ceph/src/seastar/include/seastar/core/temporary_buffer.hh

   1 /*
   2  * This file is open source software, licensed to you under the terms
   3  * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
   4  * distributed with this work for additional information regarding copyright
   5  * ownership.  You may not use this file except in compliance with the License.
   6  *
   7  * You may obtain a copy of the License at
   8  *
   9  *   http://www.apache.org/licenses/LICENSE-2.0
  10  *
  11  * Unless required by applicable law or agreed to in writing,
  12  * software distributed under the License is distributed on an
  13  * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  14  * KIND, either express or implied.  See the License for the
  15  * specific language governing permissions and limitations
  16  * under the License.
  17  */
  18 /*
  19  * Copyright (C) 2014 Cloudius Systems, Ltd.
  20  */
  21
  22 #pragma once
  23
  24 #include <seastar/core/deleter.hh>
  25 #include <seastar/util/eclipse.hh>
  26 #include <malloc.h>
  27 #include <algorithm>
  28
  29 namespace seastar {
  30
  31 /// \addtogroup memory-module
  32 /// @{
  33
  34 /// Temporary, self-managed byte buffer.
  35 ///
  36 /// A \c temporary_buffer is similar to an \c std::string or a \c std::unique_ptr<char[]>,
  37 /// but provides more flexible memory management.  A \c temporary_buffer can own the memory
  38 /// it points to, or it can be shared with another \c temporary_buffer, or point at a substring
  39 /// of a buffer.  It uses a \ref deleter to manage its memory.
  40 ///
  41 /// A \c temporary_buffer should not be held indefinitely.  It can be held while a request
  42 /// is processed, or for a similar duration, but not longer, as it can tie up more memory
  43 /// that its size indicates.
  44 ///
  45 /// A buffer can be shared: two \c temporary_buffer objects will point to the same data,
  46 /// or a subset of it.  See the \ref temporary_buffer::share() method.
  47 ///
  48 /// Unless you created a \c temporary_buffer yourself, do not modify its contents, as they
  49 /// may be shared with another user that does not expect the data to change.
  50 ///
  51 /// Use cases for a \c temporary_buffer include:
  52 ///  - passing a substring of a tcp packet for the user to consume (zero-copy
  53 ///    tcp input)
  54 ///  - passing a refcounted blob held in memory to tcp, ensuring that when the TCP ACK
  55 ///    is received, the blob is released (by decrementing its reference count) (zero-copy
  56 ///    tcp output)
  57 ///
  58 /// \tparam CharType underlying character type (must be a variant of \c char).
  59 template <typename CharType>
  60 class temporary_buffer {
  61     static_assert(sizeof(CharType) == 1, "must buffer stream of bytes");
  62     CharType* _buffer;
  63     size_t _size;
  64     deleter _deleter;
  65 public:
  66     /// Creates a \c temporary_buffer of a specified size.  The buffer is not shared
  67     /// with anyone, and is not initialized.
  68     ///
  69     /// \param size buffer size, in bytes
  70     explicit temporary_buffer(size_t size)
  71         : _buffer(static_cast<CharType*>(malloc(size * sizeof(CharType)))), _size(size)
  72         , _deleter(make_free_deleter(_buffer)) {
  73         if (size && !_buffer) {
  74             throw std::bad_alloc();
  75         }
  76     }
  77     //explicit temporary_buffer(CharType* borrow, size_t size) : _buffer(borrow), _size(size) {}
  78     /// Creates an empty \c temporary_buffer that does not point at anything.
  79     temporary_buffer()
  80         : _buffer(nullptr)
  81         , _size(0) {}
  82     temporary_buffer(const temporary_buffer&) = delete;
  83     /// Moves a \c temporary_buffer.
  84     temporary_buffer(temporary_buffer&& x) noexcept : _buffer(x._buffer), _size(x._size), _deleter(std::move(x._deleter)) {
  85         x._buffer = nullptr;
  86         x._size = 0;
  87     }
  88     /// Creates a \c temporary_buffer with a specific deleter.
  89     ///
  90     /// \param buf beginning of the buffer held by this \c temporary_buffer
  91     /// \param size size of the buffer
  92     /// \param d deleter controlling destruction of the  buffer.  The deleter
  93     ///          will be destroyed when there are no longer any users for the buffer.
  94     temporary_buffer(CharType* buf, size_t size, deleter d)
  95         : _buffer(buf), _size(size), _deleter(std::move(d)) {}
  96     /// Creates a `temporary_buffer` containing a copy of the provided data
  97     ///
  98     /// \param src  data buffer to be copied
  99     /// \param size size of data buffer in `src`
 100     temporary_buffer(const CharType* src, size_t size) : temporary_buffer(size) {
 101         std::copy_n(src, size, _buffer);
 102     }
 103     void operator=(const temporary_buffer&) = delete;
 104     /// Moves a \c temporary_buffer.
