[HICN-534] Major rework on libtransport organization

Change-Id: I361b83a18b4fd59be136d5f0817fc28e17e89884
Signed-off-by: Mauro Sardara <msardara@cisco.com>

1 files changed, 867 insertions, 0 deletions

diff --git a/libtransport/src/utils/membuf.cc b/libtransport/src/utils/membuf.cc
new file mode 100644
index 000000000..e75e85b35
--- /dev/null
+++ b/libtransport/src/utils/membuf.cc
@@ -0,0 +1,867 @@
+/*
+ * Copyright (c) 2017-2019 Cisco and/or its affiliates.
+ * Copyright 2013-present Facebook, Inc.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/*
+ * The code in this file if adapated from the IOBuf of folly:
+ * https://github.com/facebook/folly/blob/master/folly/io/IOBuf.h
+ */
+#ifdef _WIN32
+#include <hicn/transport/portability/win_portability.h>
+#endif
+
+#include <hicn/transport/utils/membuf.h>
+
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstdlib>
+#include <stdexcept>
+#include <vector>
+
+using std::unique_ptr;
+
+namespace {
+
+enum : uint16_t {
+  kHeapMagic = 0xa5a5,
+  // This memory segment contains an MemBuf that is still in use
+  kMemBufInUse = 0x01,
+  // This memory segment contains buffer data that is still in use
+  kDataInUse = 0x02,
+};
+
+enum : std::size_t {
+  // When create() is called for buffers less than kDefaultCombinedBufSize,
+  // we allocate a single combined memory segment for the MemBuf and the data
+  // together.  See the comments for createCombined()/createSeparate() for more
+  // details.
+  //
+  // (The size of 1k is largely just a guess here.  We could could probably do
+  // benchmarks of real applications to see if adjusting this number makes a
+  // difference.  Callers that know their exact use case can also explicitly
+  // call createCombined() or createSeparate().)
+  kDefaultCombinedBufSize = 1024
+};
+
+// Helper function for MemBuf::takeOwnership()
+void takeOwnershipError(bool freeOnError, void* buf,
+                        utils::MemBuf::FreeFunction freeFn, void* userData) {
+  if (!freeOnError) {
+    return;
+  }
+  if (!freeFn) {
+    free(buf);
+    return;
+  }
+  try {
+    freeFn(buf, userData);
+  } catch (...) {
+    // The user's free function is not allowed to throw.
+    // (We are already in the middle of throwing an exception, so
+    // we cannot let this exception go unhandled.)
+    abort();
+  }
+}
+
+}  // namespace
+
+namespace utils {
+
+struct MemBuf::HeapPrefix {
+  explicit HeapPrefix(uint16_t flg) : magic(kHeapMagic), flags(flg) {}
+  ~HeapPrefix() {
+    // Reset magic to 0 on destruction.  This is solely for debugging purposes
+    // to help catch bugs where someone tries to use HeapStorage after it has
+    // been deleted.
+    magic = 0;
+  }
+
+  uint16_t magic;
+  std::atomic<uint16_t> flags;
+};
+
+struct MemBuf::HeapStorage {
+  HeapPrefix prefix;
+  // The MemBuf is last in the HeapStorage object.
+  // This way operator new will work even if allocating a subclass of MemBuf
+  // that requires more space.
+  utils::MemBuf buf;
+};
+
+struct MemBuf::HeapFullStorage {
+  // Make sure jemalloc allocates from the 64-byte class.  Putting this here
+  // because HeapStorage is private so it can't be at namespace level.
+  static_assert(sizeof(HeapStorage) <= 64,
+                "MemBuf may not grow over 56 bytes!");
+
+  HeapStorage hs;
+  SharedInfo shared;
+  std::max_align_t align;
+};
+
+MemBuf::SharedInfo::SharedInfo() : freeFn(nullptr), userData(nullptr) {
+  // Use relaxed memory ordering here.  Since we are creating a new SharedInfo,
+  // no other threads should be referring to it yet.
