mfbt/BufferList.h
author Petru Lingurar <petru.lingurar@softvision.ro>
Fri, 21 Dec 2018 08:56:47 +0000
changeset 501492 65621d0fe1262af0643cec37c23b2d9ec42588ad
parent 485474 c9baf27742536134040a635d6f0b9528874d6bff
child 508163 6f3709b3878117466168c40affa7bca0b60cf75b
permissions -rw-r--r--
Bug 1513938 - Enforce a Bundle size limit and drop `privateSession` if exceeds it. r=JanH, a=jcristau The `privateSession` key would normally allow persisting the Private Browsing session across OOMs in Activity's Bundle. We need to do that to avoid storing private, sensible data on disk like we do with the normal browsing session. In some cases `privateSession` would contain a lot of data which, along with other possible concurrent transactions could overflow Binder's buffer which has a limited fixed size, currently 1Mb. To avoid this, we will drop `privateSession` from the Bundle if the resulting size is greater than a _speculative_ size of 300KBs which would mean that in the case of an OOM all Private Browsing state would be lost. Bug 1515592 is filed to investigate for a better solution. Differential Revision: https://phabricator.services.mozilla.com/D15067

/* -*- 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_BufferList_h
#define mozilla_BufferList_h

#include <algorithm>
#include "mozilla/AllocPolicy.h"
#include "mozilla/Maybe.h"
#include "mozilla/MemoryReporting.h"
#include "mozilla/Move.h"
#include "mozilla/ScopeExit.h"
#include "mozilla/Types.h"
#include "mozilla/TypeTraits.h"
#include "mozilla/Vector.h"
#include <string.h>

// BufferList represents a sequence of buffers of data. A BufferList can choose
// to own its buffers or not. The class handles writing to the buffers,
// iterating over them, and reading data out. Unlike SegmentedVector, the
// buffers may be of unequal size. Like SegmentedVector, BufferList is a nice
// way to avoid large contiguous allocations (which can trigger OOMs).

class InfallibleAllocPolicy;

namespace mozilla {

template<typename AllocPolicy>
class BufferList : private AllocPolicy
{
  // Each buffer in a BufferList has a size and a capacity. The first mSize
  // bytes are initialized and the remaining |mCapacity - mSize| bytes are free.
  struct Segment
  {
    char* mData;
    size_t mSize;
    size_t mCapacity;

    Segment(char* aData, size_t aSize, size_t aCapacity)
     : mData(aData),
       mSize(aSize),
       mCapacity(aCapacity)
    {
    }

    Segment(const Segment&) = delete;
    Segment& operator=(const Segment&) = delete;

    Segment(Segment&&) = default;
    Segment& operator=(Segment&&) = default;

    char* Start() const { return mData; }
    char* End() const { return mData + mSize; }
  };

  template<typename OtherAllocPolicy>
  friend class BufferList;

 public:
  // For the convenience of callers, all segments are required to be a multiple
  // of 8 bytes in capacity. Also, every buffer except the last one is required
  // to be full (i.e., size == capacity). Therefore, a byte at offset N within
  // the BufferList and stored in memory at an address A will satisfy
  // (N % Align == A % Align) if Align == 2, 4, or 8.
  static const size_t kSegmentAlignment = 8;

  // Allocate a BufferList. The BufferList will free all its buffers when it is
  // destroyed. If an infallible allocator is used, an initial buffer of size
  // aInitialSize and capacity aInitialCapacity is allocated automatically. This
  // data will be contiguous and can be accessed via |Start()|. If a fallible
  // alloc policy is used, aInitialSize must be 0, and the fallible |Init()|
  // method may be called instead. Subsequent buffers will be allocated with
  // capacity aStandardCapacity.
  BufferList(size_t aInitialSize,
             size_t aInitialCapacity,
             size_t aStandardCapacity,
             AllocPolicy aAP = AllocPolicy())
   : AllocPolicy(aAP),
     mOwning(true),
     mSegments(aAP),
     mSize(0),
     mStandardCapacity(aStandardCapacity)
  {
    MOZ_ASSERT(aInitialCapacity % kSegmentAlignment == 0);
    MOZ_ASSERT(aStandardCapacity % kSegmentAlignment == 0);

    if (aInitialCapacity) {
      MOZ_ASSERT((aInitialSize == 0 || IsSame<AllocPolicy, InfallibleAllocPolicy>::value),
                 "BufferList may only be constructed with an initial size when "
                 "using an infallible alloc policy");

