threadpool: make Submit return Expected instead of throwing

Unlike exceptions, which can be ignored as they require extra try-catch
blocks, returning expected errors forces callers to always handle
submission failures.

Not throwing an exception also fixes an unclean shutdown bug
#34573 since we no longer throw when attempting to Submit()
from the libevent callback http_request_cb().

src/test/fuzz/threadpool.cpp

@@ -87,10 +87,10 @@ FUZZ_TARGET(threadpool, .init = setup_threadpool_test) EXCLUSIVE_LOCKS_REQUIRED(
         std::future<void> fut;
         if (will_throw) {
             expected_fail_tasks++;
-            fut = g_pool.Submit(ThrowTask{});
+            fut = *Assert(g_pool.Submit(ThrowTask{}));
         } else {
             expected_task_counter++;
-            fut = g_pool.Submit(CounterTask{task_counter});
+            fut = *Assert(g_pool.Submit(CounterTask{task_counter}));
         }
 
         // If caller wants to wait immediately, consume the future here (safe).

src/test/threadpool_tests.cpp

@@ -44,6 +44,13 @@ BOOST_FIXTURE_TEST_SUITE(threadpool_tests, ThreadPoolFixture)
         }                                                                         \
     } while (0)
 
+// Helper to unwrap a valid pool submission
+template <typename F>
+[[nodiscard]] auto Submit(ThreadPool& pool, F&& fn)
+{
+    return std::move(*Assert(pool.Submit(std::forward<F>(fn))));
+}
+
 // Block a number of worker threads by submitting tasks that wait on `blocker_future`.
 // Returns the futures of the blocking tasks, ensuring all have started and are waiting.
 std::vector<std::future<void>> BlockWorkers(ThreadPool& threadPool, const std::shared_future<void>& blocker_future, int num_of_threads_to_block)
@@ -58,7 +65,7 @@ std::vector<std::future<void>> BlockWorkers(ThreadPool& threadPool, const std::s
     std::vector<std::future<void>> blocking_tasks;
     for (int i = 0; i < num_of_threads_to_block; i++) {
         std::promise<void>& ready = ready_promises[i];
-        blocking_tasks.emplace_back(threadPool.Submit([blocker_future, &ready]() {
+        blocking_tasks.emplace_back(Submit(threadPool, [blocker_future, &ready]() {
             ready.set_value();
             blocker_future.wait();
         }));
@@ -73,10 +80,9 @@ std::vector<std::future<void>> BlockWorkers(ThreadPool& threadPool, const std::s
 BOOST_AUTO_TEST_CASE(submit_task_before_start_fails)
 {
     ThreadPool threadPool(POOL_NAME);
-    BOOST_CHECK_EXCEPTION((void)threadPool.Submit([]{ return false; }), std::runtime_error, [&](const std::runtime_error& e) {
+    auto res = threadPool.Submit([]{ return false; });
-        BOOST_CHECK_EQUAL(e.what(), "No active workers; cannot accept new tasks");
+    BOOST_CHECK(!res);
-        return true;
+    BOOST_CHECK_EQUAL(SubmitErrorString(res.error()), "No active workers");
-    });
 }
 
 // Test 1, submit tasks and verify completion
@@ -92,7 +98,7 @@ BOOST_AUTO_TEST_CASE(submit_tasks_complete_successfully)
     std::vector<std::future<void>> futures;
     futures.reserve(num_tasks);
     for (int i = 1; i <= num_tasks; i++) {
-        futures.emplace_back(threadPool.Submit([&counter, i]() {
+        futures.emplace_back(Submit(threadPool, [&counter, i]() {
             counter.fetch_add(i, std::memory_order_relaxed);
         }));
     }
@@ -121,7 +127,7 @@ BOOST_AUTO_TEST_CASE(single_available_worker_executes_all_tasks)
 
     // Store futures to wait on
     std::vector<std::future<void>> futures(num_tasks);
-    for (auto& f : futures) f = threadPool.Submit([&counter]{ counter++; });
+    for (auto& f : futures) f = Submit(threadPool, [&counter]{ counter++; });
 
