implement connection limiter to replace manual limiting code

include/TConnectionLimiter.h

@@ -0,0 +1,62 @@
+#pragma once
+
+#include <mutex>
+#include <optional>
+#include <string>
+#include <unordered_map>
+#include <utility>
+
+class TConnectionLimiter {
+public:
+    class TGuard {
+    public:
+        TGuard() = default;
+        TGuard(TConnectionLimiter* owner, std::string ip);
+
+        TGuard(const TGuard&) = delete;
+        TGuard& operator=(const TGuard&) = delete;
+
+        ~TGuard() { Release(); }
+
+        // not threadsafe
+        TGuard(TGuard&& other) noexcept { *this = std::move(other); }
+        // not threadsafe
+        TGuard& operator=(TGuard&& other) noexcept;
+
+    private:
+        friend class TConnectionLimiter;
+
+        // not threadsafe
+        void Release();
+
+        TConnectionLimiter* mOwner { nullptr };
+        std::string mIp;
+    };
+
+    TConnectionLimiter(size_t maxPerIp, size_t maxGlobal);
+
+    [[nodiscard]] std::optional<TGuard> TryAcquire(const std::string& ip);
+
+private:
+    void Release(const std::string& ip) {
+        std::unique_lock Lock { mMutex };
+        auto It = mPerIp.find(ip);
+        if (It != mPerIp.end()) {
+            // this guard exists to avoid underflow in case something goes wrong and this gets called too many times
+            if (It->second > 0)
+                --It->second;
+            if (It->second == 0)
+                mPerIp.erase(It);
+        }
+        // this guard exists to avoid underflow in case something goes wrong and this gets called too many times
+        if (mGlobal > 0)
+            --mGlobal;
+    }
+
+    const size_t mMaxPerIp;
+    const size_t mMaxGlobal;
+
+    std::mutex mMutex { };
+    std::unordered_map<std::string, size_t> mPerIp { };
+    size_t mGlobal = 0;
+};