      AllocateSegment(aInitialSize, aInitialCapacity);
    }
  }

  BufferList(const BufferList& aOther) = delete;

  BufferList(BufferList&& aOther)
   : mOwning(aOther.mOwning),
     mSegments(std::move(aOther.mSegments)),
     mSize(aOther.mSize),
     mStandardCapacity(aOther.mStandardCapacity)
  {
    aOther.mSegments.clear();
    aOther.mSize = 0;
  }

  BufferList& operator=(const BufferList& aOther) = delete;

  BufferList& operator=(BufferList&& aOther)
  {
    Clear();

    mOwning = aOther.mOwning;
    mSegments = std::move(aOther.mSegments);
    mSize = aOther.mSize;
    aOther.mSegments.clear();
    aOther.mSize = 0;
    return *this;
  }

  ~BufferList() { Clear(); }

  // Initializes the BufferList with a segment of the given size and capacity.
  // May only be called once, before any segments have been allocated.
  bool Init(size_t aInitialSize, size_t aInitialCapacity)
  {
    MOZ_ASSERT(mSegments.empty());
    MOZ_ASSERT(aInitialCapacity != 0);
    MOZ_ASSERT(aInitialCapacity % kSegmentAlignment == 0);

    return AllocateSegment(aInitialSize, aInitialCapacity);
  }

  bool CopyFrom(const BufferList& aOther)
  {
    MOZ_ASSERT(mOwning);

    Clear();

    // We don't make an exact copy of aOther. Instead, create a single segment
    // with enough space to hold all data in aOther.
    if (!Init(aOther.mSize, (aOther.mSize + kSegmentAlignment - 1) & ~(kSegmentAlignment - 1))) {
      return false;
    }

    size_t offset = 0;
    for (const Segment& segment : aOther.mSegments) {
      memcpy(Start() + offset, segment.mData, segment.mSize);
      offset += segment.mSize;
    }
    MOZ_ASSERT(offset == mSize);

    return true;
  }

  // Returns the sum of the sizes of all the buffers.
  size_t Size() const { return mSize; }

  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf)
  {
    size_t size = mSegments.sizeOfExcludingThis(aMallocSizeOf);
    for (Segment& segment : mSegments) {
      size += aMallocSizeOf(segment.Start());
    }
    return size;
  }

  void Clear()
  {
    if (mOwning) {
      for (Segment& segment : mSegments) {
        this->free_(segment.mData, segment.mCapacity);
      }
    }
    mSegments.clear();

    mSize = 0;
  }

  // Iterates over bytes in the segments. You can advance it by as many bytes as
  // you choose.
  class IterImpl
  {
    // Invariants:
    //   (0) mSegment <= bufferList.mSegments.length()
    //   (1) mData <= mDataEnd
    //   (2) If mSegment is not the last segment, mData < mDataEnd
    uintptr_t mSegment;
    char* mData;
    char* mDataEnd;

    friend class BufferList;

  public:
    explicit IterImpl(const BufferList& aBuffers)
     : mSegment(0),
       mData(nullptr),
       mDataEnd(nullptr)
    {
      if (!aBuffers.mSegments.empty()) {
        mData = aBuffers.mSegments[0].Start();
        mDataEnd = aBuffers.mSegments[0].End();
      }
    }

    // Returns a pointer to the raw data. It is valid to access up to
    // RemainingInSegment bytes of this buffer.
    char* Data() const
    {
      MOZ_RELEASE_ASSERT(!Done());
      return mData;
    }

    // Returns true if the memory in the range [Data(), Data() + aBytes) is all
    // part of one contiguous buffer.
    bool HasRoomFor(size_t aBytes) const
    {
      MOZ_RELEASE_ASSERT(mData <= mDataEnd);
      return size_t(mDataEnd - mData) >= aBytes;
    }

