TaskSystem: Fix early shutdown

... conceptually, it was possible that a previous task
decrements the `num_threads_running_` counter before it is
incremented by the next task. Therefore, we have to unify
the queue and thread status in a single counter
(`total_workload_`) and ensure that woken threads increment
it before decrementing it for popping a queue.

3 files changed, 26 insertions, 74 deletions

diff --git a/src/buildtool/multithreading/notification_queue.hpp b/src/buildtool/multithreading/notification_queue.hpp
index 7e79aa43..5965fde0 100644
--- a/src/buildtool/multithreading/notification_queue.hpp
+++ b/src/buildtool/multithreading/notification_queue.hpp
@@ -11,87 +11,42 @@
 #include "src/buildtool/multithreading/task.hpp"
 #include "src/utils/cpp/atomic.hpp"
 
-// Flag that can block the caller until it is set. Cannot be cleared after set.
-class WaitableOneWayFlag {
-  public:
-    // Clear flag. Essentially a noop, if it was set before.
-    void Clear() {
-        if (not was_set_) {
-            initial_ = false;
-        }
-    }
-
-    // Set flag. Essentially a noop, if it was set before.
-    void Set() {
-        if (not was_set_) {
-            was_set_ = true;
-            was_set_.notify_all();
-        }
-    }
-
-    // Blocks caller until it is set, if it was ever cleared.
-    void WaitForSet() {
-        if (not was_set_ and not initial_) {
-            was_set_.wait(false);
-        }
-    }
-
-  private:
-    atomic<bool> was_set_{};
-    bool initial_{true};
-};
-
 // Counter that can block the caller until it reaches zero.
 class WaitableZeroCounter {
-    enum class Status { Init, Wait, Reached };
-
   public:
     explicit WaitableZeroCounter(std::size_t init = 0) : count_{init} {}
 
-    // Essentially a noop, if count reached zero since last wait call.
     void Decrement() {
-        if (status_ != Status::Reached and --count_ == 0) {
-            if (status_ == Status::Wait) {
-                status_ = Status::Reached;
-                status_.notify_all();
-            }
+        if (--count_ == 0) {
+            count_.notify_all();
         }
     }
 
-    // Essentially a noop, if count reached zero since last wait call.
-    void Increment() {
-        if (status_ != Status::Reached) {
-            ++count_;
-        }
-    }
+    void Increment() { ++count_; }
 
-    // Blocks caller until count reached zero, since last call to this method.
     void WaitForZero() {
-        status_ = Status::Wait;
-        if (count_ != 0) {
-            status_.wait(Status::Wait);
+        auto val = count_.load();
+        while (val != 0) {  // loop to protect against spurious wakeups
+            count_.wait(val);
+            val = count_.load();
         }
-        status_ = Status::Reached;
     }
 
   private:
-    std::atomic<std::size_t> count_{};
-    atomic<Status> status_{Status::Init};
+    atomic<std::size_t> count_{};
 };
 
 class NotificationQueue {
   public:
-    NotificationQueue(gsl::not_null<WaitableOneWayFlag*> queues_read,
-                      gsl::not_null<WaitableZeroCounter*> num_threads_running)
-        : queues_read_{std::move(queues_read)},
-          num_threads_running_{std::move(num_threads_running)} {}
+    explicit NotificationQueue(
+        gsl::not_null<WaitableZeroCounter*> total_workload)
+        : total_workload_{std::move(total_workload)} {}
 
     NotificationQueue(NotificationQueue const& other) = delete;
     NotificationQueue(NotificationQueue&& other) noexcept
         : queue_{std::move(other.queue_)},
           done_{other.done_},
-          queues_read_{std::move(other.queues_read_)},
-          num_threads_running_{std::move(other.num_threads_running_)} {}
+          total_workload_{std::move(other.total_workload_)} {}
     ~NotificationQueue() = default;
 
