feat: add self-assessment document detailing challenges and design decisions

Reviewed-on: #7

CMakeLists.txt

@@ -8,13 +8,31 @@ project(azkoyen_ipc_test LANGUAGES CXX)
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
+# Qt Setup — AUTOMOC runs moc automatically on Q_OBJECT headers
+find_package(Qt5 REQUIRED COMPONENTS Core Widgets Test)
+set(CMAKE_AUTOMOC ON)
+set(CMAKE_INCLUDE_CURRENT_DIR ON)
+
 # Core library
 add_library(core
     src/core/SysfsRead.cxx
+    src/core/UnixIpcBridge.cxx
     src/core/Producer.cxx
+    src/core/Consumer.cxx
+    include/Consumer.hpp
 )
 
 target_include_directories(core PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/include)
+target_link_libraries(core PUBLIC Qt5::Core)
+
+# Main Application
+add_executable(app
+    src/app/main.cxx
+    src/app/MainWindow.cxx
+    include/MainWindow.hpp
+)
+target_include_directories(app PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/include)
+target_link_libraries(app PRIVATE core Qt5::Widgets)
 
 #tests
 enable_testing()

README.md

@@ -25,3 +25,23 @@ The reader never throws on I/O errors; every outcome is expressed through the en
 ## Producer class / thread
 
 `Producer` ([include/Producer.hpp](include/Producer.hpp), [src/core/Producer.cxx](src/core/Producer.cxx)) runs a worker `std::thread` that periodically polls the `SysfsReader` and, when the status is `Enabled`, generates a random integer and forwards it through an injected `send_fn` callback. The polling interval is 1 second under normal conditions and 7 seconds when the sysfs file reports `ErrorTempTooHigh` (cool-down).
+
+## UnixIpcBridge
+
+>[!note]
+>why unix domain sockets? Because I have more experience with them under linux than with posix shared memmory and semaphore, and I find them easier to unit-test.
+
+`UnixIpcBridge` ([include/UnixIpcBridge.hpp](include/UnixIpcBridge.hpp), [src/core/UnixIpcBridge.cxx](src/core/UnixIpcBridge.cxx)) is a small helper that connects to a UNIX domain socket and sends a single `int` per call. It opens a new connection for each value, which keeps the protocol stateless and simple.
+
+**Tests:** [tests/test_unix_ipc.cxx](tests/test_unix_ipc.cxx) — spins up a fake socket server, sends values through the bridge, and asserts they arrive correctly.
+
+## ConsumerThread
+
+`ConsumerThread` ([include/ConsumerThread.hpp](include/ConsumerThread.hpp), [src/core/ConsumerThread.cxx](src/core/ConsumerThread.cxx)) is a `QObject` that listens on a UNIX domain socket in a background `std::thread`. On each received integer it:
+
+1. Prints the value to `stdout`.
+2. Emits the `valueReceived(int)` Qt signal.
+
+The server socket is created and bound inside `start()` **before** the thread is spawned, so the socket is guaranteed to be ready by the time `start()` returns — eliminating race conditions with the producer. Graceful shutdown is handled by `stop()`, which shuts down the file descriptor to unblock the blocking `accept()` call.
+
+**Tests:** [tests/test_consumer_thread.cxx](tests/test_consumer_thread.cxx) — uses `QSignalSpy` to verify single-value, multi-value, negative, and zero reception.