    // Returns the maximum value aBytes for which HasRoomFor(aBytes) will be
    // true.
    size_t RemainingInSegment() const
    {
      MOZ_RELEASE_ASSERT(mData <= mDataEnd);
      return mDataEnd - mData;
    }

    bool HasBytesAvailable(const BufferList& aBuffers, uint32_t aBytes) const
    {
      if (RemainingInSegment() >= aBytes) {
        return true;
      }
      aBytes -= RemainingInSegment();
      for (size_t i = mSegment + 1; i < aBuffers.mSegments.length(); i++) {
        if (aBuffers.mSegments[i].mSize >= aBytes) {
          return true;
        }
        aBytes -= aBuffers.mSegments[i].mSize;
      }

      return false;
    }

    // Advances the iterator by aBytes bytes. aBytes must be less than
    // RemainingInSegment(). If advancing by aBytes takes the iterator to the
    // end of a buffer, it will be moved to the beginning of the next buffer
    // unless it is the last buffer.
    void Advance(const BufferList& aBuffers, size_t aBytes)
    {
      const Segment& segment = aBuffers.mSegments[mSegment];
      MOZ_RELEASE_ASSERT(segment.Start() <= mData);
      MOZ_RELEASE_ASSERT(mData <= mDataEnd);
      MOZ_RELEASE_ASSERT(mDataEnd == segment.End());

      MOZ_RELEASE_ASSERT(HasRoomFor(aBytes));
      mData += aBytes;

      if (mData == mDataEnd && mSegment + 1 < aBuffers.mSegments.length()) {
        mSegment++;
        const Segment& nextSegment = aBuffers.mSegments[mSegment];
        mData = nextSegment.Start();
        mDataEnd = nextSegment.End();
        MOZ_RELEASE_ASSERT(mData < mDataEnd);
      }
    }

    // Advance the iterator by aBytes, possibly crossing segments. This function
    // returns false if it runs out of buffers to advance through. Otherwise it
    // returns true.
    bool AdvanceAcrossSegments(const BufferList& aBuffers, size_t aBytes)
    {
      size_t bytes = aBytes;
      while (bytes) {
        size_t toAdvance = std::min(bytes, RemainingInSegment());
        if (!toAdvance) {
          return false;
        }
        Advance(aBuffers, toAdvance);
        bytes -= toAdvance;
      }
      return true;
    }

    // Returns true when the iterator reaches the end of the BufferList.
    bool Done() const
    {
      return mData == mDataEnd;
    }

   private:

    // Count the bytes we would need to advance in order to reach aTarget.
    size_t BytesUntil(const BufferList& aBuffers, const IterImpl& aTarget) const {
      size_t offset = 0;

      MOZ_ASSERT(aTarget.IsIn(aBuffers));

      char* data = mData;
      for (uintptr_t segment = mSegment; segment < aTarget.mSegment; segment++) {
        offset += aBuffers.mSegments[segment].End() - data;
        data = aBuffers.mSegments[segment].mData;
      }

      MOZ_RELEASE_ASSERT(IsIn(aBuffers));
      MOZ_RELEASE_ASSERT(aTarget.mData >= data);

      offset += aTarget.mData - data;
      return offset;
    }

    bool IsIn(const BufferList& aBuffers) const {
      return mSegment < aBuffers.mSegments.length() &&
             mData >= aBuffers.mSegments[mSegment].mData &&
             mData < aBuffers.mSegments[mSegment].End();
    }
  };

  // Special convenience method that returns Iter().Data().
  char* Start()
  {
    MOZ_RELEASE_ASSERT(!mSegments.empty());
    return mSegments[0].mData;
  }
  const char* Start() const { return mSegments[0].mData; }

  IterImpl Iter() const { return IterImpl(*this); }

  // Copies aSize bytes from aData into the BufferList. The storage for these
  // bytes may be split across multiple buffers. Size() is increased by aSize.
  inline bool WriteBytes(const char* aData, size_t aSize);

