third_party/WebKit/Source/core/html/parser/HTMLParserReentryPermit.h

// Copyright 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef HTMLParserReentryPermit_h
#define HTMLParserReentryPermit_h

#include "base/macros.h"
#include "wtf/PassRefPtr.h"
#include "wtf/RefCounted.h"

namespace blink {

// The HTML spec for parsing controls reentering the parser from
// script with the "parser pause flag" and "script nesting level."
//
// The parser pause flag puts a brake on whether the tokenizer will
// produce more tokens. When the parser is paused, nested invocations
// of the tokenizer unwind.
//
// The script nesting level is incremented and decremented any time
// the parser causes script to run. The script nesting level:
//
// - May prevent document.open from blowing away the document.
//
// - Governs whether a script element becomes the "pending
//   parsing-blocking script." The pending parsing-blocking script in
//   turn affects whether document.write reenters the parser.
//
// Clearing the parser pause flag is simple: Whenever the script
// nesting level hits zero, the parser pause flag is cleared. However
// setting the parser pause flag is subtle.
//
// Processing a typical script end tag, or running a chain of pending
// parser-blocking scripts after that, does not set the parser pause
// flag. However recursively parsing end script tags, or running
// custom element constructors, does set the parser pause flag.
class HTMLParserReentryPermit final : public RefCounted<HTMLParserReentryPermit> {
public:
    static PassRefPtr<HTMLParserReentryPermit> create();
    ~HTMLParserReentryPermit() = default;

    unsigned scriptNestingLevel() const { return m_scriptNestingLevel; }
    bool parserPauseFlag() const { return m_parserPauseFlag; }
    void pause()
    {
        CHECK(m_scriptNestingLevel);
        m_parserPauseFlag = true;
    }

    class ScriptNestingLevelIncrementer final {
        STACK_ALLOCATED();
    public:
        explicit ScriptNestingLevelIncrementer(HTMLParserReentryPermit* permit)
            : m_permit(permit)
        {
            m_permit->m_scriptNestingLevel++;
        }

        ScriptNestingLevelIncrementer(ScriptNestingLevelIncrementer&&)
            = default;

        ~ScriptNestingLevelIncrementer()
        {
            m_permit->m_scriptNestingLevel--;
            if (!m_permit->m_scriptNestingLevel)
                m_permit->m_parserPauseFlag = false;
        }

    private:
        HTMLParserReentryPermit* m_permit;

        DISALLOW_COPY_AND_ASSIGN(ScriptNestingLevelIncrementer);
    };

    ScriptNestingLevelIncrementer incrementScriptNestingLevel()
    {
        return ScriptNestingLevelIncrementer(this);
    }

private:
    HTMLParserReentryPermit();

    // https://html.spec.whatwg.org/#script-nesting-level
    unsigned m_scriptNestingLevel;

    // https://html.spec.whatwg.org/#parser-pause-flag
    bool m_parserPauseFlag;

    DISALLOW_COPY_AND_ASSIGN(HTMLParserReentryPermit);
};

} // namespace blink

#endif // HTMLParserReentryPermit_h