     WAIT_FOR(futures);
     BOOST_CHECK_EQUAL(counter, num_tasks);
@@ -138,7 +144,7 @@ BOOST_AUTO_TEST_CASE(wait_for_task_to_finish)
     ThreadPool threadPool(POOL_NAME);
     threadPool.Start(NUM_WORKERS_DEFAULT);
     std::atomic<bool> flag = false;
-    std::future<void> future = threadPool.Submit([&flag]() {
+    std::future<void> future = Submit(threadPool, [&flag]() {
         UninterruptibleSleep(200ms);
         flag.store(true, std::memory_order_release);
     });
@@ -151,10 +157,10 @@ BOOST_AUTO_TEST_CASE(get_result_from_completed_task)
 {
     ThreadPool threadPool(POOL_NAME);
     threadPool.Start(NUM_WORKERS_DEFAULT);
-    std::future<bool> future_bool = threadPool.Submit([]() { return true; });
+    std::future<bool> future_bool = Submit(threadPool, []() { return true; });
     BOOST_CHECK(future_bool.get());
 
-    std::future<std::string> future_str = threadPool.Submit([]() { return std::string("true"); });
+    std::future<std::string> future_str = Submit(threadPool, []() { return std::string("true"); });
     std::string result = future_str.get();
     BOOST_CHECK_EQUAL(result, "true");
 }
@@ -170,7 +176,7 @@ BOOST_AUTO_TEST_CASE(task_exception_propagates_to_future)
     std::vector<std::future<void>> futures;
     futures.reserve(num_tasks);
     for (int i = 0; i < num_tasks; i++) {
-        futures.emplace_back(threadPool.Submit([err_msg, i]() {
+        futures.emplace_back(Submit(threadPool, [err_msg, i]() {
             throw std::runtime_error(err_msg + util::ToString(i));
         }));
     }
@@ -197,7 +203,7 @@ BOOST_AUTO_TEST_CASE(process_tasks_manually_when_workers_busy)
     int num_tasks = 20;
     std::atomic<int> counter = 0;
     for (int i = 0; i < num_tasks; i++) {
-        (void)threadPool.Submit([&counter]() {
+        (void)Submit(threadPool, [&counter]() {
             counter.fetch_add(1, std::memory_order_relaxed);
         });
     }
@@ -222,8 +228,8 @@ BOOST_AUTO_TEST_CASE(recursive_task_submission)
     threadPool.Start(NUM_WORKERS_DEFAULT);
 
     std::promise<void> signal;
-    (void)threadPool.Submit([&]() {
+    (void)Submit(threadPool, [&]() {
-        (void)threadPool.Submit([&]() {
+        (void)Submit(threadPool, [&]() {
             signal.set_value();
         });
     });
@@ -243,7 +249,7 @@ BOOST_AUTO_TEST_CASE(task_submitted_while_busy_completes)
     const auto& blocking_tasks = BlockWorkers(threadPool, blocker_future, NUM_WORKERS_DEFAULT);
 
     // Submit an extra task that should execute once a worker is free
-    std::future<bool> future = threadPool.Submit([]() { return true; });
+    std::future<bool> future = Submit(threadPool, []() { return true; });
 
     // At this point, all workers are blocked, and the extra task is queued
     BOOST_CHECK_EQUAL(threadPool.WorkQueueSize(), 1);
@@ -280,7 +286,7 @@ BOOST_AUTO_TEST_CASE(congestion_more_workers_than_cores)
     std::vector<std::future<void>> futures;
     futures.reserve(num_tasks);
     for (int i = 0; i < num_tasks; i++) {
-        futures.emplace_back(threadPool.Submit([&counter] {
+        futures.emplace_back(Submit(threadPool, [&counter] {
             counter.fetch_add(1, std::memory_order_relaxed);
         }));
     }
@@ -296,10 +302,10 @@ BOOST_AUTO_TEST_CASE(interrupt_blocks_new_submissions)
     ThreadPool threadPool(POOL_NAME);
     threadPool.Start(NUM_WORKERS_DEFAULT);
     threadPool.Interrupt();
-    BOOST_CHECK_EXCEPTION((void)threadPool.Submit([]{}), std::runtime_error, [&](const std::runtime_error& e) {
+
-        BOOST_CHECK_EQUAL(e.what(), "No active workers; cannot accept new tasks");
+    auto res = threadPool.Submit([]{});
-        return true;
+    BOOST_CHECK(!res);
-    });
+    BOOST_CHECK_EQUAL(SubmitErrorString(res.error()), "Interrupted");
 
