From 070fc34cee07db023824c76995bba43f2262d6c1 Mon Sep 17 00:00:00 2001
From: Håvard Pettersen <havardpe@yahooinc.com>
Date: Wed, 15 Feb 2023 13:10:31 +0000
Subject: use std::thread directly

also add very simple ThreadPool class to run multiple threads at once

make an effort to only join once
---
 config/src/vespa/config/helper/configfetcher.cpp   |  7 ++--
 config/src/vespa/config/helper/configfetcher.h     |  5 +--
 .../vespa/config/retriever/simpleconfigurer.cpp    | 10 +++--
 .../src/vespa/config/retriever/simpleconfigurer.h  |  2 +-
 slobrok/src/vespa/slobrok/server/slobrokserver.cpp |  4 +-
 slobrok/src/vespa/slobrok/server/slobrokserver.h   |  2 +-
 .../filestorage/operationabortingtest.cpp          |  2 +-
 vbench/src/apps/vbench/vbench.cpp                  |  3 +-
 vbench/src/tests/dispatcher/dispatcher_test.cpp    |  4 +-
 vbench/src/vbench/core/handler_thread.h            |  2 +-
 vbench/src/vbench/core/handler_thread.hpp          |  6 ++-
 vbench/src/vbench/vbench/request_scheduler.cpp     |  2 +-
 vbench/src/vbench/vbench/request_scheduler.h       |  2 +-
 vbench/src/vbench/vbench/vbench.cpp                |  2 +-
 vbench/src/vbench/vbench/vbench.h                  |  2 +-
 vbench/src/vbench/vbench/worker.cpp                |  2 +-
 vbench/src/vbench/vbench/worker.h                  |  2 +-
 vespalib/src/tests/thread/thread_test.cpp          | 39 ++++++++++--------
 vespalib/src/vespa/vespalib/testkit/test_hook.h    |  6 +--
 .../vespa/vespalib/util/simple_thread_bundle.cpp   |  2 +-
 .../src/vespa/vespalib/util/simple_thread_bundle.h |  3 +-
 .../src/vespa/vespalib/util/singleexecutor.cpp     |  2 +-
 vespalib/src/vespa/vespalib/util/singleexecutor.h  |  2 +-
 vespalib/src/vespa/vespalib/util/thread.cpp        | 32 +++------------
 vespalib/src/vespa/vespalib/util/thread.h          | 48 +++++++++++-----------
 25 files changed, 90 insertions(+), 103 deletions(-)

diff --git a/config/src/vespa/config/helper/configfetcher.cpp b/config/src/vespa/config/helper/configfetcher.cpp
index ec4ea42d0bb..db0a64b248c 100644
--- a/config/src/vespa/config/helper/configfetcher.cpp
+++ b/config/src/vespa/config/helper/configfetcher.cpp
@@ -15,7 +15,7 @@ VESPA_THREAD_STACK_TAG(config_fetcher_thread);
 
 ConfigFetcher::ConfigFetcher(std::shared_ptr<IConfigContext> context)
     : _poller(std::make_unique<ConfigPoller>(std::move(context))),
-      _thread(std::make_unique<vespalib::Thread>()),
+      _thread(),
       _closed(false),
       _started(false)
 {
@@ -36,7 +36,7 @@ ConfigFetcher::start()
             throw ConfigTimeoutException("ConfigFetcher::start timed out getting initial config");
         }
         LOG(debug, "Starting fetcher thread...");
-        *_thread = vespalib::Thread::start(*_poller, config_fetcher_thread);
+        _thread = vespalib::thread::start(*_poller, config_fetcher_thread);
         _started = true;
         LOG(debug, "Fetcher thread started");
     }
@@ -58,7 +58,8 @@ ConfigFetcher::close()
     if (!_closed) {
         _poller->close();
         if (_started)
-            _thread->join();
+            _thread.join();
+        _closed = true;
     }
 }
 