+  refcount.store(1, std::memory_order_relaxed);
+}
+
+MemBuf::SharedInfo::SharedInfo(FreeFunction fn, void* arg)
+    : freeFn(fn), userData(arg) {
+  // Use relaxed memory ordering here.  Since we are creating a new SharedInfo,
+  // no other threads should be referring to it yet.
+  refcount.store(1, std::memory_order_relaxed);
+}
+
+void* MemBuf::operator new(size_t size) {
+  size_t fullSize = offsetof(HeapStorage, buf) + size;
+  auto* storage = static_cast<HeapStorage*>(malloc(fullSize));
+
+  new (&storage->prefix) HeapPrefix(kMemBufInUse);
+  return &(storage->buf);
+}
+
+void* MemBuf::operator new(size_t /* size */, void* ptr) { return ptr; }
+
+void MemBuf::operator delete(void* ptr) {
+  auto* storageAddr = static_cast<uint8_t*>(ptr) - offsetof(HeapStorage, buf);
+  auto* storage = reinterpret_cast<HeapStorage*>(storageAddr);
+  releaseStorage(storage, kMemBufInUse);
+}
+
+void MemBuf::operator delete(void* /* ptr */, void* /* placement */) {
+  // Provide matching operator for `MemBuf::new` to avoid MSVC compilation
+  // warning (C4291) about memory leak when exception is thrown in the
+  // constructor.
+}
+
+void MemBuf::releaseStorage(HeapStorage* storage, uint16_t freeFlags) {
+  // Use relaxed memory order here.  If we are unlucky and happen to get
+  // out-of-date data the compare_exchange_weak() call below will catch
+  // it and load new data with memory_order_acq_rel.
+  auto flags = storage->prefix.flags.load(std::memory_order_acquire);
+
+  while (true) {
+    uint16_t newFlags = uint16_t(flags & ~freeFlags);
+    if (newFlags == 0) {
+      // The storage space is now unused.  Free it.
+      storage->prefix.HeapPrefix::~HeapPrefix();
+      free(storage);
+      return;
+    }
+
+    // This storage segment still contains portions that are in use.
+    // Just clear the flags specified in freeFlags for now.
+    auto ret = storage->prefix.flags.compare_exchange_weak(
+        flags, newFlags, std::memory_order_acq_rel);
+    if (ret) {
+      // We successfully updated the flags.
+      return;
+    }
+
+    // We failed to update the flags.  Some other thread probably updated them
+    // and cleared some of the other bits.  Continue around the loop to see if
+    // we are the last user now, or if we need to try updating the flags again.
+  }
+}
+
+void MemBuf::freeInternalBuf(void* /* buf */, void* userData) {
+  auto* storage = static_cast<HeapStorage*>(userData);
+  releaseStorage(storage, kDataInUse);
+}
+
+MemBuf::MemBuf(CreateOp, std::size_t capacity)
+    : next_(this),
+      prev_(this),
+      data_(nullptr),
+      length_(0),
+      flags_and_shared_info_(0) {
+  SharedInfo* info;
+  allocExtBuffer(capacity, &buf_, &info, &capacity_);
+  setSharedInfo(info);
+  data_ = buf_;
+}
+
+MemBuf::MemBuf(CopyBufferOp /* op */, const void* buf, std::size_t size,
+               std::size_t headroom, std::size_t min_tailroom)
+    : MemBuf(CREATE, headroom + size + min_tailroom) {
+  advance(headroom);
+  if (size > 0) {
+    assert(buf != nullptr);
+    memcpy(writableData(), buf, size);
+    append(size);
+  }
+}
+
+unique_ptr<MemBuf> MemBuf::create(std::size_t capacity) {
+  // For smaller-sized buffers, allocate the MemBuf, SharedInfo, and the buffer
+  // all with a single allocation.
+  //
+  // We don't do this for larger buffers since it can be wasteful if the user
+  // needs to reallocate the buffer but keeps using the same MemBuf object.
+  // In this case we can't free the data space until the MemBuf is also
+  // destroyed.  Callers can explicitly call createCombined() or
+  // createSeparate() if they know their use case better, and know if they are
+  // likely to reallocate the buffer later.