docs/self-assessment.md

@@ -0,0 +1,25 @@
+# Self-Assessment
+
+## Two Real Difficulties
+
+**1. Maintaining TDD discipline under time pressure**
+
+Sticking to a strict test-first workflow throughout the session was genuinely hard. Between the deadline and the accumulated fatigue of a full day of work beforehand, there were moments where the temptation to just write the implementation and then fill the tests was real. I did not always resist it. Some tests were written after the fact rather than before, which is something I am aware of and want to be honest about.
+
+**2. Designing testable seams at the IPC and sysfs boundaries**
+
+The components that most needed testing were also the ones most coupled to external resources: a live socket and a real sysfs path. The difficulty was finding the right abstraction level, too thin and the tests require actual kernel resources; too thick and you end up testing your mocks, not your logic. The solution was to inject the transport as a plain `std::function` callback into the producer, and to point the sysfs reader at a controlled fake file on disk. Both approaches keep the core logic testable with no sockets, no threads, and no Qt, but arriving at that boundary (deciding what to abstract and what to leave concrete) required more iteration than I anticipated.
+
+---
+
+## Alternative IPC Mechanism Considered
+
+I evaluated POSIX shared memory with semaphores as an alternative to UNIX domain sockets. The theoretical appeal is clear: no serialization, no kernel-mediated data copy, potentially lower latency. However, I am considerably less practiced with `shm_open`/`mmap`/`sem_post` than I am with socket-based communication, and more importantly, shared memory is significantly harder to unit-test in isolation. Sockets expose a clean, file-descriptor-based interface that maps naturally to mock-able abstractions. Shared memory regions and semaphore lifecycles would have added complexity to the test harness for uncertain gain at this data rate. Domain sockets were the pragmatic choice.
+
+---
+
+## Design Decision Changed Mid-Development
+
+Initially I had planned a looser boundary between the core logic and Qt, with the producer potentially depending on Qt primitives for threading or signalling. Early on, I decided to keep Qt strictly confined to the GUI layer and the consumer thread, nothing more. The producer, the sysfs reader, and the IPC bridge are plain C++ with no Qt dependency whatsoever.
+
+The reason is simple: that code could be portable. If tomorrow the producer needs to run on a microcontroller, a bare-metal embedded target, or any environment where Qt is not available or not desirable, the only thing that needs replacing is the transport callback. The core logic moves untouched. It also makes unit-testing the producer significantly cleaner and easier, no Qt test infrastructure needed, just standard C++.

include/Consumer.hpp

@@ -0,0 +1,44 @@
+#pragma once
+// ConsumerThread.hpp
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Author: Unai Blazquez <unaibg2000@gmail.com>
+#include <QObject>
+#include <atomic>
+#include <string>
+#include <thread>
+
+/// @brief Listens on a UNIX domain socket, receives integers from the
+///        producer via IPC, prints them to console, and emits a Qt signal.
+class ConsumerThread : public QObject
+{
+  Q_OBJECT
+
+ public:
+  /// @brief Construct the consumer bound to a socket path.
+  /// @param socket_path UNIX domain socket path to listen on.
+  /// @param parent Optional QObject parent for memory management.
+  explicit ConsumerThread(const std::string& socket_path,
+                          QObject* parent = nullptr);
+
+  ~ConsumerThread() override;
+
+  /// @brief Start the listener thread. The server socket is ready
+  ///        when this function returns.
+  void start();
+
+  /// @brief Stop the listener thread gracefully. Safe to call multiple times.
+  void stop();
+
+ signals:
+  /// @brief Emitted every time an integer is received from the producer.
+  void valueReceived(int value);
+
+ private:
+  /// @brief Main loop: accept → recv → print → emit. Runs in m_thread.
+  void run_loop();
+
+  std::string m_socket_path;
+  int m_server_fd = -1;
+  std::atomic<bool> m_running{false};
+  std::thread m_thread;
+};

include/MainWindow.hpp

@@ -0,0 +1,32 @@
+#pragma once
+// MainWindow.hpp
+// SPDX-License-Identifier: GPL-3.0-only
+// Author: Unai Blazquez <unaibg2000@gmail.com>
+
+#include <QLabel>
+#include <QString>
+#include <QVBoxLayout>
+#include <QWidget>
+
+/// @brief Minimal GUI window that displays the last integer received
+///        from the ConsumerThread. Never blocks — values arrive via
+///        Qt's queued signal/slot mechanism.
+class MainWindow : public QWidget
+{
+  Q_OBJECT
+
+ public:
+  explicit MainWindow(QWidget* parent = nullptr);
+
+  /// @brief Returns the current text shown in the value label (for testing).
+  QString lastDisplayedText() const;
+
+ public slots:
+  /// @brief Slot connected to ConsumerThread::valueReceived.
+  ///        Updates the label with the new value.
+  void onValueReceived(int value);
+
+ private:
+  QLabel* m_title_label;
+  QLabel* m_value_label;
+};