  // Allocates a buffer of at most |aMaxBytes| bytes and, if successful, returns
  // that buffer, and places its size in |aSize|. If unsuccessful, returns null
  // and leaves |aSize| undefined.
  inline char* AllocateBytes(size_t aMaxSize, size_t* aSize);

  // Copies possibly non-contiguous byte range starting at aIter into
  // aData. aIter is advanced by aSize bytes. Returns false if it runs out of
  // data before aSize.
  inline bool ReadBytes(IterImpl& aIter, char* aData, size_t aSize) const;

  // Return a new BufferList that shares storage with this BufferList. The new
  // BufferList is read-only. It allows iteration over aSize bytes starting at
  // aIter. Borrow can fail, in which case *aSuccess will be false upon
  // return. The borrowed BufferList can use a different AllocPolicy than the
  // original one. However, it is not responsible for freeing buffers, so the
  // AllocPolicy is only used for the buffer vector.
  template<typename BorrowingAllocPolicy>
  BufferList<BorrowingAllocPolicy> Borrow(IterImpl& aIter, size_t aSize, bool* aSuccess,
                                          BorrowingAllocPolicy aAP = BorrowingAllocPolicy()) const;

  // Return a new BufferList and move storage from this BufferList to it. The
  // new BufferList owns the buffers. Move can fail, in which case *aSuccess
  // will be false upon return. The new BufferList can use a different
  // AllocPolicy than the original one. The new OtherAllocPolicy is responsible
  // for freeing buffers, so the OtherAllocPolicy must use freeing method
  // compatible to the original one.
  template<typename OtherAllocPolicy>
  BufferList<OtherAllocPolicy> MoveFallible(bool* aSuccess, OtherAllocPolicy aAP = OtherAllocPolicy());

  // Return a new BufferList that adopts the byte range starting at Iter so that
  // range [aIter, aIter + aSize) is transplanted to the returned BufferList.
  // Contents of the buffer before aIter + aSize is left undefined.
  // Extract can fail, in which case *aSuccess will be false upon return. The
  // moved buffers are erased from the original BufferList. In case of extract
  // fails, the original BufferList is intact.  All other iterators except aIter
  // are invalidated.
  // This method requires aIter and aSize to be 8-byte aligned.
  BufferList Extract(IterImpl& aIter, size_t aSize, bool* aSuccess);

  // Return the number of bytes from 'start' to 'end', two iterators within
  // this BufferList.
  size_t RangeLength(const IterImpl& start, const IterImpl& end) const {
    MOZ_ASSERT(start.IsIn(*this) && end.IsIn(*this));
    return start.BytesUntil(*this, end);
  }

  // This takes ownership of the data
  void* WriteBytesZeroCopy(char *aData, size_t aSize, size_t aCapacity)
  {
    MOZ_ASSERT(aCapacity != 0);
    MOZ_ASSERT(aSize <= aCapacity);
    MOZ_ASSERT(mOwning);

    if (!mSegments.append(Segment(aData, aSize, aCapacity))) {
      this->free_(aData, aCapacity);
      return nullptr;
    }
    mSize += aSize;
    return aData;
  }

private:
  explicit BufferList(AllocPolicy aAP)
   : AllocPolicy(aAP),
     mOwning(false),
     mSize(0),
     mStandardCapacity(0)
  {
  }

  char* AllocateSegment(size_t aSize, size_t aCapacity)
  {
    MOZ_RELEASE_ASSERT(mOwning);
    MOZ_ASSERT(aCapacity != 0);
    MOZ_ASSERT(aSize <= aCapacity);

    char* data = this->template pod_malloc<char>(aCapacity);
    if (!data) {
      return nullptr;
    }
    if (!mSegments.append(Segment(data, aSize, aCapacity))) {
      this->free_(data, aCapacity);
      return nullptr;
    }
    mSize += aSize;
    return data;
  }

  bool mOwning;
  Vector<Segment, 1, AllocPolicy> mSegments;
  size_t mSize;
  size_t mStandardCapacity;
};

template<typename AllocPolicy>
bool
BufferList<AllocPolicy>::WriteBytes(const char* aData, size_t aSize)
{
  MOZ_RELEASE_ASSERT(mOwning);
  MOZ_RELEASE_ASSERT(mStandardCapacity);

