From 197d7565603b2e8274f5c176f73b468ce6aa46a6 Mon Sep 17 00:00:00 2001
From: Morph <39850852+Morph1984@users.noreply.github.com>
Date: Sun, 19 Mar 2023 15:17:21 -0400
Subject: bounded_threadsafe_queue: Refactor Pop

Introduces PopModes to bring waiting logic into Pop, similar to Push.
---
 src/common/bounded_threadsafe_queue.h | 202 +++++++++++-----------------------
 1 file changed, 62 insertions(+), 140 deletions(-)

diff --git a/src/common/bounded_threadsafe_queue.h b/src/common/bounded_threadsafe_queue.h
index 0fb2f42d1..bd87aa09b 100644
--- a/src/common/bounded_threadsafe_queue.h
+++ b/src/common/bounded_threadsafe_queue.h
@@ -22,52 +22,38 @@ class SPSCQueue {
     static_assert((Capacity & (Capacity - 1)) == 0, "Capacity must be a power of two.");
 
 public:
-    bool TryPush(T&& t) {
-        return Push<PushMode::Try>(std::move(t));
-    }
-
     template <typename... Args>
     bool TryEmplace(Args&&... args) {
         return Emplace<PushMode::Try>(std::forward<Args>(args)...);
     }
 
-    void PushWait(T&& t) {
-        Push<PushMode::Wait>(std::move(t));
-    }
-
     template <typename... Args>
     void EmplaceWait(Args&&... args) {
         Emplace<PushMode::Wait>(std::forward<Args>(args)...);
     }
 
     bool TryPop(T& t) {
-        return Pop(t);
+        return Pop<PopMode::Try>(t);
+    }
+
+    void PopWait(T& t) {
+        Pop<PopMode::Wait>(t);
     }
 
     void PopWait(T& t, std::stop_token stop_token) {
-        ConsumerWait(stop_token);
-        Pop(t);
+        Pop<PopMode::WaitWithStopToken>(t, stop_token);
     }
 
-    T PopWait(std::stop_token stop_token) {
-        ConsumerWait(stop_token);
+    T PopWait() {
         T t;
-        Pop(t);
+        Pop<PopMode::Wait>(t);
         return t;
     }
 
-    void Clear() {
-        while (!Empty()) {
-            Pop();
-        }
-    }
-
-    bool Empty() const {
-        return m_read_index.load() == m_write_index.load();
-    }
-
-    size_t Size() const {
-        return m_write_index.load() - m_read_index.load();
+    T PopWait(std::stop_token stop_token) {
+        T t;
+        Pop<PopMode::WaitWithStopToken>(t, stop_token);
+        return t;
     }
 
 private:
@@ -77,55 +63,27 @@ private:
         Count,
     };
 
-    template <PushMode Mode>
-    bool Push(T&& t) {
-        const size_t write_index = m_write_index.load();
-
-        if constexpr (Mode == PushMode::Try) {
-            // Check if we have free slots to write to.
-            if ((write_index - m_read_index.load()) == Capacity) {
-                return false;
-            }
-        } else if constexpr (Mode == PushMode::Wait) {
-            // Wait until we have free slots to write to.
-            std::unique_lock lock{producer_cv_mutex};
-            producer_cv.wait(lock, [this, write_index] {
-                return (write_index - m_read_index.load()) < Capacity;
-            });
-        } else {
-            static_assert(Mode < PushMode::Count, "Invalid PushMode.");
-        }
-
-        // Determine the position to write to.
-        const size_t pos = write_index % Capacity;
-
-        // Push into the queue.
-        m_data[pos] = std::move(t);
-
-        // Increment the write index.
-        ++m_write_index;
-
-        // Notify the consumer that we have pushed into the queue.
-        std::scoped_lock lock{consumer_cv_mutex};
-        consumer_cv.notify_one();
-
-        return true;
-    }
+    enum class PopMode {
+        Try,
+        Wait,
+        WaitWithStopToken,
+        Count,
+    };
 
     template <PushMode Mode, typename... Args>
     bool Emplace(Args&&... args) {
-        const size_t write_index = m_write_index.load();
+        const size_t write_index = m_write_index.load(std::memory_order::relaxed);
 
         if constexpr (Mode == PushMode::Try) {
             // Check if we have free slots to write to.
-            if ((write_index - m_read_index.load()) == Capacity) {
+            if ((write_index - m_read_index.load(std::memory_order::acquire)) == Capacity) {
                 return false;
             }
         } else if constexpr (Mode == PushMode::Wait) {
             // Wait until we have free slots to write to.
             std::unique_lock lock{producer_cv_mutex};
             producer_cv.wait(lock, [this, write_index] {
-                return (write_index - m_read_index.load()) < Capacity;
+                return (write_index - m_read_index.load(std::memory_order::acquire)) < Capacity;
             });
         } else {
             static_assert(Mode < PushMode::Count, "Invalid PushMode.");
@@ -147,34 +105,32 @@ private:
         return true;
     }
 
-    void Pop() {
-        const size_t read_index = m_read_index.load();
-
-        // Check if the queue is empty.
-        if (read_index == m_write_index.load()) {
-            return;
-        }
-
-        // Determine the position to read from.
-        const size_t pos = read_index % Capacity;
-
-        // Pop the data off the queue, deleting it.
-        std::destroy_at(std::addressof(m_data[pos]));
-
-        // Increment the read index.
-        ++m_read_index;
-
-        // Notify the producer that we have popped off the queue.
-        std::unique_lock lock{producer_cv_mutex};
-        producer_cv.notify_one();
-    }
-
-    bool Pop(T& t) {
-        const size_t read_index = m_read_index.load();
+    template <PopMode Mode>
+    bool Pop(T& t, [[maybe_unused]] std::stop_token stop_token = {}) {
+        const size_t read_index = m_read_index.load(std::memory_order::relaxed);
 
