implement connection limiter to replace manual limiting code

src/TConnectionLimiter.cpp

@@ -0,0 +1,61 @@
+#include "TConnectionLimiter.h"
+#include <mutex>
+#include <optional>
+#include "Common.h"
+
+TConnectionLimiter::TConnectionLimiter(size_t maxPerIp, size_t maxGlobal)
+    : mMaxPerIp(maxPerIp)
+    , mMaxGlobal(maxGlobal) {
+    if (maxPerIp == 0) {
+        beammp_errorf("Max connections count per IP is set to zero; the server would reject ALL connections");
+        throw std::runtime_error("Invalid maximum connections per IP setting");
+    }
+    if (maxGlobal == 0) {
+        beammp_errorf("Max connection count is set to zero; the server would reject ALL connections");
+        throw std::runtime_error("Invalid maximum connections setting");
+    }
+}
+
+std::optional<TConnectionLimiter::TGuard> TConnectionLimiter::TryAcquire(const std::string& ip) {
+    std::unique_lock Lock { mMutex };
+    if (mGlobal >= mMaxGlobal) return std::nullopt;
+    // `It` is the inserted element (so if insertion worked, its 0), or its the element that
+    // was already there, in which case we must check the ip connection limit
+    auto [It, _] = mPerIp.try_emplace(ip, 0);
+    if (It->second >= mMaxPerIp) {
+        return std::nullopt;
+    }
+    // now increment the counter finally
+    ++It->second;
+    ++mGlobal;
+    // RAII guard will drop the count once destructed
+    return TGuard(this, ip);
+}
+
+TConnectionLimiter::TGuard::TGuard(TConnectionLimiter* owner, std::string ip)
+    : mOwner(owner)
+    , mIp(std::move(ip)) {
+    beammp_debugf("Acquired connection guard for {}", ip);
+}
+
+TConnectionLimiter::TGuard& TConnectionLimiter::TGuard::operator=(TGuard&& other) noexcept {
+    // identity check
+    if (this != &other) {
+        Release();
+        mOwner = other.mOwner;
+        mIp = std::move(other.mIp);
+        other.mOwner = nullptr;
+    }
+    return *this;
+}
+
+void TConnectionLimiter::TGuard::Release() {
+    beammp_debugf("Trying to release connection guard for {} ...", mIp);
+    if (mOwner) {
+        mOwner->Release(mIp);
+        mOwner = nullptr;
+        beammp_debugf("... Released connection guard for {}", mIp);
+    } else {
+        beammp_debugf("... Connection guard for {} was already released (nothing happened)", mIp);
+    }
+}