained from nsHtml5AtomTable may be called on any
 * thread, although they only need to be called on the main thread or on the
 * thread working for the nsHtml5StreamParser when nsHtml5AtomTable belongs to
 * an nsHtml5StreamParser.
 *
 * Dynamic atoms obtained from nsHtml5AtomTable are deleted when the
 * nsHtml5AtomTable itself is destructed, which happens when the owner
 * nsHtml5Parser or nsHtml5StreamParser is destructed.
 */
class nsHtml5AtomTable {
 public:
  nsHtml5AtomTable();
  ~nsHtml5AtomTable();

  // NOTE: We rely on mRecentlyUsedParserAtoms keeping alive the returned atom,
  // but the caller is responsible to take a reference before calling GetAtom
  // again.
  nsAtom* GetAtom(const nsAString& aKey);

  /**
   * Empties the table.
   */
  void Clear() {
    for (uint32_t i = 0; i < RECENTLY_USED_PARSER_ATOMS_SIZE; ++i) {
      mRecentlyUsedParserAtoms[i] = nullptr;
    }
  }

#ifdef DEBUG
  void SetPermittedLookupEventTarget(nsISerialEventTarget* aEventTarget) {
    mPermittedLookupEventTarget = aEventTarget;
  }
#endif

 private:
  RefPtr<nsAtom> mRecentlyUsedParserAtoms[RECENTLY_USED_PARSER_ATOMS_SIZE];
#ifdef DEBUG
  nsCOMPtr<nsISerialEventTarget> mPermittedLookupEventTarget;
#endif
};

#endif  // nsHtml5AtomTable_h