146 lines
4.3 KiB
C++
146 lines
4.3 KiB
C++
/*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*
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* Copyright 2015 Danny Robson <danny@nerdcruft.net>
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*/
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#include "./circular.hpp"
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#include "../system.hpp"
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#include "../../except.hpp"
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#include "../../raii.hpp"
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#include "../../maths.hpp"
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#include "../../random.hpp"
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#include <cstring>
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#include <cstdio>
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#include <unistd.h>
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#include <sys/mman.h>
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#include <fcntl.h>
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#include <algorithm>
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using util::memory::buffer::circular;
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///////////////////////////////////////////////////////////////////////////////
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// generate a random string that could be used as a path leaf
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//
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// it looks a lot like a shitty tmpnam replacement because it is. we can't use
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// tmpnam without security warnings being emitted by binutils linker, despite
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// using it safely in this particular scenario.
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static void
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tmpname (std::string &str, size_t length)
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{
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static const char alphanum[] =
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"abcdefghijklmnopqrstuvwxyz"
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"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
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"0123456789";
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str.resize (length);
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std::generate_n (str.begin (),
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length,
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[&] (void) { return util::choose (alphanum); });
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}
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///////////////////////////////////////////////////////////////////////////////
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circular::circular (size_t bytes)
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{
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bytes = max (bytes, sizeof (value_type));
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bytes = align (bytes, pagesize ());
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int fd = -1;
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constexpr size_t RETRIES = 128;
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constexpr size_t NAME_LENGTH = 16;
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std::string name (NAME_LENGTH, '\0');
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// keep generating paths and attempting to create the shm backing. we may
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// fall through this loop upon failure, so be sure to check the validity
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// of the fd at the end.
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for (size_t i = 0; fd < 0 && i < RETRIES; ++i) {
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tmpname (name, NAME_LENGTH);
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name[0] = '/';
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fd = shm_open (name.c_str (), O_EXCL | O_CREAT | O_TRUNC | O_RDWR, 0600);
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}
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if (fd < 0)
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throw std::runtime_error ("unable to generate shm name");
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// setup a desctructor for the shm data. mmap retains a reference, so do
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// this whether we succeed or fail in the next phase.
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util::scoped_function raii ([&name] (void) { shm_unlink (name.c_str ()); });
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// embiggen to the desired size
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ftruncate (fd, bytes);
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// pre-allocate a sufficiently large virtual memory block. it doesn't
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// matter much what flags we use because we'll just be overwriting it
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// shortly.
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m_begin = reinterpret_cast<char*> (mmap (nullptr, bytes * 2, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0));
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if (MAP_FAILED == m_begin)
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errno_error::throw_code ();
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// preemptively setup an unmapping object in case the remapping fails
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util::scoped_function unmapper ([this, bytes] (void) { munmap (m_begin, bytes); });
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// overwrite the map with two adjacent copies of the memory object. this
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// must be a shared mapping for the values to propogate across segments.
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auto prot = PROT_READ | PROT_WRITE;
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auto flag = MAP_FIXED | MAP_SHARED;
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m_begin = reinterpret_cast<char*> (mmap (m_begin, bytes, prot, flag, fd, 0));
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m_end = reinterpret_cast<char*> (mmap (m_begin + bytes, bytes, prot, flag, fd, 0));
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if (m_begin == MAP_FAILED || m_end == MAP_FAILED)
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errno_error::throw_code ();
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// all went well, disarm the failsafe
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unmapper.clear ();
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}
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//-----------------------------------------------------------------------------
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circular::~circular ()
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{
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auto res = munmap (m_begin, 2 * (m_end - m_begin));
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(void)res;
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CHECK_ZERO (res);
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}
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///////////////////////////////////////////////////////////////////////////////
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char*
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circular::begin (void)
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{
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return m_begin;
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}
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//-----------------------------------------------------------------------------
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char*
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circular::end (void)
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{
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return m_end;
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}
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///////////////////////////////////////////////////////////////////////////////
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size_t
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circular::size (void) const
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{
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return m_end - m_begin;
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}
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