diff --git a/config/src/vespa/config/helper/configfetcher.h b/config/src/vespa/config/helper/configfetcher.h
index 2ed236eda68..67e8f5d134f 100644
--- a/config/src/vespa/config/helper/configfetcher.h
+++ b/config/src/vespa/config/helper/configfetcher.h
@@ -5,8 +5,7 @@
 #include <vespa/config/helper/ifetchercallback.h>
 #include <vespa/config/subscription/sourcespec.h>
 #include <atomic>
-
-namespace vespalib { class Thread; }
+#include <thread>
 
 namespace config {
 
@@ -31,7 +30,7 @@ public:
     int64_t getGeneration() const;
 private:
     std::unique_ptr<ConfigPoller> _poller;
-    std::unique_ptr<vespalib::Thread> _thread;
+    std::thread _thread;
     std::atomic<bool> _closed;
     std::atomic<bool> _started;
 };
diff --git a/config/src/vespa/config/retriever/simpleconfigurer.cpp b/config/src/vespa/config/retriever/simpleconfigurer.cpp
index 1e89f51ec03..83e56737f8d 100644
--- a/config/src/vespa/config/retriever/simpleconfigurer.cpp
+++ b/config/src/vespa/config/retriever/simpleconfigurer.cpp
@@ -25,7 +25,7 @@ SimpleConfigurer::start()
     if (!_retriever->isClosed()) {
         LOG(debug, "Polling for config");
         runConfigure();
-        _thread = vespalib::Thread::start(*this, simple_configurer_thread);
+        _thread = vespalib::thread::start(*this, simple_configurer_thread);
         _started = true;
     }
 }
@@ -38,9 +38,11 @@ SimpleConfigurer::~SimpleConfigurer()
 void
 SimpleConfigurer::close()
 {
-    _retriever->close();
-    if (_started)
-        _thread.join();
+    if (!_retriever->isClosed()) {
+        _retriever->close();
+        if (_started)
+            _thread.join();
+    }
 }
 
 void
diff --git a/config/src/vespa/config/retriever/simpleconfigurer.h b/config/src/vespa/config/retriever/simpleconfigurer.h
index aa6508c75a2..95fa12610cb 100644
--- a/config/src/vespa/config/retriever/simpleconfigurer.h
+++ b/config/src/vespa/config/retriever/simpleconfigurer.h
@@ -46,7 +46,7 @@ private:
 
     SimpleConfigRetriever::UP _retriever;
     SimpleConfigurable * const _configurable;
-    vespalib::Thread _thread;
+    std::thread _thread;
     std::atomic<bool> _started;
 };
 
diff --git a/slobrok/src/vespa/slobrok/server/slobrokserver.cpp b/slobrok/src/vespa/slobrok/server/slobrokserver.cpp
index 4a986d9ba01..4e58cbeaaeb 100644
--- a/slobrok/src/vespa/slobrok/server/slobrokserver.cpp
+++ b/slobrok/src/vespa/slobrok/server/slobrokserver.cpp
@@ -10,14 +10,14 @@ SlobrokServer::SlobrokServer(ConfigShim &shim)
     : _env(shim),
       _thread()
 {
-    _thread = vespalib::Thread::start(*this, slobrok_server_thread);
+    _thread = vespalib::thread::start(*this, slobrok_server_thread);
 }
 
 SlobrokServer::SlobrokServer(uint32_t port)
     : _env(ConfigShim(port)),
       _thread()
 {
-    _thread = vespalib::Thread::start(*this, slobrok_server_thread);
+    _thread = vespalib::thread::start(*this, slobrok_server_thread);
 }
 
 
diff --git a/slobrok/src/vespa/slobrok/server/slobrokserver.h b/slobrok/src/vespa/slobrok/server/slobrokserver.h
index 1aeba9b6be3..60a78c43d21 100644
--- a/slobrok/src/vespa/slobrok/server/slobrokserver.h
+++ b/slobrok/src/vespa/slobrok/server/slobrokserver.h
@@ -12,7 +12,7 @@ class SlobrokServer : public vespalib::Runnable
 {
 private:
     SBEnv              _env;
-    vespalib::Thread   _thread;
+    std::thread        _thread;
 