include/UnixIpcBridge.hpp

@@ -0,0 +1,31 @@
+// UnixIpcBridge.hpp
+// SPDX-License-Identifier: GPL-3.0-only
+// Author: Unai Blazquez <unaibg2000@gmail.com>
+
+#pragma once
+#include <fcntl.h>  // non‑blocking
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <unistd.h>  // close()
+
+#include <string>
+
+///@brief Small bridge to allow the producer class to send data over UNIX domain
+/// sockets
+class UnixIpcBridge
+{
+ public:
+  ///@brief constructor
+  ///@param socket path pointing the socket
+  explicit UnixIpcBridge(const std::string& socket_path);
+
+  ///@brief sending function, this goes into the producer
+  ///@param integer to send over the socket
+  void send(int value);
+
+ private:
+  std::string m_socket_path;
+  int m_socket_fd = -1;
+
+  void connect_to_consumer();
+};

src/app/MainWindow.cxx

@@ -0,0 +1,33 @@
+// MainWindow.cxx
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Author: Unai Blazquez <unaibg2000@gmail.com>
+
+#include "MainWindow.hpp"
+
+MainWindow::MainWindow(QWidget* parent) : QWidget(parent)
+{
+  setWindowTitle("Azkoyen IPC Monitor");
+  setMinimumSize(320, 120);
+
+  auto* layout = new QVBoxLayout(this);
+
+  m_title_label = new QLabel("Last received value:", this);
+  m_value_label = new QLabel("(waiting...)", this);
+
+  // Make the value label stand out a bit
+  QFont font = m_value_label->font();
+  font.setPointSize(24);
+  font.setBold(true);
+  m_value_label->setFont(font);
+  m_value_label->setAlignment(Qt::AlignCenter);
+
+  layout->addWidget(m_title_label);
+  layout->addWidget(m_value_label);
+}
+
+QString MainWindow::lastDisplayedText() const { return m_value_label->text(); }
+
+void MainWindow::onValueReceived(int value)
+{
+  m_value_label->setText(QString::number(value));
+}

src/app/main.cxx

@@ -0,0 +1,58 @@
+^// main.cxx
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Author: Unai Blazquez <unaibg2000@gmail.com>
+
+#include <QApplication>
+#include <cstdlib>
+#include <fstream>
+#include <iostream>
+
+#include "Consumer.hpp"
+#include "MainWindow.hpp"
+#include "Producer.hpp"
+#include "UnixIpcBridge.hpp"
+
+int main(int argc, char* argv[])
+{
+  QApplication app(argc, argv);
+
+  const std::string socket_path = "/tmp/azkoyen.sock";
+  const std::string sysfs_path = "./fake_sysfs_input";
+
+  // 1. Consumer — listens on the socket, emits Qt signal on receive
+  ConsumerThread consumer(socket_path);
+
+  // 2. GUI — minimal window that displays received values
+  MainWindow window;
+  window.show();
+
+  // Connect consumer signal → window slot (auto-queued across threads,
+  // so the GUI never blocks even if the producer is stuck in cool-down)
+  QObject::connect(&consumer, &ConsumerThread::valueReceived, &window,
+                   &MainWindow::onValueReceived);
+
+  consumer.start();
+
+  // 3. Bridge — sends ints over the UNIX domain socket
+  UnixIpcBridge bridge(socket_path);
+
+  // 4. Producer — reads sysfs, generates random int, sends via bridge.
+  //    Logs to a file instead of console (console is for the consumer).
+  std::ofstream log_file("producer.log", std::ios::app);
+
+  Producer producer(
+      sysfs_path, [&bridge](int value) { bridge.send(value); },
+      []() { return std::rand() % 1000; },
+      [&log_file](const std::string& msg) { log_file << msg << std::endl; });
+
+  producer.start();
+
+  // 5. Run the Qt event loop (GUI stays responsive, signals are delivered)
+  int result = app.exec();
+
+  // 6. Graceful shutdown
+  producer.stop();
+  consumer.stop();
+
+  return result;
+}