-        // Check if the queue is empty.
-        if (read_index == m_write_index.load()) {
-            return false;
+        if constexpr (Mode == PopMode::Try) {
+            // Check if the queue is empty.
+            if (read_index == m_write_index.load(std::memory_order::acquire)) {
+                return false;
+            }
+        } else if constexpr (Mode == PopMode::Wait) {
+            // Wait until the queue is not empty.
+            std::unique_lock lock{consumer_cv_mutex};
+            consumer_cv.wait(lock, [this, read_index] {
+                return read_index != m_write_index.load(std::memory_order::acquire);
+            });
+        } else if constexpr (Mode == PopMode::WaitWithStopToken) {
+            // Wait until the queue is not empty.
+            std::unique_lock lock{consumer_cv_mutex};
+            Common::CondvarWait(consumer_cv, lock, stop_token, [this, read_index] {
+                return read_index != m_write_index.load(std::memory_order::acquire);
+            });
+            if (stop_token.stop_requested()) {
+                return false;
+            }
+        } else {
+            static_assert(Mode < PopMode::Count, "Invalid PopMode.");
         }
 
         // Determine the position to read from.
@@ -193,11 +149,6 @@ private:
         return true;
     }
 
-    void ConsumerWait(std::stop_token stop_token) {
-        std::unique_lock lock{consumer_cv_mutex};
-        Common::CondvarWait(consumer_cv, lock, stop_token, [this] { return !Empty(); });
-    }
-
     alignas(128) std::atomic_size_t m_read_index{0};
     alignas(128) std::atomic_size_t m_write_index{0};
 
@@ -212,22 +163,12 @@ private:
 template <typename T, size_t Capacity = detail::DefaultCapacity>
 class MPSCQueue {
 public:
-    bool TryPush(T&& t) {
-        std::scoped_lock lock{write_mutex};
-        return spsc_queue.TryPush(std::move(t));
-    }
-
     template <typename... Args>
     bool TryEmplace(Args&&... args) {
         std::scoped_lock lock{write_mutex};
         return spsc_queue.TryEmplace(std::forward<Args>(args)...);
     }
 
-    void PushWait(T&& t) {
-        std::scoped_lock lock{write_mutex};
-        spsc_queue.PushWait(std::move(t));
-    }
-
     template <typename... Args>
     void EmplaceWait(Args&&... args) {
         std::scoped_lock lock{write_mutex};
@@ -238,24 +179,20 @@ public:
         return spsc_queue.TryPop(t);
     }
 
-    void PopWait(T& t, std::stop_token stop_token) {
-        spsc_queue.PopWait(t, stop_token);
+    void PopWait(T& t) {
+        spsc_queue.PopWait(t);
     }
 
-    T PopWait(std::stop_token stop_token) {
-        return spsc_queue.PopWait(stop_token);
-    }
-
-    void Clear() {
-        spsc_queue.Clear();
+    void PopWait(T& t, std::stop_token stop_token) {
+        spsc_queue.PopWait(t, stop_token);
     }
 
-    bool Empty() {
-        return spsc_queue.Empty();
+    T PopWait() {
+        return spsc_queue.PopWait();
     }
 
-    size_t Size() {
-        return spsc_queue.Size();
+    T PopWait(std::stop_token stop_token) {
+        return spsc_queue.PopWait(stop_token);
     }
 
 private:
@@ -266,22 +203,12 @@ private:
 template <typename T, size_t Capacity = detail::DefaultCapacity>
 class MPMCQueue {
 public:
-    bool TryPush(T&& t) {
-        std::scoped_lock lock{write_mutex};
-        return spsc_queue.TryPush(std::move(t));
-    }
-
     template <typename... Args>
     bool TryEmplace(Args&&... args) {
         std::scoped_lock lock{write_mutex};
         return spsc_queue.TryEmplace(std::forward<Args>(args)...);
     }
 
-    void PushWait(T&& t) {
-        std::scoped_lock lock{write_mutex};
-        spsc_queue.PushWait(std::move(t));
-    }
-
     template <typename... Args>
     void EmplaceWait(Args&&... args) {
         std::scoped_lock lock{write_mutex};
@@ -293,29 +220,24 @@ public:
         return spsc_queue.TryPop(t);
     }
 
-    void PopWait(T& t, std::stop_token stop_token) {
-        std::scoped_lock lock{read_mutex};
-        spsc_queue.PopWait(t, stop_token);
-    }
-
-    T PopWait(std::stop_token stop_token) {
+    void PopWait(T& t) {
         std::scoped_lock lock{read_mutex};
-        return spsc_queue.PopWait(stop_token);
+        spsc_queue.PopWait(t);
     }
 
-    void Clear() {
+    void PopWait(T& t, std::stop_token stop_token) {
         std::scoped_lock lock{read_mutex};
-        spsc_queue.Clear();
+        spsc_queue.PopWait(t, stop_token);
     }
 
-    bool Empty() {
+    T PopWait() {
         std::scoped_lock lock{read_mutex};
-        return spsc_queue.Empty();
+        return spsc_queue.PopWait();
     }
 
-    size_t Size() {
+    T PopWait(std::stop_token stop_token) {
         std::scoped_lock lock{read_mutex};
-        return spsc_queue.Size();
+        return spsc_queue.PopWait(stop_token);
     }
 
 private:
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