 public:
     SlobrokServer(ConfigShim &shim);
diff --git a/storage/src/tests/persistence/filestorage/operationabortingtest.cpp b/storage/src/tests/persistence/filestorage/operationabortingtest.cpp
index d7ecb1f30f1..85816c3ad3e 100644
--- a/storage/src/tests/persistence/filestorage/operationabortingtest.cpp
+++ b/storage/src/tests/persistence/filestorage/operationabortingtest.cpp
@@ -296,7 +296,7 @@ TEST_F(OperationAbortingTest, wait_for_current_operation_completion_for_aborted_
     auto abortCmd = makeAbortCmd(abortSet);
 
     SendTask sendTask(abortCmd, *_queueBarrier, c.top);
-    auto thread = vespalib::Thread::start(sendTask, test_thread);
+    auto thread = vespalib::thread::start(sendTask, test_thread);
 
     LOG(debug, "waiting for threads to reach barriers");
     _queueBarrier->await();
diff --git a/vbench/src/apps/vbench/vbench.cpp b/vbench/src/apps/vbench/vbench.cpp
index b5c2897207f..c73f38adaf2 100644
--- a/vbench/src/apps/vbench/vbench.cpp
+++ b/vbench/src/apps/vbench/vbench.cpp
@@ -44,12 +44,13 @@ int run(const std::string &cfg_name) {
     VBench vbench(cfg);
     NotifyDone notify(done);
     vespalib::RunnablePair runBoth(vbench, notify);
-    auto thread = vespalib::Thread::start(runBoth, vbench_thread);
+    auto thread = vespalib::thread::start(runBoth, vbench_thread);
     while (!SIG::INT.check() && !SIG::TERM.check() && !done.await(1s)) {}
     if (!done.await(vespalib::duration::zero())) {
         vbench.abort();
         done.await();
     }
+    thread.join();
     if (vbench.tainted()) {
         fprintf(stderr, "vbench failed: %s\n",
                 vbench.tainted().reason().c_str());
diff --git a/vbench/src/tests/dispatcher/dispatcher_test.cpp b/vbench/src/tests/dispatcher/dispatcher_test.cpp
index 6a3fb8d0c7c..49a41508c7b 100644
--- a/vbench/src/tests/dispatcher/dispatcher_test.cpp
+++ b/vbench/src/tests/dispatcher/dispatcher_test.cpp
@@ -30,9 +30,9 @@ TEST("dispatcher") {
     Dispatcher<int> dispatcher(dropped);
     Fetcher fetcher1(dispatcher, handler1);
     Fetcher fetcher2(dispatcher, handler2);
-    auto thread1 = vespalib::Thread::start(fetcher1, fetcher1_thread);
+    auto thread1 = vespalib::thread::start(fetcher1, fetcher1_thread);
     EXPECT_TRUE(dispatcher.waitForThreads(1, 512));
-    auto thread2 = vespalib::Thread::start(fetcher2, fetcher2_thread);
+    auto thread2 = vespalib::thread::start(fetcher2, fetcher2_thread);
     EXPECT_TRUE(dispatcher.waitForThreads(2, 512));
     EXPECT_EQUAL(-1, dropped.value);
     EXPECT_EQUAL(-1, handler1.value);
diff --git a/vbench/src/vbench/core/handler_thread.h b/vbench/src/vbench/core/handler_thread.h
index 8ece1389dfc..81a0a720720 100644
--- a/vbench/src/vbench/core/handler_thread.h
+++ b/vbench/src/vbench/core/handler_thread.h
@@ -26,7 +26,7 @@ private:
     std::condition_variable                    _cond;
     vespalib::ArrayQueue<std::unique_ptr<T> >  _queue;
     Handler<T>                                &_next;
-    vespalib::Thread                           _thread;
+    std::thread                                _thread;
     bool                                       _done;
 