 105     temporary_buffer& operator=(temporary_buffer&& x) noexcept {
 106         if (this != &x) {
 107             _buffer = x._buffer;
 108             _size = x._size;
 109             _deleter = std::move(x._deleter);
 110             x._buffer = nullptr;
 111             x._size = 0;
 112         }
 113         return *this;
 114     }
 115     /// Gets a pointer to the beginning of the buffer.
 116     const CharType* get() const { return _buffer; }
 117     /// Gets a writable pointer to the beginning of the buffer.  Use only
 118     /// when you are certain no user expects the buffer data not to change.
 119     CharType* get_write() { return _buffer; }
 120     /// Gets the buffer size.
 121     size_t size() const { return _size; }
 122     /// Gets a pointer to the beginning of the buffer.
 123     const CharType* begin() const { return _buffer; }
 124     /// Gets a pointer to the end of the buffer.
 125     const CharType* end() const { return _buffer + _size; }
 126     /// Returns the buffer, but with a reduced size.  The original
 127     /// buffer is consumed by this call and can no longer be used.
 128     ///
 129     /// \param size New size; must be smaller than current size.
 130     /// \return the same buffer, with a prefix removed.
 131     temporary_buffer prefix(size_t size) && {
 132         auto ret = std::move(*this);
 133         ret._size = size;
 134         return ret;
 135     }
 136     /// Reads a character from a specific position in the buffer.
 137     ///
 138     /// \param pos position to read character from; must be less than size.
 139     CharType operator[](size_t pos) const {
 140         return _buffer[pos];
 141     }
 142     /// Checks whether the buffer is empty.
 143     bool empty() const { return !size(); }
 144     /// Checks whether the buffer is not empty.
 145     explicit operator bool() const { return size(); }
 146     /// Create a new \c temporary_buffer object referring to the same
 147     /// underlying data.  The underlying \ref deleter will not be destroyed
 148     /// until both the original and the clone have been destroyed.
 149     ///
 150     /// \return a clone of the buffer object.
 151     temporary_buffer share() {
 152         return temporary_buffer(_buffer, _size, _deleter.share());
 153     }
 154     /// Create a new \c temporary_buffer object referring to a substring of the
 155     /// same underlying data.  The underlying \ref deleter will not be destroyed
 156     /// until both the original and the clone have been destroyed.
 157     ///
 158     /// \param pos Position of the first character to share.
 159     /// \param len Length of substring to share.
 160     /// \return a clone of the buffer object, referring to a substring.
 161     temporary_buffer share(size_t pos, size_t len) {
 162         auto ret = share();
 163         ret._buffer += pos;
 164         ret._size = len;
 165         return ret;
 166     }
 167     /// Clone the current \c temporary_buffer object into a new one.
 168     /// This creates a temporary buffer with the same length and data but not
 169     /// pointing to the memory of the original object.
 170     temporary_buffer clone() const {
 171         return {_buffer, _size};
 172     }
 173     /// Remove a prefix from the buffer.  The underlying data
 174     /// is not modified.
 175     ///
 176     /// \param pos Position of first character to retain.
 177     void trim_front(size_t pos) {
 178         _buffer += pos;
 179         _size -= pos;
 180     }
 181     /// Remove a suffix from the buffer.  The underlying data
 182     /// is not modified.
 183     ///
 184     /// \param pos Position of first character to drop.
 185     void trim(size_t pos) {
 186         _size = pos;
 187     }
 188     /// Stops automatic memory management.  When the \c temporary_buffer
 189     /// object is destroyed, the underlying \ref deleter will not be called.
 190     /// Instead, it is the caller's responsibility to destroy the deleter object
 191     /// when the data is no longer needed.
 192     ///
 193     /// \return \ref deleter object managing the data's lifetime.
 194     deleter release() {
 195         return std::move(_deleter);
 196     }
 197     /// Creates a \c temporary_buffer object with a specified size, with
 198     /// memory aligned to a specific boundary.
 199     ///
 200     /// \param alignment Required alignment; must be a power of two.
 201     /// \param size Required size.
 202     /// \return a new \c temporary_buffer object.
 203     static temporary_buffer aligned(size_t alignment, size_t size) {
 204         void *ptr = nullptr;
 205         auto ret = ::posix_memalign(&ptr, alignment, size * sizeof(CharType));
 206         auto buf = static_cast<CharType*>(ptr);
 207         if (ret) {
 208             throw std::bad_alloc();
 209         }
 210         return temporary_buffer(buf, size, make_free_deleter(buf));
 211     }
 212
 213     /// Compare contents of this buffer with another buffer for equality
 214     ///
 215     /// \param o buffer to compare with
 216     /// \return true if and only if contents are the same
 217     bool operator==(const temporary_buffer<char>& o) const {
 218         return size() == o.size() && std::equal(begin(), end(), o.begin());
 219     }
 220
 221     /// Compare contents of this buffer with another buffer for inequality
 222     ///
 223     /// \param o buffer to compare with
 224     /// \return true if and only if contents are not the same
 225     bool operator!=(const temporary_buffer<char>& o) const {
 226         return !(*this == o);
 227     }
 228 };
 229
 230 /// @}
 231
 232 }