Make ThreadPool idle timeout configurable at runtime (Fix #2481)

Author: yhirose

README.md

@@ -824,6 +824,15 @@ svr.new_task_queue = [] { return new ThreadPool(/*base_threads=*/12, /*max_threa
 Default limit is 0 (unlimited). Once the limit is reached, the listener
 will shutdown the client connection.
 
+#### Idle timeout for dynamic threads
+
+The idle timeout for dynamic threads can also be set at runtime via the
+fourth parameter (in seconds):
+
+```cpp
+svr.new_task_queue = [] { return new ThreadPool(/*base_threads=*/8, /*max_threads=*/64, /*max_queued_requests=*/0, /*idle_timeout_sec=*/10); };
+```
+
 ### Override the default thread pool with yours
 
 You can supply your own thread pool implementation according to your need.

httplib.h

@@ -1541,7 +1541,9 @@ class TaskQueue {
 
 class ThreadPool final : public TaskQueue {
 public:
-  explicit ThreadPool(size_t n, size_t max_n = 0, size_t mqr = 0);
+  explicit ThreadPool(
+      size_t n, size_t max_n = 0, size_t mqr = 0,
+      time_t idle_timeout_sec = CPPHTTPLIB_THREAD_POOL_IDLE_TIMEOUT);
   ThreadPool(const ThreadPool &) = delete;
   ~ThreadPool() override = default;
 
@@ -1556,6 +1558,7 @@ class ThreadPool final : public TaskQueue {
   size_t base_thread_count_;
   size_t max_thread_count_;
   size_t max_queued_requests_;
+  time_t idle_timeout_sec_;
   size_t idle_thread_count_;
 
   bool shutdown_;
@@ -10304,8 +10307,10 @@ inline ssize_t detail::BodyReader::read(char *buf, size_t len) {
 }
 
 // ThreadPool implementation
-inline ThreadPool::ThreadPool(size_t n, size_t max_n, size_t mqr)
-    : base_thread_count_(n), max_queued_requests_(mqr), idle_thread_count_(0),
+inline ThreadPool::ThreadPool(size_t n, size_t max_n, size_t mqr,
+                              time_t idle_timeout_sec)
+    : base_thread_count_(n), max_queued_requests_(mqr),
+      idle_timeout_sec_(idle_timeout_sec), idle_thread_count_(0),
       shutdown_(false) {
 #ifndef CPPHTTPLIB_NO_EXCEPTIONS
   if (max_n != 0 && max_n < n) {
@@ -10415,9 +10420,9 @@ inline void ThreadPool::worker(bool is_dynamic) {
       idle_thread_count_++;
 
       if (is_dynamic) {
-        auto has_work = cond_.wait_for(
-            lock, std::chrono::seconds(CPPHTTPLIB_THREAD_POOL_IDLE_TIMEOUT),
-            [&] { return !jobs_.empty() || shutdown_; });
+        auto has_work =
+            cond_.wait_for(lock, std::chrono::seconds(idle_timeout_sec_),
+                           [&] { return !jobs_.empty() || shutdown_; });
         if (!has_work) {
           // Timed out with no work - exit this dynamic thread
           idle_thread_count_--;

test/test.cc

@@ -13476,6 +13476,44 @@ TEST(TaskQueueTest, IncreaseAtomicIntegerWithQueueLimit) {
   EXPECT_TRUE(queued_count >= qlimit);
 }
 
+TEST(TaskQueueTest, IdleTimeoutAtRuntime) {
+  // Use a short idle timeout so a dynamic thread spawns, times out and
+  // exits during the test, and the pool keeps working afterwards.
+  std::unique_ptr<TaskQueue> task_queue{new ThreadPool{
+      /*num_threads=*/1, /*max_threads=*/2, /*max_queued_requests=*/0,
+      /*idle_timeout_sec=*/1}};
+
+  std::atomic_uint count{0};
+  std::condition_variable cv;
+  std::mutex mtx;
+  bool release = false;
+
+  // Block the base thread so the second task spawns a dynamic thread.
+  EXPECT_TRUE(task_queue->enqueue([&] {
+    std::unique_lock<std::mutex> lock(mtx);
+    while (!release) {
+      cv.wait(lock);
+    }
+    count++;
+  }));
+  EXPECT_TRUE(task_queue->enqueue([&] { count++; }));
+
+  // Let the dynamic thread finish its task and exceed the idle timeout.
+  std::this_thread::sleep_for(std::chrono::milliseconds(1500));
+
+  {
+    std::unique_lock<std::mutex> lock(mtx);
+    release = true;
+  }
+  cv.notify_all();
+
+  // The pool must still accept and run tasks after the dynamic thread exited.
+  EXPECT_TRUE(task_queue->enqueue([&] { count++; }));
+
+  task_queue->shutdown();
+  EXPECT_EQ(3u, count.load());
+}
+
 TEST(TaskQueueTest, MaxQueuedRequests) {
   static constexpr unsigned int qlimit{3};
   std::unique_ptr<TaskQueue> task_queue{new ThreadPool{1, 1, qlimit}};