     void run() override;
diff --git a/vbench/src/vbench/core/handler_thread.hpp b/vbench/src/vbench/core/handler_thread.hpp
index 1a99861ea81..3373e196ab7 100644
--- a/vbench/src/vbench/core/handler_thread.hpp
+++ b/vbench/src/vbench/core/handler_thread.hpp
@@ -31,13 +31,15 @@ HandlerThread<T>::HandlerThread(Handler<T> &next, init_fun_t init_fun)
       _thread(),
       _done(false)
 {
-    _thread = vespalib::Thread::start(*this, init_fun);
+    _thread = vespalib::thread::start(*this, init_fun);
 }
 
 template <typename T>
 HandlerThread<T>::~HandlerThread()
 {
-    join();
+    if (!_done) {
+        join();
+    }
     assert(_queue.empty());
 }
 
diff --git a/vbench/src/vbench/vbench/request_scheduler.cpp b/vbench/src/vbench/vbench/request_scheduler.cpp
index cde31ec07b8..136f2bed98b 100644
--- a/vbench/src/vbench/vbench/request_scheduler.cpp
+++ b/vbench/src/vbench/vbench/request_scheduler.cpp
@@ -71,7 +71,7 @@ void
 RequestScheduler::start()
 {
     _timer.reset();
-    _thread = vespalib::Thread::start(*this, vbench_request_scheduler_thread);
+    _thread = vespalib::thread::start(*this, vbench_request_scheduler_thread);
 }
 
 RequestScheduler &
diff --git a/vbench/src/vbench/vbench/request_scheduler.h b/vbench/src/vbench/vbench/request_scheduler.h
index b1d525eb691..1483a86a458 100644
--- a/vbench/src/vbench/vbench/request_scheduler.h
+++ b/vbench/src/vbench/vbench/request_scheduler.h
@@ -26,7 +26,7 @@ private:
     TimeQueue<Request>      _queue;
     DroppedTagger           _droppedTagger;
     Dispatcher<Request>     _dispatcher;
-    vespalib::Thread        _thread;
+    std::thread             _thread;
     HttpConnectionPool      _connectionPool;
     std::vector<Worker::UP> _workers;
     std::mutex              _lock;
diff --git a/vbench/src/vbench/vbench/vbench.cpp b/vbench/src/vbench/vbench/vbench.cpp
index 8a8bceccefb..58d0f9e0345 100644
--- a/vbench/src/vbench/vbench/vbench.cpp
+++ b/vbench/src/vbench/vbench/vbench.cpp
@@ -100,7 +100,7 @@ VBench::run()
 {
     _scheduler->start();
     for (size_t i = 0; i < _inputs.size(); ++i) {
-        _inputs[i]->thread = vespalib::Thread::start(*_inputs[i]->generator, vbench_inputchain_generator);
+        _inputs[i]->thread = vespalib::thread::start(*_inputs[i]->generator, vbench_inputchain_generator);
     }
     for (size_t i = 0; i < _inputs.size(); ++i) {
         _inputs[i]->thread.join();
diff --git a/vbench/src/vbench/vbench/vbench.h b/vbench/src/vbench/vbench/vbench.h
index 2b7fcf0cd88..f355beddce5 100644
--- a/vbench/src/vbench/vbench/vbench.h
+++ b/vbench/src/vbench/vbench/vbench.h
@@ -26,7 +26,7 @@ private:
         using UP = std::unique_ptr<InputChain>;
         std::vector<Tagger::UP>           taggers;
         Generator::UP                     generator;
-        vespalib::Thread                  thread;
+        std::thread                       thread;
     };
     NativeFactory                _factory;
     std::vector<Analyzer::UP>    _analyzers;
diff --git a/vbench/src/vbench/vbench/worker.cpp b/vbench/src/vbench/vbench/worker.cpp
index eabd17ae73f..d9ba039eb53 100644
--- a/vbench/src/vbench/vbench/worker.cpp
+++ b/vbench/src/vbench/vbench/worker.cpp
@@ -30,7 +30,7 @@ Worker::Worker(Provider<Request> &provider, Handler<Request> &next,
       _pool(pool),
       _timer(timer)
 {
-    _thread = vespalib::Thread::start(*this, vbench_worker_thread);
+    _thread = vespalib::thread::start(*this, vbench_worker_thread);
 }
 
