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diff --git a/xpcom/build/IOInterposer.h b/xpcom/build/IOInterposer.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 mozilla_IOInterposer_h
+#define mozilla_IOInterposer_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/TimeStamp.h"
+#include "nsString.h"
+
+namespace mozilla {
+
+/**
+ * Interface for I/O interposer observers. This is separate from the
+ * IOInterposer because we have multiple uses for these observations.
+ */
+class IOInterposeObserver {
+ public:
+  enum Operation {
+    OpNone = 0,
+    OpCreateOrOpen = (1 << 0),
+    OpRead = (1 << 1),
+    OpWrite = (1 << 2),
+    OpFSync = (1 << 3),
+    OpStat = (1 << 4),
+    OpClose = (1 << 5),
+    OpNextStage =
+        (1 << 6),  // Meta - used when leaving startup, entering shutdown
+    OpWriteFSync = (OpWrite | OpFSync),
+    OpAll = (OpCreateOrOpen | OpRead | OpWrite | OpFSync | OpStat | OpClose),
+    OpAllWithStaging = (OpAll | OpNextStage)
+  };
+
+  /** A representation of an I/O observation  */
+  class Observation {
+   protected:
+    /**
+     * This constructor is for use by subclasses that are intended to take
+     * timing measurements via RAII. The |aShouldReport| parameter may be
+     * used to make the measurement and reporting conditional on the
+     * satisfaction of an arbitrary predicate that was evaluated
+     * in the subclass. Note that IOInterposer::IsObservedOperation() is
+     * always ANDed with aShouldReport, so the subclass does not need to
+     * include a call to that function explicitly.
+     */
+    Observation(Operation aOperation, const char* aReference,
+                bool aShouldReport = true);
+
+   public:
+    /**
+     * Since this constructor accepts start and end times, it does *not* take
+     * its own timings, nor does it report itself.
+     */
+    Observation(Operation aOperation, const TimeStamp& aStart,
+                const TimeStamp& aEnd, const char* aReference);
+
+    /**
+     * Operation observed, this is one of the individual Operation values.
+     * Combinations of these flags are only used when registering observers.
+     */
+    Operation ObservedOperation() const { return mOperation; }
+
+    /**
+     * Return the observed operation as a human-readable string.
+     */
+    const char* ObservedOperationString() const;
+
+    /** Time at which the I/O operation was started */
+    TimeStamp Start() const { return mStart; }
+
+    /**
+     * Time at which the I/O operation ended, for asynchronous methods this is
+     * the time at which the call initiating the asynchronous request returned.
+     */
+    TimeStamp End() const { return mEnd; }
+
+    /**
+     * Duration of the operation, for asynchronous I/O methods this is the
+     * duration of the call initiating the asynchronous request.
+     */
+    TimeDuration Duration() const { return mEnd - mStart; }
+
+    /**
+     * IO reference, function name or name of component (sqlite) that did IO
+     * this is in addition the generic operation. This attribute may be platform
+     * specific, but should only take a finite number of distinct values.
+     * E.g. sqlite-commit, CreateFile, NtReadFile, fread, fsync, mmap, etc.
+     * I.e. typically the platform specific function that did the IO.
+     */
+    const char* Reference() const { return mReference; }
+
+    virtual const char* FileType() const { return "File"; }
+
+    /** Request filename associated with the I/O operation, empty if unknown */
+    virtual void Filename(nsAString& aString) { aString.Truncate(); }
+
+    virtual ~Observation() = default;
+
+   protected:
+    void Report();
+
+    Operation mOperation;
+    TimeStamp mStart;
+    TimeStamp mEnd;
+    const char* mReference;  // Identifies the source of the Observation
+    bool mShouldReport;      // Measure and report if true
+  };
+
+  /**
+   * Invoked whenever an implementation of the IOInterposeObserver should
+   * observe aObservation. Implement this and do your thing...
+   * But do consider if it is wise to use IO functions in this method, they are
+   * likely to cause recursion :)
+   * At least, see PoisonIOInterposer.h and register your handle as a debug file
+   * even, if you don't initialize the poison IO interposer, someone else might.
+   *
+   * Remark: Observations may occur on any thread.
+   */
+  virtual void Observe(Observation& aObservation) = 0;
+
+  virtual ~IOInterposeObserver() = default;
+
+ protected:
+  /**
+   * We don't use NS_IsMainThread() because we need to be able to determine the
+   * main thread outside of XPCOM Initialization. IOInterposer observers should
+   * call this function instead.
+   */
+  static bool IsMainThread();
+};
+
+/**
+ * These functions are responsible for ensuring that events are routed to the
+ * appropriate observers.
+ */
+namespace IOInterposer {
+
+/**
+ * This function must be called from the main-thread when no other threads are
+ * running before any of the other methods on this class may be used.
+ *
+ * IO reports can however, safely assume that IsObservedOperation() will
+ * return false until the IOInterposer is initialized.
