Fix marking files modified externally as modified

It replaces 59c90e00c1
and d0c537ca75

Task-number: QTCREATORBUG-17048
Change-Id: Ief4b1b72f2e5e7b1711be05d4ea8c03bbbf48fdf
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/libs/utils/guard.cpp

@@ -0,0 +1,113 @@
+/****************************************************************************
+**
+** Copyright (C) 2016 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#include "guard.h"
+#include "qtcassert.h"
+
+/*! \class Utils::Guard
+
+  \brief The Guard class implements a recursive guard with locking mechanism.
+
+  It may be used as an alternative to QSignalBlocker.
+  QSignalBlocker blocks all signals of the object
+  which is usually not desirable. It may also block signals
+  which are needed internally by the object itself.
+  The Guard and GuardLocker classes don't block signals at all.
+
+  When calling a object's method which may in turn emit a signal
+  which you are connected to, and you want to ignore
+  this notification, you should keep the Guard object
+  as your class member and declare the GuardLocker object
+  just before calling the mentioned method, like:
+
+  \code
+  class MyClass : public QObject
+  {
+  \dots
+  private:
+      Guard updateGuard; // member of your class
+  };
+
+  \dots
+
+  void MyClass::updateOtherObject()
+  {
+  GuardLocker updatelocker(updateGuard);
+  otherObject->update(); // this may trigger a signal
+  }
+  \endcode
+
+  Inside a slot which is connected to the other's object signal
+  you may check if the guard is locked and ignore the further
+  operations in this case:
+
+  \code
+  void MyClass::otherObjectUpdated()
+  {
+  if (updateGuard.isLocked)
+      return;
+
+  // we didn't trigger the update
+  // so do update now
+  \dots
+  }
+  \endcode
+
+  The GuardLock unlocks the Guard in it's destructor.
+
+  The Guard object is recursive, you may declare many GuardLocker
+  objects for the same Guard instance and the Guard will be locked
+  as long as at least one GuardLocker object created for the Guard
+  is in scope.
+*/
+
+namespace Utils {
+
+Guard::Guard()
+{
+}
+
+Guard::~Guard()
+{
+    QTC_CHECK(m_lockCount == 0);
+}
+
+bool Guard::isLocked() const
+{
+    return m_lockCount;
+}
+
+GuardLocker::GuardLocker(Guard &guard)
+    : m_guard(guard)
+{
+    ++m_guard.m_lockCount;
+}
+
+GuardLocker::~GuardLocker()
+{
+    --m_guard.m_lockCount;
+}
+
+} // namespace Utils