gecko/xpcom/build/IOInterposer.h

/* 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 "mozilla/XPCOM.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),
    OpWriteFSync = (OpWrite | OpFSync),
    OpAll = (OpCreateOrOpen | OpRead | OpWrite | OpFSync | OpStat | OpClose)
  };

  /** 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 either OpRead, OpWrite or OpFSync,
     * combinations of these flags are only used when registering observers.
     */
    Operation ObservedOperation() const
    {
      return mOperation;
    }

    /** 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;
    }

    /** Request filename associated with the I/O operation, null if unknown */
    virtual const char16_t* Filename()
    {
      return nullptr;
    }

    virtual ~Observation()
    {
    }

  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()
  {
  }

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();
};

#ifdef MOZ_ENABLE_PROFILER_SPS

/**
 * Class offering the public static IOInterposer API.
 *
 * This class is responsible for ensuring that events are routed to the
 * appropriate observers. Methods Init() and Clear() should only be called from
 * the main-thread at startup and shutdown, respectively.
 *
 * Remark: Instances of this class will never be created, you should consider it
 * to be a namespace containing static functions. The class is created to
 * facilitate to a private static instance variable sObservedOperations.
 * As we want to access this from an inline static methods, we have to do this
 * trick.
 */
class IOInterposer MOZ_FINAL
{
  // Track whether or not a given type of observation is being observed
  static IOInterposeObserver::Operation sObservedOperations;

  // No instance of class should be created, they'd be empty anyway.
  IOInterposer();
public:

  /**
   * 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.
   */
  static void 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.
   */
  static void Clear();

  /**
   * 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.
   */
  static 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 not report IO when no observers are registered.
   */
  static inline bool IsObservedOperation(IOInterposeObserver::Operation aOp) {
    // The quick reader may observe that no locks are being employed here,
    // hence, the result of the operations is truly undefined. However, most
    // computers will usually return either true or false, which is good enough.
    // If we occasionally report more or less IO than is being observed than
    // that is not a problem.
    return (sObservedOperations & aOp);
  }

  /**
   * Register IOInterposeObserver, the observer object will receive all
   * observations for the given operation aOp.
   *
   * Remark: Init() must be called before observers are registered.
   */
  static void Register(IOInterposeObserver::Operation aOp,
                       IOInterposeObserver* aObserver);

  /**
   * 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)
   *
   * Remark: Init() must be called before observers are unregistered.
   */
  static void Unregister(IOInterposeObserver::Operation aOp,
                         IOInterposeObserver* aObserver);

  /**
   * Registers the current thread with the IOInterposer.
   *
   * @param aIsMainThread true if IOInterposer should treat the current thread
   *                      as the main thread.
   */
  static void
  RegisterCurrentThread(bool aIsMainThread = false);
};

#else /* MOZ_ENABLE_PROFILER_SPS */

class IOInterposer MOZ_FINAL
{
  IOInterposer();
public:
  static inline void Init()                                               {}
  static inline void Clear()                                              {}
  static inline void Report(IOInterposeObserver::Observation& aOb)        {}
  static inline void Register(IOInterposeObserver::Operation aOp,
                              IOInterposeObserver* aObserver)             {}
  static inline void Unregister(IOInterposeObserver::Operation aOp,
                                IOInterposeObserver* aObserver)           {}
  static inline bool IsObservedOperation(IOInterposeObserver::Operation aOp) {
    return false;
  }
  static inline void RegisterCurrentThread(bool)                          {}
};

#endif /* MOZ_ENABLE_PROFILER_SPS */

class IOInterposerInit
{
public:
  IOInterposerInit()
  {
    IOInterposer::Init();
  }

  // No destructor needed at the moment -- this stuff stays active for the
  // life of the process. This may change in the future.
};

} // namespace mozilla

#endif // mozilla_IOInterposer_h