src/core/Consumer.cxx

@@ -0,0 +1,104 @@
+// ConsumerThread.cxx
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Author: Unai Blazquez <unaibg2000@gmail.com>
+
+#include "Consumer.hpp"
+
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <unistd.h>
+
+#include <cstring>
+#include <iostream>
+#include <stdexcept>
+
+ConsumerThread::ConsumerThread(const std::string& socket_path, QObject* parent)
+    : QObject(parent), m_socket_path(socket_path)
+{
+}
+
+ConsumerThread::~ConsumerThread() { stop(); }
+
+void ConsumerThread::start()
+{
+  // Remove stale socket from previous runs
+  unlink(m_socket_path.c_str());
+
+  // Create, bind and listen BEFORE spawning the thread so the socket
+  // is guaranteed ready when start() returns — no race with the producer.
+  m_server_fd = socket(AF_UNIX, SOCK_STREAM, 0);
+  if (m_server_fd < 0)
+  {
+    throw std::runtime_error("ConsumerThread: socket() failed");
+  }
+
+  struct sockaddr_un addr = {};
+  addr.sun_family = AF_UNIX;
+  std::strncpy(addr.sun_path, m_socket_path.c_str(), sizeof(addr.sun_path) - 1);
+
+  if (bind(m_server_fd, reinterpret_cast<sockaddr*>(&addr), sizeof(addr)) < 0)
+  {
+    close(m_server_fd);
+    m_server_fd = -1;
+    throw std::runtime_error("ConsumerThread: bind() failed");
+  }
+
+  if (listen(m_server_fd, 5) < 0)
+  {
+    close(m_server_fd);
+    m_server_fd = -1;
+    throw std::runtime_error("ConsumerThread: listen() failed");
+  }
+
+  m_running.store(true);
+  m_thread = std::thread(&ConsumerThread::run_loop, this);
+}
+
+void ConsumerThread::stop()
+{
+  if (!m_running.exchange(false))
+  {
+    return;  // already stopped or never started
+  }
+
+  // Shutdown the server fd to unblock the blocking accept() call
+  if (m_server_fd >= 0)
+  {
+    shutdown(m_server_fd, SHUT_RDWR);
+    close(m_server_fd);
+    m_server_fd = -1;
+  }
+
+  if (m_thread.joinable())
+  {
+    m_thread.join();
+  }
+
+  unlink(m_socket_path.c_str());
+}
+
+void ConsumerThread::run_loop()
+{
+  while (m_running.load())
+  {
+    int client_fd = accept(m_server_fd, nullptr, nullptr);
+    if (client_fd < 0)
+    {
+      // accept() failed — most likely stop() closed the fd
+      break;
+    }
+
+    int value = 0;
+    ssize_t n = recv(client_fd, &value, sizeof(value), MSG_WAITALL);
+    close(client_fd);
+
+    if (n == static_cast<ssize_t>(sizeof(value)))
+    {
+      // 1) Print to console (spec requirement)
+      std::cout << "ConsumerThread received: " << value << std::endl;
+
+      // 2) Emit Qt signal (spec requirement)
+      emit valueReceived(value);
+    }
+  }
+}