+  if (capacity <= kDefaultCombinedBufSize) {
+    return createCombined(capacity);
+  }
+  return createSeparate(capacity);
+}
+
+unique_ptr<MemBuf> MemBuf::createCombined(std::size_t capacity) {
+  // To save a memory allocation, allocate space for the MemBuf object, the
+  // SharedInfo struct, and the data itself all with a single call to malloc().
+  size_t requiredStorage = offsetof(HeapFullStorage, align) + capacity;
+  size_t mallocSize = requiredStorage;
+  auto* storage = static_cast<HeapFullStorage*>(malloc(mallocSize));
+
+  new (&storage->hs.prefix) HeapPrefix(kMemBufInUse | kDataInUse);
+  new (&storage->shared) SharedInfo(freeInternalBuf, storage);
+
+  uint8_t* bufAddr = reinterpret_cast<uint8_t*>(&storage->align);
+  uint8_t* storageEnd = reinterpret_cast<uint8_t*>(storage) + mallocSize;
+  size_t actualCapacity = size_t(storageEnd - bufAddr);
+  unique_ptr<MemBuf> ret(new (&storage->hs.buf) MemBuf(
+      InternalConstructor(), packFlagsAndSharedInfo(0, &storage->shared),
+      bufAddr, actualCapacity, bufAddr, 0));
+  return ret;
+}
+
+unique_ptr<MemBuf> MemBuf::createSeparate(std::size_t capacity) {
+  return std::make_unique<MemBuf>(CREATE, capacity);
+}
+
+unique_ptr<MemBuf> MemBuf::createChain(size_t totalCapacity,
+                                       std::size_t maxBufCapacity) {
+  unique_ptr<MemBuf> out =
+      create(std::min(totalCapacity, size_t(maxBufCapacity)));
+  size_t allocatedCapacity = out->capacity();
+
+  while (allocatedCapacity < totalCapacity) {
+    unique_ptr<MemBuf> newBuf = create(
+        std::min(totalCapacity - allocatedCapacity, size_t(maxBufCapacity)));
+    allocatedCapacity += newBuf->capacity();
+    out->prependChain(std::move(newBuf));
+  }
+
+  return out;
+}
+
+MemBuf::MemBuf(TakeOwnershipOp, void* buf, std::size_t capacity,
+               std::size_t length, FreeFunction freeFn, void* userData,
+               bool freeOnError)
+    : next_(this),
+      prev_(this),
+      data_(static_cast<uint8_t*>(buf)),
+      buf_(static_cast<uint8_t*>(buf)),
+      length_(length),
+      capacity_(capacity),
+      flags_and_shared_info_(
+          packFlagsAndSharedInfo(flag_free_shared_info, nullptr)) {
+  try {
+    setSharedInfo(new SharedInfo(freeFn, userData));
+  } catch (...) {
+    takeOwnershipError(freeOnError, buf, freeFn, userData);
+    throw;
+  }
+}
+
+unique_ptr<MemBuf> MemBuf::takeOwnership(void* buf, std::size_t capacity,
+                                         std::size_t length,
+                                         FreeFunction freeFn, void* userData,
+                                         bool freeOnError) {
+  try {
+    // TODO: We could allocate the MemBuf object and SharedInfo all in a single
+    // memory allocation.  We could use the existing HeapStorage class, and
+    // define a new kSharedInfoInUse flag.  We could change our code to call
+    // releaseStorage(flag_free_shared_info) when this flag_free_shared_info,
+    // rather than directly calling delete.
+    //
+    // Note that we always pass freeOnError as false to the constructor.
+    // If the constructor throws we'll handle it below.  (We have to handle
+    // allocation failures from std::make_unique too.)
+    return std::make_unique<MemBuf>(TAKE_OWNERSHIP, buf, capacity, length,
+                                    freeFn, userData, false);
+  } catch (...) {
+    takeOwnershipError(freeOnError, buf, freeFn, userData);
+    throw;
+  }
+}
+
+MemBuf::MemBuf(WrapBufferOp, const void* buf, std::size_t capacity) noexcept
+    : MemBuf(InternalConstructor(), 0,
+             // We cast away the const-ness of the buffer here.
