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 #include <cstddef>
  29
  30 namespace seastar {
  31
  32 /// \addtogroup memory-module
  33 /// @{
  34
  35 /// Temporary, self-managed byte buffer.
  36 ///
  37 /// A \c temporary_buffer is similar to an \c std::string or a \c std::unique_ptr<char[]>,
  38 /// but provides more flexible memory management.  A \c temporary_buffer can own the memory
  39 /// it points to, or it can be shared with another \c temporary_buffer, or point at a substring
  40 /// of a buffer.  It uses a \ref deleter to manage its memory.
  41 ///
  42 /// A \c temporary_buffer should not be held indefinitely.  It can be held while a request
  43 /// is processed, or for a similar duration, but not longer, as it can tie up more memory
  44 /// that its size indicates.
  45 ///
  46 /// A buffer can be shared: two \c temporary_buffer objects will point to the same data,
  47 /// or a subset of it.  See the \ref temporary_buffer::share() method.
  48 ///
  49 /// Unless you created a \c temporary_buffer yourself, do not modify its contents, as they
  50 /// may be shared with another user that does not expect the data to change.
  51 ///
  52 /// Use cases for a \c temporary_buffer include:
  53 ///  - passing a substring of a tcp packet for the user to consume (zero-copy
  54 ///    tcp input)
  55 ///  - passing a refcounted blob held in memory to tcp, ensuring that when the TCP ACK
  56 ///    is received, the blob is released (by decrementing its reference count) (zero-copy
  57 ///    tcp output)
  58 ///
  59 /// \tparam CharType underlying character type (must be a variant of \c char).
  60 template <typename CharType>
  61 class temporary_buffer {
  62     static_assert(sizeof(CharType) == 1, "must buffer stream of bytes");
  63     CharType* _buffer;
  64     size_t _size;
  65     deleter _deleter;
  66 public:
  67     /// Creates a \c temporary_buffer of a specified size.  The buffer is not shared
  68     /// with anyone, and is not initialized.
  69     ///
  70     /// \param size buffer size, in bytes
  71     explicit temporary_buffer(size_t size)
  72         : _buffer(static_cast<CharType*>(malloc(size * sizeof(CharType)))), _size(size)
  73         , _deleter(make_free_deleter(_buffer)) {
  74         if (size && !_buffer) {
  75             throw std::bad_alloc();
  76         }
  77     }
  78     //explicit temporary_buffer(CharType* borrow, size_t size) : _buffer(borrow), _size(size) {}
  79     /// Creates an empty \c temporary_buffer that does not point at anything.
  80     temporary_buffer()
  81         : _buffer(nullptr)
  82         , _size(0) {}
  83     temporary_buffer(const temporary_buffer&) = delete;
  84
  85     /// Moves a \c temporary_buffer.
  86     temporary_buffer(temporary_buffer&& x) noexcept : _buffer(x._buffer), _size(x._size), _deleter(std::move(x._deleter)) {
  87         x._buffer = nullptr;
  88         x._size = 0;
  89     }
  90
  91     /// Creates a \c temporary_buffer with a specific deleter.
  92     ///
  93     /// \param buf beginning of the buffer held by this \c temporary_buffer
  94     /// \param size size of the buffer
  95     /// \param d deleter controlling destruction of the  buffer.  The deleter
  96     ///          will be destroyed when there are no longer any users for the buffer.
  97     temporary_buffer(CharType* buf, size_t size, deleter d)
  98         : _buffer(buf), _size(size), _deleter(std::move(d)) {}
  99     /// Creates a `temporary_buffer` containing a copy of the provided data
 100     ///
 101     /// \param src  data buffer to be copied
 102     /// \param size size of data buffer in `src`
 103     temporary_buffer(const CharType* src, size_t size) : temporary_buffer(size) {
 104         std::copy_n(src, size, _buffer);
 105     }
 106     void operator=(const temporary_buffer&) = delete;
 107     /// Moves a \c temporary_buffer.
 108     temporary_buffer& operator=(temporary_buffer&& x) noexcept {
 109         if (this != &x) {
 110             _buffer = x._buffer;
 111             _size = x._size;
 112             _deleter = std::move(x._deleter);
 113             x._buffer = nullptr;
 114             x._size = 0;
 115         }
 116         return *this;
 117     }
 118     /// Gets a pointer to the beginning of the buffer.
 119     const CharType* get() const { return _buffer; }
 120     /// Gets a writable pointer to the beginning of the buffer.  Use only
 121     /// when you are certain no user expects the buffer data not to change.
 122     CharType* get_write() { return _buffer; }
 123     /// Gets the buffer size.