 } // namespace vbench
diff --git a/vbench/src/vbench/vbench/worker.h b/vbench/src/vbench/vbench/worker.h
index d2bcfae637b..9cd7b04fdfe 100644
--- a/vbench/src/vbench/vbench/worker.h
+++ b/vbench/src/vbench/vbench/worker.h
@@ -20,7 +20,7 @@ namespace vbench {
 class Worker : public vespalib::Runnable
 {
 private:
-    vespalib::Thread    _thread;
+    std::thread         _thread;
     Provider<Request>  &_provider;
     Handler<Request>   &_next;
     HttpConnectionPool &_pool;
diff --git a/vespalib/src/tests/thread/thread_test.cpp b/vespalib/src/tests/thread/thread_test.cpp
index f7e8753fd86..9533fe1c190 100644
--- a/vespalib/src/tests/thread/thread_test.cpp
+++ b/vespalib/src/tests/thread/thread_test.cpp
@@ -19,30 +19,33 @@ void my_fun(bool *was_run) {
     *was_run = true;
 }
 
+Runnable::init_fun_t wrap(Runnable::init_fun_t init, bool *init_called) {
+    return [=](Runnable &target)
+           {
+               *init_called = true;
+               return init(target);
+           };
+}
+
 TEST("run vespalib::Runnable with init function") {
     Agent agent;
-    {
-        auto thread = Thread::start(agent, test_agent_thread);
-    }
+    bool init_called = false;
+    auto thread = thread::start(agent, wrap(test_agent_thread, &init_called));
+    thread.join();
+    EXPECT_TRUE(init_called);
     EXPECT_TRUE(agent.was_run);
 }
 
-TEST("run custom function") {
-    bool was_run = false;
-    {
-        auto thread = Thread::start(my_fun, &was_run);
-    }
-    EXPECT_TRUE(was_run);
-}
-
-TEST("join multiple times (including destructor)") {
+TEST("use thread pool to run multiple things") {
+    Agent agent;
+    bool init_called = false;
     bool was_run = false;
-    {
-        auto thread = Thread::start(my_fun, &was_run);
-        thread.join();
-        thread.join();
-        thread.join();
-    }
+    ThreadPool pool;
+    pool.start(my_fun, &was_run);
+    pool.start(agent, wrap(test_agent_thread, &init_called));
+    pool.join();
+    EXPECT_TRUE(init_called);
+    EXPECT_TRUE(agent.was_run);
     EXPECT_TRUE(was_run);
 }
 