+             // This is okay since MemBuf users must use unshare() to create a
+             // copy of this buffer before writing to the buffer.
+             static_cast<uint8_t*>(const_cast<void*>(buf)), capacity,
+             static_cast<uint8_t*>(const_cast<void*>(buf)), capacity) {}
+
+unique_ptr<MemBuf> MemBuf::wrapBuffer(const void* buf, std::size_t capacity) {
+  return std::make_unique<MemBuf>(WRAP_BUFFER, buf, capacity);
+}
+
+MemBuf MemBuf::wrapBufferAsValue(const void* buf,
+                                 std::size_t capacity) noexcept {
+  return MemBuf(WrapBufferOp::WRAP_BUFFER, buf, capacity);
+}
+
+MemBuf::MemBuf() noexcept {}
+
+MemBuf::MemBuf(MemBuf&& other) noexcept
+    : data_(other.data_),
+      buf_(other.buf_),
+      length_(other.length_),
+      capacity_(other.capacity_),
+      flags_and_shared_info_(other.flags_and_shared_info_) {
+  // Reset other so it is a clean state to be destroyed.
+  other.data_ = nullptr;
+  other.buf_ = nullptr;
+  other.length_ = 0;
+  other.capacity_ = 0;
+  other.flags_and_shared_info_ = 0;
+
+  // If other was part of the chain, assume ownership of the rest of its chain.
+  // (It's only valid to perform move assignment on the head of a chain.)
+  if (other.next_ != &other) {
+    next_ = other.next_;
+    next_->prev_ = this;
+    other.next_ = &other;
+
+    prev_ = other.prev_;
+    prev_->next_ = this;
+    other.prev_ = &other;
+  }
+}
+
+MemBuf::MemBuf(const MemBuf& other) { *this = other.cloneAsValue(); }
+
+MemBuf::MemBuf(InternalConstructor, uintptr_t flagsAndSharedInfo, uint8_t* buf,
+               std::size_t capacity, uint8_t* data, std::size_t length) noexcept
+    : next_(this),
+      prev_(this),
+      data_(data),
+      buf_(buf),
+      length_(length),
+      capacity_(capacity),
+      flags_and_shared_info_(flagsAndSharedInfo) {
+  assert(data >= buf);
+  assert(data + length <= buf + capacity);
+}
+
+MemBuf::~MemBuf() {
+  // Destroying an MemBuf destroys the entire chain.
+  // Users of MemBuf should only explicitly delete the head of any chain.
+  // The other elements in the chain will be automatically destroyed.
+  while (next_ != this) {
+    // Since unlink() returns unique_ptr() and we don't store it,
+    // it will automatically delete the unlinked element.
+    (void)next_->unlink();
+  }
+
+  decrementRefcount();
+}
+
+MemBuf& MemBuf::operator=(MemBuf&& other) noexcept {
+  if (this == &other) {
+    return *this;
+  }
+
+  // If we are part of a chain, delete the rest of the chain.
+  while (next_ != this) {
+    // Since unlink() returns unique_ptr() and we don't store it,
+    // it will automatically delete the unlinked element.
+    (void)next_->unlink();
+  }
+
+  // Decrement our refcount on the current buffer
+  decrementRefcount();
+
+  // Take ownership of the other buffer's data
+  data_ = other.data_;
+  buf_ = other.buf_;
+  length_ = other.length_;
+  capacity_ = other.capacity_;
+  flags_and_shared_info_ = other.flags_and_shared_info_;
+  // Reset other so it is a clean state to be destroyed.
+  other.data_ = nullptr;
+  other.buf_ = nullptr;
+  other.length_ = 0;
+  other.capacity_ = 0;
+  other.flags_and_shared_info_ = 0;
+
+  // If other was part of the chain, assume ownership of the rest of its chain.
+  // (It's only valid to perform move assignment on the head of a chain.)
