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/* Compatibility read-write lock
(C) 2016-2017 Niall Douglas <http://www.nedproductions.biz/> (11 commits)
File Created: April 2016
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 in the accompanying file
Licence.txt or 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.
Distributed under the Boost Software License, Version 1.0.
(See accompanying file Licence.txt or copy at
http://www.boost.org/LICENSE_1_0.txt)
*/
#ifndef LLFIO_SHARED_FS_MUTEX_LOCK_FILES_HPP
#define LLFIO_SHARED_FS_MUTEX_LOCK_FILES_HPP
#include "../../file_handle.hpp"
#include "base.hpp"
#include "quickcpplib/algorithm/small_prng.hpp"
//! \file lock_files.hpp Provides algorithm::shared_fs_mutex::lock_files
LLFIO_V2_NAMESPACE_BEGIN
namespace algorithm
{
namespace shared_fs_mutex
{
/*! \class lock_files
\brief Many entity exclusive compatibility file system based lock
This is a very simple many entity shared mutex likely to work almost anywhere without surprises.
It works by trying to exclusively create a file called the hex of the entity id. If it fails to
exclusively create any file, it deletes all previously created files, randomises the order
and tries locking them again until success. The only real reason to use this implementation
is its excellent compatibility with almost everything, most users will want byte_ranges instead.
- Compatible with all networked file systems.
- Linear complexity to number of concurrent users.
- Exponential complexity to number of contended entities being concurrently locked.
- Requests for shared locks are treated as if for exclusive locks.
Caveats:
- No ability to sleep until a lock becomes free, so CPUs are spun at 100%.
- On POSIX only sudden process exit with locks held will deadlock all other users by leaving stale
files around.
- Costs a file descriptor per entity locked.
- Sudden power loss during use will deadlock first user after reboot, again due to stale files.
- Currently this implementation does not permit more than one lock() per instance as the lock
information is stored as member data. Creating multiple instances referring to the same path
works fine. This could be fixed easily, but it would require a memory allocation per lock and
user demand that this is actually a problem in practice.
- Leaves many 16 character long hexadecimal named files in the supplied directory which may
confuse users. Tip: create a hidden lockfile directory.
Fixing the stale lock file problem could be quite trivial - simply byte range lock the first byte
in the lock file to detect when a lock file is stale. However in this situation using the
byte_ranges algorithm would be far superior, so implementing stale lock file clean up is left up
to the user.
*/
class lock_files : public shared_fs_mutex
{
const path_handle &_path;
std::vector<file_handle> _hs;
explicit lock_files(const path_handle &o)
: _path(o)
{
}
public:
//! The type of an entity id
using entity_type = shared_fs_mutex::entity_type;
//! The type of a sequence of entities
using entities_type = shared_fs_mutex::entities_type;
//! No copy construction
lock_files(const lock_files &) = delete;
//! No copy assignment
lock_files &operator=(const lock_files &) = delete;
~lock_files() = default;
//! Move constructor
lock_files(lock_files &&o) noexcept : _path(o._path), _hs(std::move(o._hs)) {}
//! Move assign
lock_files &operator=(lock_files &&o) noexcept
{
this->~lock_files();
new(this) lock_files(std::move(o));
return *this;
}
//! Initialises a shared filing system mutex using the directory at \em lockdir which MUST stay valid for the duration of this lock.
LLFIO_MAKE_FREE_FUNCTION
static result<lock_files> fs_mutex_lock_files(const path_handle &lockdir) noexcept
{
LLFIO_LOG_FUNCTION_CALL(0);
return lock_files(lockdir);
}
//! Return the path to the directory being used for this lock
const path_handle &path() const noexcept { return _path; }
protected:
LLFIO_HEADERS_ONLY_VIRTUAL_SPEC result<void> _lock(entities_guard &out, deadline d, bool spin_not_sleep) noexcept final
{
LLFIO_LOG_FUNCTION_CALL(this);
std::chrono::steady_clock::time_point began_steady;
std::chrono::system_clock::time_point end_utc;
if(d)
{
if((d).steady)
{
began_steady = std::chrono::steady_clock::now();
}
else
{
end_utc = (d).to_time_point();
}
}
size_t n;
// Create a set of paths to files to exclusively create
std::vector<std::string> entity_paths(out.entities.size());
for(n = 0; n < out.entities.size(); n++)
{
auto v = out.entities[n].value;
entity_paths[n] = QUICKCPPLIB_NAMESPACE::algorithm::string::to_hex_string(span<char>(reinterpret_cast<char *>(&v), 8));
}
_hs.resize(out.entities.size());
do
{
auto was_contended = static_cast<size_t>(-1);
{
auto undo = undoer([&] {
// 0 to (n-1) need to be closed
if(n > 0)
{
--n;
// Now 0 to n needs to be closed
for(; n > 0; n--)
{
(void) _hs[n].close(); // delete on close semantics deletes the file
}
(void) _hs[0].close();
}
});
for(n = 0; n < out.entities.size(); n++)
{
auto ret = file_handle::file(_path, entity_paths[n], file_handle::mode::write, file_handle::creation::only_if_not_exist, file_handle::caching::temporary, file_handle::flag::unlink_on_first_close);
if(ret.has_error())
{
const auto &ec = ret.error();
if(ec != errc::resource_unavailable_try_again && ec != errc::file_exists)
{
return std::move(ret).error();
}
// Collided with another locker
was_contended = n;
break;
}
_hs[n] = std::move(ret.value());
}
if(n == out.entities.size())
{
undo.dismiss();
}
}
if(n != out.entities.size())
{
if(d)
{
if((d).steady)
{
if(std::chrono::steady_clock::now() >= (began_steady + std::chrono::nanoseconds((d).nsecs)))
{
return errc::timed_out;
}
}
else
{
if(std::chrono::system_clock::now() >= end_utc)
{
return errc::timed_out;
}
}
}
// Move was_contended to front and randomise rest of out.entities
std::swap(out.entities[was_contended], out.entities[0]);
auto front = out.entities.begin();
++front;
QUICKCPPLIB_NAMESPACE::algorithm::small_prng::random_shuffle(front, out.entities.end());
// Sleep for a very short time
if(!spin_not_sleep)
{
std::this_thread::yield();
}
}
} while(n < out.entities.size());
return success();
}
public:
LLFIO_HEADERS_ONLY_VIRTUAL_SPEC void unlock(entities_type /*entities*/, unsigned long long /*hint*/) noexcept final
{
LLFIO_LOG_FUNCTION_CALL(this);
for(auto &i : _hs)
{
(void) i.close(); // delete on close semantics deletes the file
}
}
};
} // namespace shared_fs_mutex
} // namespace algorithm
LLFIO_V2_NAMESPACE_END
#endif
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