diff --git a/vespalib/src/vespa/vespalib/testkit/test_hook.h b/vespalib/src/vespa/vespalib/testkit/test_hook.h
index 8a5c6c1e684..0ff5ba11709 100644
--- a/vespalib/src/vespa/vespalib/testkit/test_hook.h
+++ b/vespalib/src/vespa/vespalib/testkit/test_hook.h
@@ -78,10 +78,9 @@ protected:
         Barrier barrier(num_threads);
         std::vector<FixtureUP> fixtures;
         std::vector<ThreadUP> threads;
-        std::vector<Thread> thread_handles;
+        ThreadPool pool;
         threads.reserve(num_threads);
         fixtures.reserve(num_threads);
-        thread_handles.reserve(num_threads - 1);
         for (size_t i = 0; i < num_threads; ++i) {
             FixtureUP fixture_up(new T(fixture));
             fixture_up->thread_id = i;
@@ -90,10 +89,11 @@ protected:
             fixtures.push_back(std::move(fixture_up));
         }
         for (size_t i = 1; i < num_threads; ++i) {
-            thread_handles.push_back(Thread::start([&target = *threads[i]](){ target.threadEntry(); }));
+            pool.start([&target = *threads[i]](){ target.threadEntry(); });
         }
         threads[0]->threadEntry();
         latch.await();
+        pool.join();
         bool result = true;
         for (size_t i = 0; i < num_threads; ++i) {
             result = result && threads[i]->getResult();
diff --git a/vespalib/src/vespa/vespalib/util/simple_thread_bundle.cpp b/vespalib/src/vespa/vespalib/util/simple_thread_bundle.cpp
index becb5d2ab74..958bb58f34a 100644
--- a/vespalib/src/vespa/vespalib/util/simple_thread_bundle.cpp
+++ b/vespalib/src/vespa/vespalib/util/simple_thread_bundle.cpp
@@ -182,7 +182,7 @@ SimpleThreadBundle::Worker::Worker(Signal &s, Runnable::init_fun_t init_fun, Run
     signal(s),
     hook(std::move(h))
 {
-    thread = Thread::start(*this, std::move(init_fun));
+    thread = thread::start(*this, std::move(init_fun));
 }
 
 void
diff --git a/vespalib/src/vespa/vespalib/util/simple_thread_bundle.h b/vespalib/src/vespa/vespalib/util/simple_thread_bundle.h
index b5f371d2de3..536474d9300 100644
--- a/vespalib/src/vespa/vespalib/util/simple_thread_bundle.h
+++ b/vespalib/src/vespa/vespalib/util/simple_thread_bundle.h
@@ -113,7 +113,7 @@ public:
 private:
     struct Worker : Runnable {
         using UP = std::unique_ptr<Worker>;
-        Thread thread;
+        std::thread thread;
         Signal &signal;
         Runnable::UP hook;
         Worker(Signal &s, init_fun_t init_fun, Runnable::UP h);
@@ -136,4 +136,3 @@ public:
 };
 
 } // namespace vespalib
-
diff --git a/vespalib/src/vespa/vespalib/util/singleexecutor.cpp b/vespalib/src/vespa/vespalib/util/singleexecutor.cpp
index 298226d8805..fa1aef61167 100644
--- a/vespalib/src/vespa/vespalib/util/singleexecutor.cpp
+++ b/vespalib/src/vespa/vespalib/util/singleexecutor.cpp
@@ -38,7 +38,7 @@ SingleExecutor::SingleExecutor(init_fun_t func, uint32_t reservedQueueSize, bool
     if ( ! isQueueSizeHard) {
         _overflow = std::make_unique<ArrayQueue<Task::UP>>();
     }
-    _thread = Thread::start(*this, func);
+    _thread = thread::start(*this, func);
 }
 
 SingleExecutor::~SingleExecutor() {
diff --git a/vespalib/src/vespa/vespalib/util/singleexecutor.h b/vespalib/src/vespa/vespalib/util/singleexecutor.h
index 051f506e90a..e8b7b0c44ad 100644
--- a/vespalib/src/vespa/vespalib/util/singleexecutor.h
+++ b/vespalib/src/vespa/vespalib/util/singleexecutor.h
@@ -70,7 +70,7 @@ private:
     std::mutex                  _mutex;
     std::condition_variable     _consumerCondition;
     std::condition_variable     _producerCondition;
-    vespalib::Thread            _thread;
+    std::thread                 _thread;
     std::atomic<bool>           _stopped;
     ExecutorIdleTracker         _idleTracker;
     ThreadIdleTracker           _threadIdleTracker;
diff --git a/vespalib/src/vespa/vespalib/util/thread.cpp b/vespalib/src/vespa/vespalib/util/thread.cpp
index cad24c5bcda..1c8fef53ba3 100644
--- a/vespalib/src/vespa/vespalib/util/thread.cpp
+++ b/vespalib/src/vespa/vespalib/util/thread.cpp
@@ -2,33 +2,13 @@
 