+ *
+ * Remark, it's safe to call this method multiple times, so just call it when
+ * you to utilize IO interposing.
+ *
+ * Using the IOInterposerInit class is preferred to calling this directly.
+ */
+bool Init();
+
+/**
+ * This function must be called from the main thread, and furthermore
+ * it must be called when no other threads are executing. Effectively
+ * restricting us to calling it only during shutdown.
+ *
+ * Callers should take care that no other consumers are subscribed to events,
+ * as these events will stop when this function is called.
+ *
+ * In practice, we don't use this method as the IOInterposer is used for
+ * late-write checks.
+ */
+void Clear();
+
+/**
+ * This function immediately disables IOInterposer functionality in a fast,
+ * thread-safe manner. Primarily for use by the crash reporter.
+ */
+void Disable();
+
+/**
+ * This function re-enables IOInterposer functionality in a fast, thread-safe
+ * manner.  Primarily for use by the crash reporter.
+ */
+void Enable();
+
+/**
+ * Report IO to registered observers.
+ * Notice that the reported operation must be either OpRead, OpWrite or
+ * OpFSync. You are not allowed to report an observation with OpWriteFSync or
+ * OpAll, these are just auxiliary values for use with Register().
+ *
+ * If the IO call you're reporting does multiple things, write and fsync, you
+ * can choose to call Report() twice once with write and once with FSync. You
+ * may not call Report() with OpWriteFSync! The Observation::mOperation
+ * attribute is meant to be generic, not perfect.
+ *
+ * Notice that there is no reason to report an observation with an operation
+ * which is not being observed. Use IsObservedOperation() to check if the
+ * operation you are about to report is being observed. This is especially
+ * important if you are constructing expensive observations containing
+ * filename and full-path.
+ *
+ * Remark: Init() must be called before any IO is reported. But
+ * IsObservedOperation() will return false until Init() is called.
+ */
+void Report(IOInterposeObserver::Observation& aObservation);
+
+/**
+ * Return whether or not an operation is observed. Reporters should not
+ * report operations that are not being observed by anybody. This mechanism
+ * allows us to avoid reporting I/O when no observers are registered.
+ */
+bool IsObservedOperation(IOInterposeObserver::Operation aOp);
+
+/**
+ * Register IOInterposeObserver, the observer object will receive all
+ * observations for the given operation aOp.
+ *
+ * Remarks:
+ * - Init() must be called before observers are registered.
+ * - The IOInterposeObserver object should be static, because it could still be
+ *   used on another thread shortly after Unregister().
+ */
+void Register(IOInterposeObserver::Operation aOp,
+              IOInterposeObserver* aStaticObserver);
+
+/**
+ * Unregister an IOInterposeObserver for a given operation
+ * Remark: It is always safe to unregister for all operations, even if yoú
+ * didn't register for them all.
+ * I.e. IOInterposer::Unregister(IOInterposeObserver::OpAll, aObserver)
+ *
+ * Remarks:
+ * - Init() must be called before observers are registered.
+ * - The IOInterposeObserver object should be static, because it could still be
+ *   used on another thread shortly after this Unregister() call.
+ */
+void Unregister(IOInterposeObserver::Operation aOp,
+                IOInterposeObserver* aStaticObserver);
+
+/**
+ * Registers the current thread with the IOInterposer. This must be done to
+ * ensure that per-thread data is created in an orderly fashion.
+ * We could have written this to initialize that data lazily, however this
+ * could have unintended consequences if a thread that is not aware of
+ * IOInterposer was implicitly registered: its per-thread data would never
+ * be deleted because it would not know to unregister itself.
+ *
+ * @param aIsMainThread true if IOInterposer should treat the current thread
+ *                      as the main thread.
+ */
+void RegisterCurrentThread(bool aIsMainThread = false);
+
+/**
+ * Unregisters the current thread with the IOInterposer. This is important
+ * to call when a thread is shutting down because it cleans up data that
+ * is stored in a TLS slot.
+ */
+void UnregisterCurrentThread();
+
+/**
+ * Called to inform observers that the process has transitioned out of the
+ * startup stage or into the shutdown stage. Main thread only.
+ */
+void EnteringNextStage();
+
+}  // namespace IOInterposer
+
+class MOZ_RAII AutoIOInterposer {
+ public:
+  AutoIOInterposer() = default;
+
+  void Init() {
+#if defined(EARLY_BETA_OR_EARLIER)
+    IOInterposer::Init();
+#endif
+  }
+
+  ~AutoIOInterposer() {
+#if defined(EARLY_BETA_OR_EARLIER)
+    IOInterposer::Clear();
+#endif
+  }
+};
+
+class MOZ_RAII AutoIOInterposerDisable final {
+ public:
+  explicit AutoIOInterposerDisable() { IOInterposer::Disable(); }
+  ~AutoIOInterposerDisable() { IOInterposer::Enable(); }
+
+ private:
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_IOInterposer_h