libhttpserver/src/httpserver/detail/route_cache.hpp at master · etr/libhttpserver · GitHub

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This file is part of libhttpserver

This library is free software; you can redistribute it and/or

modify it under the terms of the GNU Lesser General Public

License as published by the Free Software Foundation; either

version 2.1 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,

but WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU

Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public

License along with this library; if not, write to the Free Software

Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301

USA

// LRU cache fronting the 3-tier route table.

// Plain std::mutex (not shared_mutex) because every cache touch — the

// LRU promotion on a hit included — is a write (std::list::splice).

// Lock-order discipline: route_table_mutex_ is always acquired BEFORE

// the cache's internal mutex when both are held.

// Internal header — only reachable when compiling libhttpserver.

#if !defined(HTTPSERVER_COMPILATION)

#error "route_cache.hpp is internal; only reachable when compiling libhttpserver."

#endif

#ifndef SRC_HTTPSERVER_DETAIL_ROUTE_CACHE_HPP_

#define SRC_HTTPSERVER_DETAIL_ROUTE_CACHE_HPP_

#include <cstddef>

#include <cstdint>

#include <functional>

#include <list>

#include <mutex>

#include <stdexcept>

#include <string>

#include <string_view>

#include <unordered_map>

#include <utility>

#include <vector>

#include "httpserver/http_method.hpp"

#include "httpserver/detail/route_entry.hpp"

namespace httpserver {

namespace detail {

// (method, path) cache key. Hashed by combining the path's string hash

// with the method enum value via the boost-style mix-shift.

struct cache_key {

http_method method = http_method::get;

std::string path;

friend bool operator==(const cache_key& a, const cache_key& b) noexcept {

return a.method == b.method && a.path == b.path;

}

};

struct cache_key_hash {

// Golden-ratio mix constant: reduces hash clustering vs. plain XOR.

static constexpr std::size_t kHashMix = 0x9e3779b97f4a7c15ULL;

// Keep this formula in sync with the inline string_view copy in

// route_cache::find_by_view below -- the two must agree for the

// manual bucket probe to land where insert() put the entry.

std::size_t operator()(const cache_key& k) const noexcept {

std::size_t h1 = std::hash<std::string>{}(k.path);

std::size_t h2 = static_cast<std::size_t>(k.method);

return h1 ^ (h2 + kHashMix + (h1 << 6) + (h1 >> 2));

}

};

// cache_value: the hit payload. Carries a copy of the route_entry (one

// shared_ptr ref-bump for the class arm; std::function copy for the

// lambda arm) and the parameter capture vector so the cache hit can

// replay parameter binding without re-walking the segment trie.

struct cache_value {

route_entry entry;

// Read in src/webserver.cpp at the cache-hit replay site

// (`result.captured_params = std::move(cached.captured_params)`); cppcheck

// analyses each TU in isolation and does not see the cross-TU read.

// cppcheck-suppress unusedStructMember

std::vector<std::pair<std::string, std::string>> captured_params;

};

// route_cache: bounded LRU front-end for the tier chain. Bounded to a

// configurable max-size (default 256 per architecture spec). Insertion

// of a new key evicts the back of the LRU list when the size cap is

// reached. find() promotes the hit to the front via splice().

class route_cache {

public:

explicit route_cache(std::size_t max_entries = 256)

: max_entries_(max_entries) {

if (max_entries == 0) {

throw std::invalid_argument(

"route_cache max_entries must be > 0");

}

// Find by key; returns true on hit and copies the value into `out`.

// Promotes the hit to the front of the LRU list as a side effect.

bool find(const cache_key& key, cache_value& out) {

std::lock_guard<std::mutex> lock(mutex_);

auto it = map_.find(key);

if (it == map_.end()) return false;

list_.splice(list_.begin(), list_, it->second);

out = it->second->second;

return true;

}

// Zero-allocation warm-path variant: looks up without constructing a

// cache_key (avoids copying `path` into a std::string on every call,

// including every warm cache hit). Uses a compatible hash computed

// from (method, string_view) and a heterogeneous equality check.

// On hit, copies the value into `out` and promotes the entry.

// std::hash<std::string_view> produces the same hash as

// std::hash<std::string> for identical character sequences (C++17

// standard guarantee), so the probe always lands on the correct

// bucket.

bool find_by_view(http_method method, std::string_view path,

cache_value& out) {

// Compute the same hash as cache_key_hash without owning `path`

// (mirror of cache_key_hash::operator() -- keep in sync).

std::size_t h1 = std::hash<std::string_view>{}(path);

std::size_t h2 = static_cast<std::size_t>(method);

std::size_t bucket_hash = h1 ^ (h2 + cache_key_hash::kHashMix

+ (h1 << 6) + (h1 >> 2));

std::lock_guard<std::mutex> lock(mutex_);

// Empty-cache early-out. On libc++ a default-constructed

// unordered_map has bucket_count() == 0, and calling begin(0) /

// end(0) on it dereferences a null bucket-list pointer (UB);

// the first request against a fresh server hits exactly this.

if (map_.bucket_count() == 0) {

return false;

}

// Walk the target bucket manually via the bucket-iterator

// overloads begin(b)/end(b) so no full cache_key is constructed.

// ASSUMPTION: bucket index == hash % bucket_count(). The

// standard does not mandate that mapping, but libstdc++ and

// libc++ both use it; on an implementation that maps

// differently this probe would merely report a miss (the miss

// path re-walks the tiers and re-inserts), costing hit rate,

// never correctness.

std::size_t b = bucket_hash % map_.bucket_count();

for (auto it = map_.begin(b), end = map_.end(b); it != end; ++it) {

if (it->first.method == method && it->first.path == path) {

// Promote using the mutable bucket iterator directly —

// avoids a second map_.find() call on every cache hit.

list_.splice(list_.begin(), list_, it->second);

out = it->second->second;

return true;

}

return false;

}

// Insert (or replace) the entry for `key`. Evicts the LRU back if

// the size cap is reached.

void insert(const cache_key& key, cache_value value) {

std::lock_guard<std::mutex> lock(mutex_);

auto it = map_.find(key);

if (it != map_.end()) {

// Replace in place; promote.

it->second->second = std::move(value);

list_.splice(list_.begin(), list_, it->second);

return;

}

list_.emplace_front(key, std::move(value));

map_[key] = list_.begin();

if (map_.size() > max_entries_) {

auto& back = list_.back();

map_.erase(back.first);

list_.pop_back();

}

// Note: not noexcept — std::lock_guard can throw std::system_error

// (though in practice it does not on POSIX under normal conditions).

void clear() {

std::lock_guard<std::mutex> lock(mutex_);

map_.clear();

list_.clear();

}

std::size_t size() const {

std::lock_guard<std::mutex> lock(mutex_);

return map_.size();

}

private:

using list_t = std::list<std::pair<cache_key, cache_value>>;

mutable std::mutex mutex_;

std::size_t max_entries_;

list_t list_;

std::unordered_map<cache_key, typename list_t::iterator,

cache_key_hash> map_;

};

} // namespace detail

} // namespace httpserver

#endif // SRC_HTTPSERVER_DETAIL_ROUTE_CACHE_HPP_

libhttpserver/src/httpserver/detail/route_cache.hpp at master · etr/libhttpserver · GitHub

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