src/core/UnixIpcBridge.cxx

@@ -0,0 +1,49 @@
+// UnixIpcBridge.cxx
+// SPDX-License-Identifier: GPL-3.0-only
+// Author: Unai Blazquez <unaibg2000@gmail.com>
+#include "UnixIpcBridge.hpp"
+
+#include <cstring>
+#include <stdexcept>
+
+UnixIpcBridge::UnixIpcBridge(const std::string& socket_path)
+    : m_socket_path(socket_path)
+{
+}
+
+void UnixIpcBridge::send(int value)
+{
+  connect_to_consumer();
+
+  ssize_t n = ::send(m_socket_fd, &value, sizeof(value), 0);
+  if (n != sizeof(value))
+  {
+    close(m_socket_fd);
+    m_socket_fd = -1;
+    throw std::runtime_error("UnixIpcBridge::send: failed to write value");
+  }
+
+  close(m_socket_fd);
+  m_socket_fd = -1;
+}
+
+void UnixIpcBridge::connect_to_consumer()
+{
+  m_socket_fd = socket(AF_UNIX, SOCK_STREAM, 0);
+  if (m_socket_fd < 0)
+  {
+    throw std::runtime_error("UnixIpcBridge: socket() failed");
+  }
+
+  struct sockaddr_un addr = {};
+  addr.sun_family = AF_UNIX;
+  std::strncpy(addr.sun_path, m_socket_path.c_str(), sizeof(addr.sun_path) - 1);
+
+  if (connect(m_socket_fd, reinterpret_cast<sockaddr*>(&addr), sizeof(addr)) <
+      0)
+  {
+    close(m_socket_fd);
+    m_socket_fd = -1;
+    throw std::runtime_error("UnixIpcBridge: connect() failed");
+  }
+}

tests/CMakeLists.txt

@@ -25,3 +25,51 @@ target_link_libraries(test_producer
 
 add_test(NAME test_producer COMMAND test_producer)
 
+add_executable(test_ipc
+    test_unix_ipc.cxx
+)
+
+target_link_libraries(test_ipc
+    PRIVATE
+        core
+        gtest
+        gtest_main
+)
+
+add_test(NAME test_ipc COMMAND test_ipc)
+
+
+add_executable(test_consumer
+    test_consumer.cxx
+)
+
+target_link_libraries(test_consumer
+    PRIVATE
+        core
+        gtest
+        gtest_main
+        Qt5::Core
+        Qt5::Test
+)
+
+add_test(NAME test_consumer COMMAND test_consumer)
+
+add_executable(test_main_window
+    test_main_window.cxx
+    ${CMAKE_SOURCE_DIR}/src/app/MainWindow.cxx
+    ${CMAKE_SOURCE_DIR}/include/MainWindow.hpp
+)
+
+target_include_directories(test_main_window PRIVATE ${CMAKE_SOURCE_DIR}/include)
+
+target_link_libraries(test_main_window
+    PRIVATE
+        core
+        gtest
+        gtest_main
+        Qt5::Core
+        Qt5::Widgets
+        Qt5::Test
+)
+
+add_test(NAME test_main_window COMMAND test_main_window)

