Implement BoundedChannel and multithreaded logging infra

tooling/pipeline/includes/concurrency/bounded_channel.h

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+#ifndef BIERGARTEN_PIPELINE_INCLUDES_CONCURRENCY_BOUNDED_CHANNEL_H_
+#define BIERGARTEN_PIPELINE_INCLUDES_CONCURRENCY_BOUNDED_CHANNEL_H_
+
+#include <condition_variable>
+#include <cstddef>
+#include <mutex>
+#include <optional>
+#include <queue>
+
+/**
+ * @file bounded_channel.h
+ * @brief Thread-safe, bounded multi-producer/multi-consumer synchronous channel.
+ *
+ * Intent: Enables asynchronous inter-thread communication with backpressure.
+ * Models a synchronous channel where producers/consumers block on capacity limits.
+ */
+
+/**
+ * @class BoundedChannel
+ * @brief MPMC channel with fixed capacity and blocking semantics.
+ *
+ * Producers block when buffer is full; consumers block when empty.
+ * Close() unblocks all waiters and signals channel exhaustion.
+ */
+template <typename T>
+class BoundedChannel {
+  // -------------------------------------------------------------------------
+  // Internal state — all access must be guarded by mutex_.
+  // -------------------------------------------------------------------------
+
+  std::queue<T> queue_;
+
+  std::mutex mutex_;
+
+  std::condition_variable not_full_;
+
+  std::condition_variable not_empty_;
+
+  std::size_t capacity_;
+
+  bool closed_ = false;
+
+ public:
+  /**
+   * @brief Construct a bounded channel with the given capacity.
+   * @param capacity Maximum number of items the channel may hold.
+   */
+  explicit BoundedChannel(std::size_t capacity) : capacity_(capacity) {}
+
+  /**
+   * @brief Send an item into the channel. Blocks when the channel is full.
+   * @param item Move-only item to enqueue.
+   */
+  void Send(T item);
+
+  /**
+   * @brief Receive an item from the channel. Blocks when the channel is
+   * empty.
+   * @return std::optional<T> containing the item, or std::nullopt when the
+   * channel is closed and drained.
+   */
+  std::optional<T> Receive();
+
+  /**
+   * @brief Close the channel and unblock all waiting threads. Idempotent.
+   */
+  void Close();
+};
+
+// Include the template implementation
+#include "bounded_channel.tcc"
+
+#endif  // BIERGARTEN_PIPELINE_INCLUDES_CONCURRENCY_BOUNDED_CHANNEL_H_

tooling/pipeline/includes/concurrency/bounded_channel.tcc

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+#include "bounded_channel.h"
+
+template <typename T>
+void BoundedChannel<T>::Send(T item) {
+  // Acquire exclusive ownership of the mutex; released automatically on scope exit.
+  std::unique_lock lock(mutex_);
+
+  // Block until there is space in the queue or the channel has been closed.
+  // The predicate guards against spurious wakeups.
+  not_full_.wait(lock, [&] { return queue_.size() < capacity_ || closed_; });
+
+  // If the channel was closed while waiting, discard the item and return.
+  if (closed_) return;
+
+  // Move the item into the queue to avoid an unnecessary copy.
+  queue_.push(std::move(item));
+
+  // Wake one blocked Receive() call to signal that data is now available.
+  not_empty_.notify_one();
+}
+
+template <typename T>
+std::optional<T> BoundedChannel<T>::Receive() {
+  // Acquire exclusive ownership of the mutex.
+  std::unique_lock lock(mutex_);
+
+  // Block until the queue is non-empty or the channel has been closed.
+  // The predicate guards against spurious wakeups.
+  not_empty_.wait(lock, [&] { return !queue_.empty() || closed_; });
+
+  // If woken due to closure and no items remain, signal exhaustion via nullopt.
+  if (queue_.empty()) return std::nullopt;
+
+  // Move the front item out of the queue to avoid an unnecessary copy.
+  T item = std::move(queue_.front());
+  queue_.pop();
+
+  // Wake one blocked Send() call to signal that a slot has opened.
+  not_full_.notify_one();
+
+  return item;
+}
+
+template <typename T>
+void BoundedChannel<T>::Close() {
+  // Acquire exclusive ownership of the mutex to ensure visibility of the flag.
+  std::unique_lock lock(mutex_);
+
+  // Mark the channel as closed; subsequent Send() calls will be dropped.
+  closed_ = true;
+
+  // Wake all blocked Send() callers so they can observe the closed flag and exit.
+  not_full_.notify_all();
+
+  // Wake all blocked Receive() callers so they can drain remaining items or return nullopt.
+  not_empty_.notify_all();
+}