toolkit/xre/nsUpdateMutex.h

   1 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim:set ts=2 sw=2 sts=2 et cindent: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef nsUpdateMutex_h__
   8 #define nsUpdateMutex_h__
   9
  10 #include "nsIUpdateService.h"
  11 #include "nsProfileLock.h"
  12 #include "mozilla/StaticMutex.h"
  13
  14 /**
  15  * A primitive object type suitable for acquiring the update mutex. It is
  16  * composed of two parts:
  17  *  - a nsProfileLock taken on the update directory, to ensure that if two
  18  *    instances running from the same application path try to acquire the
  19  *    update mutex simultaneously, only one of them succeeds;
  20  *  - a StaticMutex, to ensure that even within the same instance of the
  21  *    application, it is never possible to successfully acquire two
  22  *    UpdateMutexImpl objects simultaneously.
  23  *
  24  * While the second part is not strictly required, it makes reasoning about
  25  * these objects easier, and it helps us simulate an acquisition coming from
  26  * another instance in tests.
  27  *
  28  * Contrary to a nsIUpdateMutex object, an UpdateMutexImpl object does not
  29  * keep track of whether it is currently locked or unlocked. Therefore, it is
  30  * the responsibility of the caller to guarantee the following:
  31  *  - a call to Unlock() must only occur after a matching successful call to
  32  *    TryLock();
  33  *  - no second call to TryLock() should ever occur after a successful first
  34  *    call to TryLock(), unless a call to Unlock() occured in the middle.
  35  */
  36 class MOZ_CAPABILITY("mutex") UpdateMutexImpl {
  37  public:
  38   [[nodiscard]] bool TryLock() MOZ_TRY_ACQUIRE(true);
  39
  40   void Unlock() MOZ_CAPABILITY_RELEASE();
  41
  42  private:
  43   static mozilla::StaticMutex sInProcessMutex;
  44
  45   nsProfileLock mCrossProcessLock;
  46 };
  47
  48 /**
  49  * An XPCOM wrapper for the UpdateMutexImpl primitive type, achieving the same
  50  * goals but through a safe XPCOM-compatible nsIUpdateMutex interface.
  51  *
  52  * Contrary to UpdateMutexImpl objects, nsUpdateMutex objects track whether
  53  * they are currently locked or unlocked. It is therefore always safe to call
  54  * TryLock() or Unlock() on a nsUpdateMutex object.
  55  *
  56  * See nsIUpdateMutex in nsUpdateService.idl for more details.
  57  */
  58 class nsUpdateMutex final : public nsIUpdateMutex {
  59  public:
  60   explicit nsUpdateMutex() = default;
  61
  62   NS_DECL_THREADSAFE_ISUPPORTS
  63   NS_DECL_NSIUPDATEMUTEX
  64
  65  private:
  66   UpdateMutexImpl mUpdateMutexImpl;
  67   bool mIsLocked{};
  68
  69   virtual ~nsUpdateMutex() {
  70     if (mIsLocked) {
  71       Unlock();
  72     }
  73   }
  74 };
  75
  76 #endif  // nsUpdateMutex_h__