  size_t copied = 0;
  while (copied < aSize) {
    size_t toCopy;
    char* data = AllocateBytes(aSize - copied, &toCopy);
    if (!data) {
      return false;
    }
    memcpy(data, aData + copied, toCopy);
    copied += toCopy;
  }

  return true;
}

template<typename AllocPolicy>
char*
BufferList<AllocPolicy>::AllocateBytes(size_t aMaxSize, size_t* aSize)
{
  MOZ_RELEASE_ASSERT(mOwning);
  MOZ_RELEASE_ASSERT(mStandardCapacity);

  if (!mSegments.empty()) {
    Segment& lastSegment = mSegments.back();

    size_t capacity = lastSegment.mCapacity - lastSegment.mSize;
    if (capacity) {
      size_t size = std::min(aMaxSize, capacity);
      char* data = lastSegment.mData + lastSegment.mSize;

      lastSegment.mSize += size;
      mSize += size;

      *aSize = size;
      return data;
    }
  }

  size_t size = std::min(aMaxSize, mStandardCapacity);
  char* data = AllocateSegment(size, mStandardCapacity);
  if (data) {
    *aSize = size;
  }
  return data;
}

template<typename AllocPolicy>
bool
BufferList<AllocPolicy>::ReadBytes(IterImpl& aIter, char* aData, size_t aSize) const
{
  size_t copied = 0;
  size_t remaining = aSize;
  while (remaining) {
    size_t toCopy = std::min(aIter.RemainingInSegment(), remaining);
    if (!toCopy) {
      // We've run out of data in the last segment.
      return false;
    }
    memcpy(aData + copied, aIter.Data(), toCopy);
    copied += toCopy;
    remaining -= toCopy;

    aIter.Advance(*this, toCopy);
  }

  return true;
}

template<typename AllocPolicy> template<typename BorrowingAllocPolicy>
BufferList<BorrowingAllocPolicy>
BufferList<AllocPolicy>::Borrow(IterImpl& aIter, size_t aSize, bool* aSuccess,
                                BorrowingAllocPolicy aAP) const
{
  BufferList<BorrowingAllocPolicy> result(aAP);

  size_t size = aSize;
  while (size) {
    size_t toAdvance = std::min(size, aIter.RemainingInSegment());

    if (!toAdvance || !result.mSegments.append(typename BufferList<BorrowingAllocPolicy>::Segment(aIter.mData, toAdvance, toAdvance))) {
      *aSuccess = false;
      return result;
    }
    aIter.Advance(*this, toAdvance);
    size -= toAdvance;
  }

  result.mSize = aSize;
  *aSuccess = true;
  return result;
}

template<typename AllocPolicy> template<typename OtherAllocPolicy>
BufferList<OtherAllocPolicy>
BufferList<AllocPolicy>::MoveFallible(bool* aSuccess, OtherAllocPolicy aAP)
{
  BufferList<OtherAllocPolicy> result(0, 0, mStandardCapacity, aAP);

  IterImpl iter = Iter();
  while (!iter.Done()) {
    size_t toAdvance = iter.RemainingInSegment();

    if (!toAdvance || !result.mSegments.append(typename BufferList<OtherAllocPolicy>::Segment(iter.mData, toAdvance, toAdvance))) {
      *aSuccess = false;
      result.mSegments.clear();
      return result;
    }
    iter.Advance(*this, toAdvance);
  }

  result.mSize = mSize;
  mSegments.clear();
  mSize = 0;
  *aSuccess = true;
  return result;
}

template<typename AllocPolicy>
BufferList<AllocPolicy>
BufferList<AllocPolicy>::Extract(IterImpl& aIter, size_t aSize, bool* aSuccess)
{
  MOZ_RELEASE_ASSERT(aSize);
  MOZ_RELEASE_ASSERT(mOwning);
  MOZ_ASSERT(aSize % kSegmentAlignment == 0);
  MOZ_ASSERT(intptr_t(aIter.mData) % kSegmentAlignment == 0);

  auto failure = [this, aSuccess]() {
    *aSuccess = false;
    return BufferList(0, 0, mStandardCapacity);
  };