     // Reset pool
     threadPool.Stop();
@@ -310,7 +316,7 @@ BOOST_AUTO_TEST_CASE(interrupt_blocks_new_submissions)
     std::atomic<int> counter{0};
     std::promise<void> blocker;
     const auto blocking_tasks = BlockWorkers(threadPool, blocker.get_future().share(), 1);
-    threadPool.Submit([&threadPool, &counter]{
+    Submit(threadPool, [&threadPool, &counter]{
         threadPool.Interrupt();
         counter.fetch_add(1, std::memory_order_relaxed);
     }).get();

src/util/threadpool.h

@@ -7,6 +7,7 @@
 
 #include <sync.h>
 #include <tinyformat.h>
+#include <util/expected.h>
 #include <util/check.h>
 #include <util/thread.h>
 
@@ -15,8 +16,8 @@
 #include <functional>
 #include <future>
 #include <queue>
-#include <stdexcept>
 #include <thread>
+#include <type_traits>
 #include <utility>
 #include <vector>
 
@@ -143,28 +144,39 @@ public:
         // Note: m_interrupt is left true until next Start()
     }
 
+    enum class SubmitError {
+        Inactive,
+        Interrupted,
+    };
+
     /**
      * @brief Enqueues a new task for asynchronous execution.
      *
-     * Returns a `std::future` that provides the task's result or propagates
+     * @param  fn Callable to execute asynchronously.
-     * any exception it throws.
+     * @return On success, a future containing fn's result.
-     * Note: Ignoring the returned future requires guarding the task against
+     *         On failure, an error indicating why the task was rejected:
-     * uncaught exceptions, as they would otherwise be silently discarded.
+     *         - SubmitError::Inactive: Pool has no workers (never started or already stopped).
+     *         - SubmitError::Interrupted: Pool task acceptance has been interrupted.
+     *
+     * Thread-safe: Can be called from any thread, including within the provided 'fn' callable.
+     *
+     * @warning Ignoring the returned future requires guarding the task against
+     *          uncaught exceptions, as they would otherwise be silently discarded.
      */
-    template <class F> [[nodiscard]] EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
+    template <class F>
-    auto Submit(F&& fn)
+    [[nodiscard]] util::Expected<std::future<std::invoke_result_t<F>>, SubmitError> Submit(F&& fn) noexcept EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
     {
-        std::packaged_task task{std::forward<F>(fn)};
+        std::packaged_task<std::invoke_result_t<F>()> task{std::forward<F>(fn)};
         auto future{task.get_future()};
         {
             LOCK(m_mutex);
-            if (m_interrupt || m_workers.empty()) {
+            if (m_workers.empty()) return util::Unexpected{SubmitError::Inactive};
-                throw std::runtime_error("No active workers; cannot accept new tasks");
+            if (m_interrupt) return util::Unexpected{SubmitError::Interrupted};
-            }
+
             m_work_queue.emplace(std::move(task));
         }
         m_cv.notify_one();
-        return future;
+        return {std::move(future)};
     }
 
     /**
@@ -208,4 +220,15 @@ public:
     }
 };
 
+constexpr std::string_view SubmitErrorString(const ThreadPool::SubmitError err) noexcept {
+    switch (err) {
+        case ThreadPool::SubmitError::Inactive:
+            return "No active workers";
+        case ThreadPool::SubmitError::Interrupted:
+            return "Interrupted";
+    }
+    Assume(false); // Unreachable
+    return "Unknown error";
+}
+
 #endif // BITCOIN_UTIL_THREADPOOL_H