tests/test_consumer.cxx

@@ -0,0 +1,120 @@
+#include <gtest/gtest.h>
+
+#include <QCoreApplication>
+#include <QSignalSpy>
+
+#include "Consumer.hpp"
+#include "UnixIpcBridge.hpp"
+
+// QSignalSpy needs a QCoreApplication to dispatch queued signals
+static int argc_ = 0;
+static QCoreApplication app_(argc_, nullptr);
+
+TEST(ConsumerThreadTest, ReceivesSingleValue)
+{
+  const std::string sock = "/tmp/test_ct_single.sock";
+
+  ConsumerThread consumer(sock);
+
+  // QSignalSpy records every emission of the given signal
+  QSignalSpy spy(&consumer, &ConsumerThread::valueReceived);
+  consumer.start();
+
+  UnixIpcBridge bridge(sock);
+  bridge.send(42);
+
+  // spy.wait() pumps the event loop for up to 1s until a signal arrives
+  spy.wait(1000);
+  consumer.stop();
+
+  ASSERT_EQ(spy.count(), 1);
+  EXPECT_EQ(spy.at(0).at(0).toInt(), 42);
+}
+
+TEST(ConsumerThreadTest, ReceivesMultipleValues)
+{
+  const std::string sock = "/tmp/test_ct_multi.sock";
+
+  ConsumerThread consumer(sock);
+  QSignalSpy spy(&consumer, &ConsumerThread::valueReceived);
+  consumer.start();
+
+  constexpr int kMessages = 5;
+  for (int i = 0; i < kMessages; ++i)
+  {
+    UnixIpcBridge bridge(sock);
+    bridge.send(i * 10);
+    // Small delay so the consumer can re-enter accept() between sends
+    std::this_thread::sleep_for(std::chrono::milliseconds(10));
+  }
+
+  // Wait until all signals arrive (or timeout after 5s)
+  for (int attempt = 0; spy.count() < kMessages && attempt < 50; ++attempt)
+  {
+    spy.wait(100);
+  }
+
+  consumer.stop();
+
+  ASSERT_EQ(spy.count(), kMessages);
+  for (int i = 0; i < kMessages; ++i)
+  {
+    EXPECT_EQ(spy.at(i).at(0).toInt(), i * 10);
+  }
+}
+
+TEST(ConsumerThreadTest, ReceivesNegativeAndZero)
+{
+  // Zero
+  {
+    const std::string sock = "/tmp/test_ct_zero.sock";
+    ConsumerThread consumer(sock);
+    QSignalSpy spy(&consumer, &ConsumerThread::valueReceived);
+    consumer.start();
+
+    UnixIpcBridge bridge(sock);
+    bridge.send(0);
+
+    spy.wait(1000);
+    consumer.stop();
+
+    ASSERT_EQ(spy.count(), 1);
+    EXPECT_EQ(spy.at(0).at(0).toInt(), 0);
+  }
+
+  // Negative
+  {
+    const std::string sock = "/tmp/test_ct_neg.sock";
+    ConsumerThread consumer(sock);
+    QSignalSpy spy(&consumer, &ConsumerThread::valueReceived);
+    consumer.start();
+
+    UnixIpcBridge bridge(sock);
+    bridge.send(-999);
+
+    spy.wait(1000);
+    consumer.stop();
+
+    ASSERT_EQ(spy.count(), 1);
+    EXPECT_EQ(spy.at(0).at(0).toInt(), -999);
+  }
+}
+
+TEST(ConsumerThreadTest, StopsCleanlyWithoutDeadlock)
+{
+  const std::string sock = "/tmp/test_ct_stop.sock";
+
+  ConsumerThread consumer(sock);
+  consumer.start();
+  // stop() must return without hanging, even with no connections
+  consumer.stop();
+}
+
+TEST(ConsumerThreadTest, StopsCleanlyWhenNeverStarted)
+{
+  const std::string sock = "/tmp/test_ct_nostart.sock";
+
+  ConsumerThread consumer(sock);
+  // stop() on a consumer that was never started must not crash
+  consumer.stop();
+}

tests/test_main_window.cxx

@@ -0,0 +1,47 @@
+#include <gtest/gtest.h>
+
+#include <QApplication>
+#include <QLabel>
+#include <QSignalSpy>
+#include <cstdlib>
+
+#include "MainWindow.hpp"
+
+// QWidget-based tests need a full QApplication (not QCoreApplication).
+// Use offscreen platform so tests run headless in containers.
+static int argc_ = 0;
+static char* argv_[] = {nullptr};
+static struct SetupOffscreen
+{
+  SetupOffscreen() { qputenv("QT_QPA_PLATFORM", "offscreen"); }
+} setup_offscreen_;
+static QApplication app_(argc_, argv_);
+
+TEST(MainWindowTest, LabelUpdatesOnValueReceived)
+{
+  MainWindow window;
+  // Simulate receiving a value from ConsumerThread
+  window.onValueReceived(42);
+
+  // The label should display the received value
+  EXPECT_NE(window.lastDisplayedText().toStdString().find("42"),
+            std::string::npos);
+}
+
+TEST(MainWindowTest, LabelUpdatesMultipleTimes)
+{
+  MainWindow window;
+  window.onValueReceived(10);
+  window.onValueReceived(20);
+  window.onValueReceived(30);
+
+  // Label should show the most recent value
+  EXPECT_NE(window.lastDisplayedText().toStdString().find("30"),
+            std::string::npos);
+}
+
+TEST(MainWindowTest, WindowTitleIsSet)
+{
+  MainWindow window;
+  EXPECT_FALSE(window.windowTitle().isEmpty());
+}