  // Number of segments we'll need to copy data from to satisfy the request.
  size_t segmentsNeeded = 0;
  // If this is None then the last segment is a full segment, otherwise we need
  // to copy this many bytes.
  Maybe<size_t> lastSegmentSize;
  {
    // Copy of the iterator to walk the BufferList and see how many segments we
    // need to copy.
    IterImpl iter = aIter;
    size_t remaining = aSize;
    while (!iter.Done() && remaining &&
           remaining >= iter.RemainingInSegment()) {
      remaining -= iter.RemainingInSegment();
      iter.Advance(*this, iter.RemainingInSegment());
      segmentsNeeded++;
    }

    if (remaining) {
      if (iter.Done()) {
        // We reached the end of the BufferList and there wasn't enough data to
        // satisfy the request.
        return failure();
      }
      lastSegmentSize.emplace(remaining);
      // The last block also counts as a segment. This makes the conditionals
      // on segmentsNeeded work in the rest of the function.
      segmentsNeeded++;
    }
  }

  BufferList result(0, 0, mStandardCapacity);
  if (!result.mSegments.reserve(segmentsNeeded + lastSegmentSize.isSome())) {
    return failure();
  }

  // Copy the first segment, it's special because we can't just steal the
  // entire Segment struct from this->mSegments.
  size_t firstSegmentSize = std::min(aSize, aIter.RemainingInSegment());
  if (!result.WriteBytes(aIter.Data(), firstSegmentSize)) {
    return failure();
  }
  aIter.Advance(*this, firstSegmentSize);
  segmentsNeeded--;

  // The entirety of the request wasn't in the first segment, now copy the
  // rest.
  if (segmentsNeeded) {
    char* finalSegment = nullptr;
    // Pre-allocate the final segment so that if this fails, we return before
    // we delete the elements from |this->mSegments|.
    if (lastSegmentSize.isSome()) {
      MOZ_RELEASE_ASSERT(mStandardCapacity >= *lastSegmentSize);
      finalSegment = this->template pod_malloc<char>(mStandardCapacity);
      if (!finalSegment) {
        return failure();
      }
    }

    size_t copyStart = aIter.mSegment;
    // Copy segments from this over to the result and remove them from our
    // storage. Not needed if the only segment we need to copy is the last
    // partial one.
    size_t segmentsToCopy = segmentsNeeded - lastSegmentSize.isSome();
    for (size_t i = 0; i < segmentsToCopy; ++i) {
      result.mSegments.infallibleAppend(
        Segment(mSegments[aIter.mSegment].mData,
                mSegments[aIter.mSegment].mSize,
                mSegments[aIter.mSegment].mCapacity));
      aIter.Advance(*this, aIter.RemainingInSegment());
    }
    // Due to the way IterImpl works, there are two cases here: (1) if we've
    // consumed the entirety of the BufferList, then the iterator is pointed at
    // the end of the final segment, (2) otherwise it is pointed at the start
    // of the next segment. We want to verify that we really consumed all
    // |segmentsToCopy| segments.
    MOZ_RELEASE_ASSERT(
      (aIter.mSegment == copyStart + segmentsToCopy) ||
      (aIter.Done() && aIter.mSegment == copyStart + segmentsToCopy - 1));
    mSegments.erase(mSegments.begin() + copyStart,
                    mSegments.begin() + copyStart + segmentsToCopy);

    // Reset the iter's position for what we just deleted.
    aIter.mSegment -= segmentsToCopy;

    if (lastSegmentSize.isSome()) {
      // We called reserve() on result.mSegments so infallibleAppend is safe.
      result.mSegments.infallibleAppend(
        Segment(finalSegment, 0, mStandardCapacity));
      bool r = result.WriteBytes(aIter.Data(), *lastSegmentSize);
      MOZ_RELEASE_ASSERT(r);
      aIter.Advance(*this, *lastSegmentSize);
    }
  }

  mSize -= aSize;
  result.mSize = aSize;

  *aSuccess = true;
  return result;
}

} // namespace mozilla

#endif /* mozilla_BufferList_h */