+  if (other.next_ != &other) {
+    next_ = other.next_;
+    next_->prev_ = this;
+    other.next_ = &other;
+
+    prev_ = other.prev_;
+    prev_->next_ = this;
+    other.prev_ = &other;
+  }
+
+  return *this;
+}
+
+MemBuf& MemBuf::operator=(const MemBuf& other) {
+  if (this != &other) {
+    *this = MemBuf(other);
+  }
+  return *this;
+}
+
+bool MemBuf::empty() const {
+  const MemBuf* current = this;
+  do {
+    if (current->length() != 0) {
+      return false;
+    }
+    current = current->next_;
+  } while (current != this);
+  return true;
+}
+
+size_t MemBuf::countChainElements() const {
+  size_t numElements = 1;
+  for (MemBuf* current = next_; current != this; current = current->next_) {
+    ++numElements;
+  }
+  return numElements;
+}
+
+std::size_t MemBuf::computeChainDataLength() const {
+  std::size_t fullLength = length_;
+  for (MemBuf* current = next_; current != this; current = current->next_) {
+    fullLength += current->length_;
+  }
+  return fullLength;
+}
+
+void MemBuf::prependChain(unique_ptr<MemBuf>&& iobuf) {
+  // Take ownership of the specified MemBuf
+  MemBuf* other = iobuf.release();
+
+  // Remember the pointer to the tail of the other chain
+  MemBuf* otherTail = other->prev_;
+
+  // Hook up prev_->next_ to point at the start of the other chain,
+  // and other->prev_ to point at prev_
+  prev_->next_ = other;
+  other->prev_ = prev_;
+
+  // Hook up otherTail->next_ to point at us,
+  // and prev_ to point back at otherTail,
+  otherTail->next_ = this;
+  prev_ = otherTail;
+}
+
+unique_ptr<MemBuf> MemBuf::clone() const {
+  return std::make_unique<MemBuf>(cloneAsValue());
+}
+
+unique_ptr<MemBuf> MemBuf::cloneOne() const {
+  return std::make_unique<MemBuf>(cloneOneAsValue());
+}
+
+unique_ptr<MemBuf> MemBuf::cloneCoalesced() const {
+  return std::make_unique<MemBuf>(cloneCoalescedAsValue());
+}
+
+unique_ptr<MemBuf> MemBuf::cloneCoalescedWithHeadroomTailroom(
+    std::size_t new_headroom, std::size_t new_tailroom) const {
+  return std::make_unique<MemBuf>(
+      cloneCoalescedAsValueWithHeadroomTailroom(new_headroom, new_tailroom));
+}
+
+MemBuf MemBuf::cloneAsValue() const {
+  auto tmp = cloneOneAsValue();
+
+  for (MemBuf* current = next_; current != this; current = current->next_) {
+    tmp.prependChain(current->cloneOne());
+  }
+
+  return tmp;
+}
+
+MemBuf MemBuf::cloneOneAsValue() const {
+  if (SharedInfo* info = sharedInfo()) {
+    setFlags(flag_maybe_shared);
+    info->refcount.fetch_add(1, std::memory_order_acq_rel);
+  }
+  return MemBuf(InternalConstructor(), flags_and_shared_info_, buf_, capacity_,
+                data_, length_);
+}
+
+MemBuf MemBuf::cloneCoalescedAsValue() const {
+  const std::size_t new_headroom = headroom();
+  const std::size_t new_tailroom = prev()->tailroom();
+  return cloneCoalescedAsValueWithHeadroomTailroom(new_headroom, new_tailroom);
+}
+
+MemBuf MemBuf::cloneCoalescedAsValueWithHeadroomTailroom(
+    std::size_t new_headroom, std::size_t new_tailroom) const {
+  if (!isChained()) {
+    return cloneOneAsValue();
+  }
+  // Coalesce into newBuf
+  const std::size_t new_length = computeChainDataLength();
+  const std::size_t new_capacity = new_length + new_headroom + new_tailroom;
+  MemBuf newBuf{CREATE, new_capacity};
+  newBuf.advance(new_headroom);
+
+  auto current = this;
+  do {
+    if (current->length() > 0) {
+      memcpy(newBuf.writableTail(), current->data(), current->length());
+      newBuf.append(current->length());
+    }
+    current = current->next();
+  } while (current != this);
+
+  return newBuf;
+}
+
+void MemBuf::unshareOneSlow() {
+  // Allocate a new buffer for the data
+  uint8_t* buf;
+  SharedInfo* sharedInfo;
+  std::size_t actualCapacity;
+  allocExtBuffer(capacity_, &buf, &sharedInfo, &actualCapacity);
+
+  // Copy the data
+  // Maintain the same amount of headroom.  Since we maintained the same
+  // minimum capacity we also maintain at least the same amount of tailroom.
