parser/html/nsHtml5StreamListener.h

   1 /* This Source Code Form is subject to the terms of the Mozilla Public
   2  * License, v. 2.0. If a copy of the MPL was not distributed with this
   3  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   4
   5 #ifndef nsHtml5StreamListener_h
   6 #define nsHtml5StreamListener_h
   7
   8 #include "nsIStreamListener.h"
   9 #include "nsIThreadRetargetableStreamListener.h"
  10 #include "nsHtml5StreamParser.h"
  11 #include "mozilla/ReentrantMonitor.h"
  12
  13 /**
  14  * The purpose of this class is to reconcile the problem that
  15  * nsHtml5StreamParser is a cycle collection participant, which means that it
  16  * can only be refcounted on the main thread, but
  17  * nsIThreadRetargetableStreamListener can be refcounted from another thread,
  18  * so nsHtml5StreamParser being an nsIThreadRetargetableStreamListener was
  19  * a memory corruption problem.
  20  *
  21  * mDelegate is an nsHtml5StreamParserPtr, which releases the object that it
  22  * points to from a runnable on the main thread. DropDelegate() is only called
  23  * on the main thread. This call will finish before the main-thread derefs the
  24  * nsHtml5StreamListener itself, so there is no risk of another thread making
  25  * the refcount of nsHtml5StreamListener go to zero and running the destructor
  26  * concurrently. Other than that, the thread-safe nsISupports implementation
  27  * takes care of the destructor not running concurrently from different
  28  * threads, so there is no need to have a mutex around nsHtml5StreamParserPtr to
  29  * prevent it from double-releasing nsHtml5StreamParser.
  30  */
  31 class nsHtml5StreamListener : public nsIThreadRetargetableStreamListener {
  32  public:
  33   explicit nsHtml5StreamListener(nsHtml5StreamParser* aDelegate);
  34
  35   NS_DECL_THREADSAFE_ISUPPORTS
  36   NS_DECL_NSIREQUESTOBSERVER
  37   NS_DECL_NSISTREAMLISTENER
  38   NS_DECL_NSITHREADRETARGETABLESTREAMLISTENER
  39
  40   // Main-thread-only
  41   nsHtml5StreamParser* GetDelegate();
  42
  43   // Main-thread-only
  44   void DropDelegate();
  45
  46  private:
  47   void DropDelegateImpl();
  48   virtual ~nsHtml5StreamListener();
  49
  50   // ReentrantMonitor instead of Mutex, because `GetDelegate()`
  51   // can be called from within the Necko callbacks when Necko events
  52   // are delivered on the main thread.
  53   mozilla::ReentrantMonitor mDelegateMonitor MOZ_UNANNOTATED;
  54   // Owning pointer with manually-managed refcounting, protected by
  55   // mDelegateMonitor.   Access to it is Atomic, which avoids getting a lock
  56   // to check if it's set or to return the pointer.   Access to the data within
  57   // it needs the monitor.   MOZ_PT_GUARDED_BY() can't be used with Atomic<...*>
  58   mozilla::Atomic<nsHtml5StreamParser*, mozilla::ReleaseAcquire> mDelegate;
  59 };
  60
  61 #endif  // nsHtml5StreamListener_h