The HTML parser synchronously creates custom elements

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 "platform/heap/Handle.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 GarbageCollected<HTMLParserReentryPermit> {
+public:
+    HTMLParserReentryPermit()
+        : m_scriptNestingLevel(0)
+        , m_parserPauseFlag(false) {}
+
+    // TODO(dominicc): platform/heap/BlinkGCAPIReference.md says this type
+    // should not be traced, but compilation errors result unless it is.
+    DEFINE_INLINE_TRACE() {}
+
+    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:
+        UntracedMember<HTMLParserReentryPermit> m_permit;
+
+        DISALLOW_COPY_AND_ASSIGN(ScriptNestingLevelIncrementer);
+    };
+
+    ScriptNestingLevelIncrementer incrementScriptNestingLevel()
+    {
+        return ScriptNestingLevelIncrementer(this);
+    }
+
+private:
+    // 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