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diff --git a/xpcom/build/IOInterposerPrivate.h b/xpcom/build/IOInterposerPrivate.h
new file mode 100644
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+++ b/xpcom/build/IOInterposerPrivate.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef xpcom_build_IOInterposerPrivate_h
+#define xpcom_build_IOInterposerPrivate_h
+
+/* This header file contains declarations for helper classes that are
+   to be used exclusively by IOInterposer and its observers. This header
+   file is not to be used by anything else and MUST NOT be exported! */
+
+#include <prcvar.h>
+#include <prlock.h>
+
+namespace mozilla {
+namespace IOInterposer {
+
+/**
+ * The following classes are simple wrappers for PRLock and PRCondVar.
+ * IOInterposer and friends use these instead of Mozilla::Mutex et al because
+ * of the fact that IOInterposer is permitted to run until the process
+ * terminates; we can't use anything that plugs into leak checkers or deadlock
+ * detectors because IOInterposer will outlive those and generate false
+ * positives.
+ */
+
+class Monitor
+{
+public:
+  Monitor()
+    : mLock(PR_NewLock())
+    , mCondVar(PR_NewCondVar(mLock))
+  {
+  }
+
+  ~Monitor()
+  {
+    PR_DestroyCondVar(mCondVar);
+    mCondVar = nullptr;
+    PR_DestroyLock(mLock);
+    mLock = nullptr;
+  }
+
+  void Lock()
+  {
+    PR_Lock(mLock);
+  }
+
+  void Unlock()
+  {
+    PR_Unlock(mLock);
+  }
+
+  bool Wait(PRIntervalTime aTimeout = PR_INTERVAL_NO_TIMEOUT)
+  {
+    return PR_WaitCondVar(mCondVar, aTimeout) == PR_SUCCESS;
+  }
+
+  bool Notify()
+  {
+    return PR_NotifyCondVar(mCondVar) == PR_SUCCESS;
+  }
+
+private:
+  PRLock*    mLock;
+  PRCondVar* mCondVar;
+};
+
+class MonitorAutoLock
+{
+public:
+  explicit MonitorAutoLock(Monitor& aMonitor)
+    : mMonitor(aMonitor)
+  {
+    mMonitor.Lock();
+  }
+
+  ~MonitorAutoLock()
+  {
+    mMonitor.Unlock();
+  }
+
+  bool Wait(PRIntervalTime aTimeout = PR_INTERVAL_NO_TIMEOUT)
+  {
+    return mMonitor.Wait(aTimeout);
+  }
+
+  bool Notify()
+  {
+    return mMonitor.Notify();
+  }
+
+private:
+  Monitor&  mMonitor;
+};
+
+class MonitorAutoUnlock
+{
+public:
+  explicit MonitorAutoUnlock(Monitor& aMonitor)
+    : mMonitor(aMonitor)
+  {
+    mMonitor.Unlock();
+  }
+
+  ~MonitorAutoUnlock()
+  {
+    mMonitor.Lock();
+  }
+
+private:
+  Monitor&  mMonitor;
+};
+
+class Mutex
+{
+public:
+  Mutex()
+    : mPRLock(PR_NewLock())
+  {
+  }
+
+  ~Mutex()
+  {
+    PR_DestroyLock(mPRLock);
+    mPRLock = nullptr;
+  }
+
+  void Lock()
+  {
+    PR_Lock(mPRLock);
+  }
+
+  void Unlock()
+  {
+    PR_Unlock(mPRLock);
+  }
+
+private:
+  PRLock*   mPRLock;
+};
+
+class AutoLock
+{
+public:
+  explicit AutoLock(Mutex& aLock)
+    : mLock(aLock)
+  {
+    mLock.Lock();
+  }
+
+  ~AutoLock()
+  {
+    mLock.Unlock();
+  }
+
+private:
+  Mutex&     mLock;
+};
+
+} // namespace IOInterposer
+} // namespace mozilla
+
+#endif // xpcom_build_IOInterposerPrivate_h
+