 #include "thread.h"
 
-namespace vespalib {
+namespace vespalib::thread {
 
-Thread &
-Thread::operator=(Thread &&rhs) noexcept
-{
-    // may call std::terminate
-    _thread = std::move(rhs._thread);
-    return *this;
+std::thread start(Runnable &runnable, Runnable::init_fun_t init_fun_in) {
+    return std::thread([&runnable, init_fun = std::move(init_fun_in)]()
+                       {
+                           init_fun(runnable);
+                       });
 }
 
-void
-Thread::join()
-{
-    if (_thread.joinable()) {
-        _thread.join();
-    }
 }
-
-Thread::~Thread()
-{
-    join();
-}
-
-Thread
-Thread::start(Runnable &runnable, Runnable::init_fun_t init_fun)
-{
-    return start([&runnable, init_fun](){ init_fun(runnable); });
-}
-
-} // namespace vespalib
diff --git a/vespalib/src/vespa/vespalib/util/thread.h b/vespalib/src/vespa/vespalib/util/thread.h
index 6db1c1d54b4..d919b8f2ab7 100644
--- a/vespalib/src/vespa/vespalib/util/thread.h
+++ b/vespalib/src/vespa/vespalib/util/thread.h
@@ -8,36 +8,36 @@
 
 namespace vespalib {
 
+namespace thread {
+[[nodiscard]] std::thread start(Runnable &runnable, Runnable::init_fun_t init_fun);
+}
+
 /**
- * Thin thread abstraction that takes some things from std::thread
- * (not allowed to assign to a running thread), some things from
- * std::jthread (destructor does automatic join) and some things from
- * now deprecated thread pools (the join function can be called
- * multiple times and will only join the underlying thread if it is
- * joinable). Enables starting a thread either by using a runnable and
- * an init function or by forwarding directly to the std::thread
- * constructor. Note that this class does not handle cancellation.
+ * Keeps track of multiple running threads. Calling join will join all
+ * currently running threads. All threads must be joined before
+ * destructing the pool itself. This class is not thread safe.
  **/
-class Thread
-{
+class ThreadPool {
 private:
-    std::thread _thread;
-    Thread(std::thread &&thread) noexcept : _thread(std::move(thread)) {}
+    std::vector<std::thread> _threads;
 public:
-    Thread() noexcept : _thread() {}
-    Thread(const Thread &rhs) = delete;
-    Thread(Thread &&rhs) noexcept : Thread(std::move(rhs._thread)) {}
-    std::thread::id get_id() const noexcept { return _thread.get_id(); }
-    Thread &operator=(const Thread &rhs) = delete;
-    Thread &operator=(Thread &&rhs) noexcept;
-    void join();
-    ~Thread();
-    [[nodiscard]] static Thread start(Runnable &runnable, Runnable::init_fun_t init_fun_t);
+    ThreadPool() noexcept : _threads() {}
+    void start(Runnable &runnable, Runnable::init_fun_t init_fun) {
+        _threads.reserve(_threads.size() + 1);
+        _threads.push_back(thread::start(runnable, std::move(init_fun)));
+    }
     template<typename F, typename... Args>
     requires std::invocable<F,Args...>
-    [[nodiscard]] static Thread start(F &&f, Args && ... args) {
-        return Thread(std::thread(std::forward<F>(f), std::forward<Args>(args)...));
+    void start(F &&f, Args && ... args) {
+        _threads.reserve(_threads.size() + 1);
+        _threads.emplace_back(std::forward<F>(f), std::forward<Args>(args)...);
     };
+    void join() {
+        for (auto &thread: _threads) {
+            thread.join();
+        }
+        _threads.clear();
+    }
 };
 
-} // namespace vespalib
+}
-- 
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