ThreadPool: optional limit for jobs queue (#1741)

For very busy servers, the internal jobs queue where accepted
sockets are enqueued can grow without limit.
This is a problem for two reasons:
 - queueing too much work causes the server to respond with huge latency,
   resulting in repetead timeouts on the clients; it is definitely
   better to reject the connection early, so that the client
   receives the backpressure signal as soon as the queue is
   becoming too large
 - the jobs list can eventually cause an out of memory condition

httplib.h

@@ -653,7 +653,7 @@ public:
   TaskQueue() = default;
   virtual ~TaskQueue() = default;
 
-  virtual void enqueue(std::function<void()> fn) = 0;
+  virtual bool enqueue(std::function<void()> fn) = 0;
   virtual void shutdown() = 0;
 
   virtual void on_idle() {}
@@ -661,7 +661,8 @@ public:
 
 class ThreadPool : public TaskQueue {
 public:
-  explicit ThreadPool(size_t n) : shutdown_(false) {
+  explicit ThreadPool(size_t n, size_t mqr = 0)
+      : shutdown_(false), max_queued_requests_(mqr) {
     while (n) {
       threads_.emplace_back(worker(*this));
       n--;
@@ -671,13 +672,17 @@ public:
   ThreadPool(const ThreadPool &) = delete;
   ~ThreadPool() override = default;
 
-  void enqueue(std::function<void()> fn) override {
+  bool enqueue(std::function<void()> fn) override {
     {
       std::unique_lock<std::mutex> lock(mutex_);
+      if (max_queued_requests_ > 0 && jobs_.size() >= max_queued_requests_) {
+        return false;
+      }
       jobs_.push_back(std::move(fn));
     }
 
     cond_.notify_one();
+    return true;
   }
 
   void shutdown() override {
@@ -727,6 +732,7 @@ private:
   std::list<std::function<void()>> jobs_;
 
   bool shutdown_;
+  size_t max_queued_requests_ = 0;
 
   std::condition_variable cond_;
   std::mutex mutex_;
@@ -6319,7 +6325,11 @@ inline bool Server::listen_internal() {
 #endif
       }
 
-      task_queue->enqueue([this, sock]() { process_and_close_socket(sock); });
+      if (!task_queue->enqueue(
+              [this, sock]() { process_and_close_socket(sock); })) {
+        detail::shutdown_socket(sock);
+        detail::close_socket(sock);
+      }
     }
 
     task_queue->shutdown();