ThreadPool: optional limit for jobs queue

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

README.md

@@ -433,6 +433,17 @@ If you want to set the thread count at runtime, there is no convenient way... Bu
 svr.new_task_queue = [] { return new ThreadPool(12); };
 ```
 
+You can also provide an optional parameter to limit the maximum number
+of pending requests, i.e. requests `accept()`ed by the listener but
+still waiting to be serviced by worker threads.
+
+```cpp
+svr.new_task_queue = [] { return new ThreadPool(/*num_threads=*/12, /*max_queued_requests=*/18); };
+```
+
+Default limit is 0 (unlimited). Once the limit is reached, the listener
+will shutdown the client connection.
+
 ### Override the default thread pool with yours
 
 You can supply your own thread pool implementation according to your need.
@@ -444,8 +455,10 @@ public:
     pool_.start_with_thread_count(n);
   }
 
-  virtual void enqueue(std::function<void()> fn) override {
-    pool_.enqueue(fn);
+  virtual bool enqueue(std::function<void()> fn) override {
+    /* Return true if the task was actually enqueued, or false
+     * if the caller must drop the corresponding connection. */
+    return pool_.enqueue(fn);
   }
 
   virtual void shutdown() override {

httplib.h

@@ -582,15 +582,15 @@ class TaskQueue {
   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() {}
 };
 
 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--;
@@ -600,13 +600,17 @@ class ThreadPool : public TaskQueue {
   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 {
@@ -656,6 +660,7 @@ class ThreadPool : public TaskQueue {
   std::list<std::function<void()>> jobs_;
 
   bool shutdown_;
+  size_t max_queued_requests_ = 0;
 
   std::condition_variable cond_;
   std::mutex mutex_;
@@ -6242,7 +6247,10 @@ 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();

test/test.cc

@@ -6506,18 +6506,40 @@ TEST(SocketStream, is_writable_INET) {
 #endif // #ifndef _WIN32
 
 TEST(TaskQueueTest, IncreaseAtomicInteger) {
-  static constexpr unsigned int number_of_task{1000000};
+  static constexpr unsigned int number_of_tasks{1000000};
   std::atomic_uint count{0};
   std::unique_ptr<TaskQueue> task_queue{
       new ThreadPool{CPPHTTPLIB_THREAD_POOL_COUNT}};
 
-  for (unsigned int i = 0; i < number_of_task; ++i) {
-    task_queue->enqueue(
+  for (unsigned int i = 0; i < number_of_tasks; ++i) {
+    auto queued = task_queue->enqueue(
         [&count] { count.fetch_add(1, std::memory_order_relaxed); });
+    EXPECT_TRUE(queued);
   }
 
   EXPECT_NO_THROW(task_queue->shutdown());
-  EXPECT_EQ(number_of_task, count.load());
+  EXPECT_EQ(number_of_tasks, count.load());
+}
+
+TEST(TaskQueueTest, IncreaseAtomicIntegerWithQueueLimit) {
+  static constexpr unsigned int number_of_tasks{1000000};
+  static constexpr unsigned int qlimit{2};
+  unsigned int queued_count{0};
+  std::atomic_uint count{0};
+  std::unique_ptr<TaskQueue> task_queue{
+      new ThreadPool{/*num_threads=*/1, qlimit}};
+
+  for (unsigned int i = 0; i < number_of_tasks; ++i) {
+    if (task_queue->enqueue(
+        [&count] { count.fetch_add(1, std::memory_order_relaxed); })) {
+      queued_count++;
+    }
+  }
+
+  EXPECT_NO_THROW(task_queue->shutdown());
+  EXPECT_EQ(queued_count, count.load());
+  EXPECT_TRUE(queued_count <= number_of_tasks);
+  EXPECT_TRUE(queued_count >= qlimit);
 }
 
 TEST(RedirectTest, RedirectToUrlWithQueryParameters) {