     [[nodiscard]] auto operator=(NotificationQueue const& other)
@@ -110,9 +65,9 @@ class NotificationQueue {
             return !queue_.empty() || done_;
         };
         if (not there_is_something_to_pop_or_we_are_done()) {
-            num_threads_running_->Decrement();
+            total_workload_->Decrement();
             ready_.wait(lock, there_is_something_to_pop_or_we_are_done);
-            num_threads_running_->Increment();
+            total_workload_->Increment();
         }
 
         if (queue_.empty()) {
@@ -120,7 +75,7 @@ class NotificationQueue {
         }
         auto t = std::move(queue_.front());
         queue_.pop_front();
-        queues_read_->Set();
+        total_workload_->Decrement();
         return t;
     }
 
@@ -133,7 +88,7 @@ class NotificationQueue {
         }
         auto t = std::move(queue_.front());
         queue_.pop_front();
-        queues_read_->Set();
+        total_workload_->Decrement();
         return t;
     }
 
@@ -145,7 +100,7 @@ class NotificationQueue {
             std::unique_lock lock{mutex_};
             queue_.emplace_back(std::forward<FunctionType>(f));
         }
-        queues_read_->Clear();
+        total_workload_->Increment();
         ready_.notify_one();
     }
 
@@ -160,7 +115,7 @@ class NotificationQueue {
             }
             queue_.emplace_back(std::forward<FunctionType>(f));
         }
-        queues_read_->Clear();
+        total_workload_->Increment();
         ready_.notify_one();
         return true;
     }
@@ -181,8 +136,7 @@ class NotificationQueue {
     bool done_{false};
     std::mutex mutex_{};
     std::condition_variable ready_{};
-    gsl::not_null<WaitableOneWayFlag*> queues_read_;
-    gsl::not_null<WaitableZeroCounter*> num_threads_running_;
+    gsl::not_null<WaitableZeroCounter*> total_workload_;
 };
 
 #endif  // INCLUDED_SRC_BUILDTOOL_MULTITHREADING_NOTIFICATION_QUEUE_HPP
diff --git a/src/buildtool/multithreading/task_system.cpp b/src/buildtool/multithreading/task_system.cpp
index 8c976a2f..4af57d0d 100644
--- a/src/buildtool/multithreading/task_system.cpp
+++ b/src/buildtool/multithreading/task_system.cpp
@@ -7,9 +7,9 @@ TaskSystem::TaskSystem() : TaskSystem(std::thread::hardware_concurrency()) {}
 
 TaskSystem::TaskSystem(std::size_t number_of_threads)
     : thread_count_{std::max(1UL, number_of_threads)},
-      num_threads_running_{thread_count_} {
+      total_workload_{thread_count_} {
     for (std::size_t index = 0; index < thread_count_; ++index) {
-        queues_.emplace_back(&queues_read_, &num_threads_running_);
+        queues_.emplace_back(&total_workload_);
     }
     for (std::size_t index = 0; index < thread_count_; ++index) {
         threads_.emplace_back([&, index]() { Run(index); });
@@ -19,11 +19,10 @@ TaskSystem::TaskSystem(std::size_t number_of_threads)
 TaskSystem::~TaskSystem() {
     // When starting a new task system all spawned threads will immediately go
     // to sleep and wait for tasks. Even after adding some tasks, it can take a
-    // while until the first thread wakes up. Therefore, we first need to wait
-    // for the queues being read, before we can wait for all threads to become
-    // idle.
-    queues_read_.WaitForSet();
-    num_threads_running_.WaitForZero();
+    // while until the first thread wakes up. Therefore, we need to wait for the
+    // total workload (number of active threads _and_ total number of queued
+    // tasks) to become zero.
+    total_workload_.WaitForZero();
     for (auto& q : queues_) {
         q.done();
     }
diff --git a/src/buildtool/multithreading/task_system.hpp b/src/buildtool/multithreading/task_system.hpp
index 20950142..6387988a 100644
--- a/src/buildtool/multithreading/task_system.hpp
+++ b/src/buildtool/multithreading/task_system.hpp
@@ -54,8 +54,7 @@ class TaskSystem {
     std::vector<std::thread> threads_{};
     std::vector<NotificationQueue> queues_{};
     std::atomic<std::size_t> index_{0};
-    WaitableOneWayFlag queues_read_{};
-    WaitableZeroCounter num_threads_running_{};
+    WaitableZeroCounter total_workload_{};
 
     static constexpr std::size_t kNumberOfAttempts = 5;