 124     size_t size() const { return _size; }
 125     /// Gets a pointer to the beginning of the buffer.
 126     const CharType* begin() const { return _buffer; }
 127     /// Gets a pointer to the end of the buffer.
 128     const CharType* end() const { return _buffer + _size; }
 129     /// Returns the buffer, but with a reduced size.  The original
 130     /// buffer is consumed by this call and can no longer be used.
 131     ///
 132     /// \param size New size; must be smaller than current size.
 133     /// \return the same buffer, with a prefix removed.
 134     temporary_buffer prefix(size_t size) && {
 135         auto ret = std::move(*this);
 136         ret._size = size;
 137         return ret;
 138     }
 139     /// Reads a character from a specific position in the buffer.
 140     ///
 141     /// \param pos position to read character from; must be less than size.
 142     CharType operator[](size_t pos) const {
 143         return _buffer[pos];
 144     }
 145     /// Checks whether the buffer is empty.
 146     bool empty() const { return !size(); }
 147     /// Checks whether the buffer is not empty.
 148     explicit operator bool() const { return size(); }
 149     /// Create a new \c temporary_buffer object referring to the same
 150     /// underlying data.  The underlying \ref deleter will not be destroyed
 151     /// until both the original and the clone have been destroyed.
 152     ///
 153     /// \return a clone of the buffer object.
 154     temporary_buffer share() {
 155         return temporary_buffer(_buffer, _size, _deleter.share());
 156     }
 157     /// Create a new \c temporary_buffer object referring to a substring of the
 158     /// same underlying data.  The underlying \ref deleter will not be destroyed
 159     /// until both the original and the clone have been destroyed.
 160     ///
 161     /// \param pos Position of the first character to share.
 162     /// \param len Length of substring to share.
 163     /// \return a clone of the buffer object, referring to a substring.
 164     temporary_buffer share(size_t pos, size_t len) {
 165         auto ret = share();
 166         ret._buffer += pos;
 167         ret._size = len;
 168         return ret;
 169     }
 170     /// Clone the current \c temporary_buffer object into a new one.
 171     /// This creates a temporary buffer with the same length and data but not
 172     /// pointing to the memory of the original object.
 173     temporary_buffer clone() const {
 174         return {_buffer, _size};
 175     }
 176     /// Remove a prefix from the buffer.  The underlying data
 177     /// is not modified.
 178     ///
 179     /// \param pos Position of first character to retain.
 180     void trim_front(size_t pos) {
 181         _buffer += pos;
 182         _size -= pos;
 183     }
 184     /// Remove a suffix from the buffer.  The underlying data
 185     /// is not modified.
 186     ///
 187     /// \param pos Position of first character to drop.
 188     void trim(size_t pos) {
 189         _size = pos;
 190     }
 191     /// Stops automatic memory management.  When the \c temporary_buffer
 192     /// object is destroyed, the underlying \ref deleter will not be called.
 193     /// Instead, it is the caller's responsibility to destroy the deleter object
 194     /// when the data is no longer needed.
 195     ///
 196     /// \return \ref deleter object managing the data's lifetime.
 197     deleter release() {
 198         return std::move(_deleter);
 199     }
 200     /// Creates a \c temporary_buffer object with a specified size, with
 201     /// memory aligned to a specific boundary.
 202     ///
 203     /// \param alignment Required alignment; must be a power of two and a multiple of sizeof(void *).
 204     /// \param size Required size; must be a multiple of alignment.
 205     /// \return a new \c temporary_buffer object.
 206     static temporary_buffer aligned(size_t alignment, size_t size) {
 207         void *ptr = nullptr;
 208         auto ret = ::posix_memalign(&ptr, alignment, size * sizeof(CharType));
 209         auto buf = static_cast<CharType*>(ptr);
 210         if (ret) {
 211             throw std::bad_alloc();
 212         }
 213         return temporary_buffer(buf, size, make_free_deleter(buf));
 214     }
 215
 216     /// Compare contents of this buffer with another buffer for equality
 217     ///
 218     /// \param o buffer to compare with
 219     /// \return true if and only if contents are the same
 220     bool operator==(const temporary_buffer<char>& o) const {
 221         return size() == o.size() && std::equal(begin(), end(), o.begin());
 222     }
 223
 224     /// Compare contents of this buffer with another buffer for inequality
 225     ///
 226     /// \param o buffer to compare with
 227     /// \return true if and only if contents are not the same
 228     bool operator!=(const temporary_buffer<char>& o) const {
 229         return !(*this == o);
 230     }
 231 };
 232
 233 /// @}
 234
 235 }