+  std::size_t headlen = headroom();
+  if (length_ > 0) {
+    assert(data_ != nullptr);
+    memcpy(buf + headlen, data_, length_);
+  }
+
+  // Release our reference on the old buffer
+  decrementRefcount();
+  // Make sure flag_maybe_shared and flag_free_shared_info are all cleared.
+  setFlagsAndSharedInfo(0, sharedInfo);
+
+  // Update the buffer pointers to point to the new buffer
+  data_ = buf + headlen;
+  buf_ = buf;
+}
+
+void MemBuf::unshareChained() {
+  // unshareChained() should only be called if we are part of a chain of
+  // multiple MemBufs.  The caller should have already verified this.
+  assert(isChained());
+
+  MemBuf* current = this;
+  while (true) {
+    if (current->isSharedOne()) {
+      // we have to unshare
+      break;
+    }
+
+    current = current->next_;
+    if (current == this) {
+      // None of the MemBufs in the chain are shared,
+      // so return without doing anything
+      return;
+    }
+  }
+
+  // We have to unshare.  Let coalesceSlow() do the work.
+  coalesceSlow();
+}
+
+void MemBuf::markExternallyShared() {
+  MemBuf* current = this;
+  do {
+    current->markExternallySharedOne();
+    current = current->next_;
+  } while (current != this);
+}
+
+void MemBuf::makeManagedChained() {
+  assert(isChained());
+
+  MemBuf* current = this;
+  while (true) {
+    current->makeManagedOne();
+    current = current->next_;
+    if (current == this) {
+      break;
+    }
+  }
+}
+
+void MemBuf::coalesceSlow() {
+  // coalesceSlow() should only be called if we are part of a chain of multiple
+  // MemBufs.  The caller should have already verified this.
+
+  // Compute the length of the entire chain
+  std::size_t new_length = 0;
+  MemBuf* end = this;
+  do {
+    new_length += end->length_;
+    end = end->next_;
+  } while (end != this);
+
+  coalesceAndReallocate(new_length, end);
+  // We should be only element left in the chain now
+}
+
+void MemBuf::coalesceSlow(size_t max_length) {
+  // coalesceSlow() should only be called if we are part of a chain of multiple
+  // MemBufs.  The caller should have already verified this.
+
+  // Compute the length of the entire chain
+  std::size_t new_length = 0;
+  MemBuf* end = this;
+  while (true) {
+    new_length += end->length_;
+    end = end->next_;
+    if (new_length >= max_length) {
+      break;
+    }
+    if (end == this) {
+      throw std::overflow_error(
+          "attempted to coalesce more data than "
+          "available");
+    }
+  }
+
+  coalesceAndReallocate(new_length, end);
+  // We should have the requested length now
+}
+
+void MemBuf::coalesceAndReallocate(size_t new_headroom, size_t new_length,
+                                   MemBuf* end, size_t new_tailroom) {
+  std::size_t new_capacity = new_length + new_headroom + new_tailroom;
+
+  // Allocate space for the coalesced buffer.
+  // We always convert to an external buffer, even if we happened to be an
+  // internal buffer before.
+  uint8_t* newBuf;
+  SharedInfo* newInfo;
+  std::size_t actualCapacity;
+  allocExtBuffer(new_capacity, &newBuf, &newInfo, &actualCapacity);
+
+  // Copy the data into the new buffer
+  uint8_t* new_data = newBuf + new_headroom;
+  uint8_t* p = new_data;
+  MemBuf* current = this;
+  size_t remaining = new_length;
+  do {
+    if (current->length_ > 0) {
+      assert(current->length_ <= remaining);
+      assert(current->data_ != nullptr);
+      remaining -= current->length_;
+      memcpy(p, current->data_, current->length_);
+      p += current->length_;
+    }
+    current = current->next_;
+  } while (current != end);
+  assert(remaining == 0);
+
+  // Point at the new buffer
+  decrementRefcount();
+
+  // Make sure flag_maybe_shared and flag_free_shared_info are all cleared.
+  setFlagsAndSharedInfo(0, newInfo);
+
+  capacity_ = actualCapacity;
+  buf_ = newBuf;
+  data_ = new_data;
+  length_ = new_length;
+
+  // Separate from the rest of our chain.
+  // Since we don't store the unique_ptr returned by separateChain(),
+  // this will immediately delete the returned subchain.
+  if (isChained()) {
+    (void)separateChain(next_, current->prev_);
+  }
+}
+
+void MemBuf::decrementRefcount() {
+  // Externally owned buffers don't have a SharedInfo object and aren't managed
+  // by the reference count
+  SharedInfo* info = sharedInfo();
+  if (!info) {
+    return;
+  }
+
+  // Decrement the refcount
+  uint32_t newcnt = info->refcount.fetch_sub(1, std::memory_order_acq_rel);
+  // Note that fetch_sub() returns the value before we decremented.
+  // If it is 1, we were the only remaining user; if it is greater there are
+  // still other users.
+  if (newcnt > 1) {
+    return;
+  }
+
+  // We were the last user.  Free the buffer
+  freeExtBuffer();
+
+  // Free the SharedInfo if it was allocated separately.
+  //
+  // This is only used by takeOwnership().
+  //
+  // To avoid this special case handling in decrementRefcount(), we could have
+  // takeOwnership() set a custom freeFn() that calls the user's free function
+  // then frees the SharedInfo object.  (This would require that
+  // takeOwnership() store the user's free function with its allocated
+  // SharedInfo object.)  However, handling this specially with a flag seems
+  // like it shouldn't be problematic.
+  if (flags() & flag_free_shared_info) {
+    delete sharedInfo();
+  }
+}
+
+void MemBuf::reserveSlow(std::size_t min_headroom, std::size_t min_tailroom) {
+  size_t new_capacity = (size_t)length_ + min_headroom + min_tailroom;
+
+  // // reserveSlow() is dangerous if anyone else is sharing the buffer, as we
+  // may
+  // // reallocate and free the original buffer.  It should only ever be called
+  // if
+  // // we are the only user of the buffer.
+
+  // We'll need to reallocate the buffer.
+  // There are a few options.
+  // - If we have enough total room, move the data around in the buffer
+  //   and adjust the data_ pointer.
+  // - If we're using an internal buffer, we'll switch to an external
+  //   buffer with enough headroom and tailroom.
+  // - If we have enough headroom (headroom() >= min_headroom) but not too much
+  //   (so we don't waste memory), we can try:
+  //   - If we don't have too much to copy, we'll use realloc() (note that
+  //   realloc might have to copy
+  //     headroom + data + tailroom)
+  // - Otherwise, bite the bullet and reallocate.
+  if (headroom() + tailroom() >= min_headroom + min_tailroom) {
+    uint8_t* new_data = writableBuffer() + min_headroom;
+    std::memmove(new_data, data_, length_);
+    data_ = new_data;
+    return;
+  }
+
+  size_t new_allocated_capacity = 0;
+  uint8_t* new_buffer = nullptr;
+  std::size_t new_headroom = 0;
+  std::size_t old_headroom = headroom();
+
+  // If we have a buffer allocated with malloc and we just need more tailroom,
+  // try to use realloc()/xallocx() to grow the buffer in place.
+  SharedInfo* info = sharedInfo();
+  if (info && (info->freeFn == nullptr) && length_ != 0 &&
+      old_headroom >= min_headroom) {
+    size_t head_slack = old_headroom - min_headroom;
+    new_allocated_capacity = goodExtBufferSize(new_capacity + head_slack);
+
+    size_t copySlack = capacity() - length_;
+    if (copySlack * 2 <= length_) {
+      void* p = realloc(buf_, new_allocated_capacity);
+      if (TRANSPORT_EXPECT_FALSE(p == nullptr)) {
+        throw std::bad_alloc();
+      }
+      new_buffer = static_cast<uint8_t*>(p);
+      new_headroom = old_headroom;
+    }
+  }
+
+  // None of the previous reallocation strategies worked (or we're using
+  // an internal buffer).  malloc/copy/free.
+  if (new_buffer == nullptr) {
+    new_allocated_capacity = goodExtBufferSize(new_capacity);
+    new_buffer = static_cast<uint8_t*>(malloc(new_allocated_capacity));
+    if (length_ > 0) {
+      assert(data_ != nullptr);
+      memcpy(new_buffer + min_headroom, data_, length_);
+    }
+    if (sharedInfo()) {
+      freeExtBuffer();
+    }
+    new_headroom = min_headroom;
+  }
+
+  std::size_t cap;
+  initExtBuffer(new_buffer, new_allocated_capacity, &info, &cap);
+
+  if (flags() & flag_free_shared_info) {
+    delete sharedInfo();
+  }
+
+  setFlagsAndSharedInfo(0, info);
+  capacity_ = cap;
+  buf_ = new_buffer;
+  data_ = new_buffer + new_headroom;
+  // length_ is unchanged
+}
+
+void MemBuf::freeExtBuffer() {
+  SharedInfo* info = sharedInfo();
+
+  if (info->freeFn) {
+    try {
+      info->freeFn(buf_, info->userData);
+    } catch (...) {
+      // The user's free function should never throw.  Otherwise we might
+      // throw from the MemBuf destructor.  Other code paths like coalesce()
+      // also assume that decrementRefcount() cannot throw.
+      abort();
+    }
+  } else {
+    free(buf_);
+  }
+}
+
+void MemBuf::allocExtBuffer(std::size_t minCapacity, uint8_t** bufReturn,
+                            SharedInfo** infoReturn,
+                            std::size_t* capacityReturn) {
+  size_t mallocSize = goodExtBufferSize(minCapacity);
+  uint8_t* buf = static_cast<uint8_t*>(malloc(mallocSize));
+  initExtBuffer(buf, mallocSize, infoReturn, capacityReturn);
+  *bufReturn = buf;
+}
+
+size_t MemBuf::goodExtBufferSize(std::size_t minCapacity) {
+  // Determine how much space we should allocate.  We'll store the SharedInfo
+  // for the external buffer just after the buffer itself.  (We store it just
+  // after the buffer rather than just before so that the code can still just
+  // use free(buf_) to free the buffer.)
+  size_t minSize = static_cast<size_t>(minCapacity) + sizeof(SharedInfo);
+  // Add room for padding so that the SharedInfo will be aligned on an 8-byte
+  // boundary.
+  minSize = (minSize + 7) & ~7;
+
+  // Use goodMallocSize() to bump up the capacity to a decent size to request
+  // from malloc, so we can use all of the space that malloc will probably give
+  // us anyway.
+  return minSize;
+}
+
+void MemBuf::initExtBuffer(uint8_t* buf, size_t mallocSize,
+                           SharedInfo** infoReturn,
+                           std::size_t* capacityReturn) {
+  // Find the SharedInfo storage at the end of the buffer
+  // and construct the SharedInfo.
+  uint8_t* infoStart = (buf + mallocSize) - sizeof(SharedInfo);
+  SharedInfo* sharedInfo = new (infoStart) SharedInfo;
+
+  *capacityReturn = std::size_t(infoStart - buf);
+  *infoReturn = sharedInfo;
+}
+
+}  // namespace utils
\ No newline at end of file