tests/test_unix_ipc.cxx

@@ -0,0 +1,220 @@
+#include <gtest/gtest.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <unistd.h>
+
+#include <atomic>
+#include <chrono>
+#include <cstring>
+#include <stdexcept>
+#include <string>
+#include <thread>
+#include <vector>
+
+#include "UnixIpcBridge.hpp"
+
+// ---------------------------------------------------------------------------
+// Helper: a minimal UNIX-domain socket server that accepts one connection,
+// reads `count` ints, then tears down cleanly.  Uses a ready-flag so the
+// client never races against bind/listen.
+// ---------------------------------------------------------------------------
+class FakeConsumer
+{
+ public:
+  explicit FakeConsumer(const std::string& path, int count = 1)
+      : m_path(path), m_count(count)
+  {
+    // Remove stale socket from any previous failed run
+    unlink(m_path.c_str());
+  }
+
+  /// Start the server on a background thread.
+  void start()
+  {
+    m_thread = std::thread([this] { run(); });
+
+    // Spin until the server signals it is listening (bounded wait).
+    auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(5);
+    while (!m_ready.load(std::memory_order_acquire))
+    {
+      if (std::chrono::steady_clock::now() > deadline)
+      {
+        throw std::runtime_error("FakeConsumer: server failed to start");
+      }
+      std::this_thread::sleep_for(std::chrono::milliseconds(1));
+    }
+  }
+
+  /// Block until the server thread finishes.
+  void join() { m_thread.join(); }
+
+  /// Values received in order.
+  const std::vector<int>& received() const { return m_received; }
+
+  ~FakeConsumer() { unlink(m_path.c_str()); }
+
+ private:
+  void run()
+  {
+    m_server_fd = socket(AF_UNIX, SOCK_STREAM, 0);
+    ASSERT_GE(m_server_fd, 0) << "socket() failed: " << strerror(errno);
+
+    struct sockaddr_un addr = {};
+    addr.sun_family = AF_UNIX;
+    std::strncpy(addr.sun_path, m_path.c_str(), sizeof(addr.sun_path) - 1);
+
+    ASSERT_EQ(
+        bind(m_server_fd, reinterpret_cast<sockaddr*>(&addr), sizeof(addr)), 0)
+        << "bind() failed: " << strerror(errno);
+    ASSERT_EQ(listen(m_server_fd, 1), 0)
+        << "listen() failed: " << strerror(errno);
+
+    // Signal that we are ready to accept connections.
+    m_ready.store(true, std::memory_order_release);
+
+    int client_fd = accept(m_server_fd, nullptr, nullptr);
+    ASSERT_GE(client_fd, 0) << "accept() failed: " << strerror(errno);
+
+    for (int i = 0; i < m_count; ++i)
+    {
+      int value = 0;
+      ssize_t n = recv(client_fd, &value, sizeof(value), MSG_WAITALL);
+      ASSERT_EQ(n, static_cast<ssize_t>(sizeof(value)))
+          << "recv() short read on message " << i;
+      m_received.push_back(value);
+    }
+
+    close(client_fd);
+    close(m_server_fd);
+  }
+
+  std::string m_path;
+  int m_count;
+  int m_server_fd = -1;
+  std::atomic<bool> m_ready{false};
+  std::thread m_thread;
+  std::vector<int> m_received;
+};
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+/// Sends a single integer value and verifies the consumer receives it.
+TEST(UnixIpcBridgeTest, SendsSingleInt)
+{
+  const std::string sock = "/tmp/test_ipc_single.sock";
+
+  FakeConsumer consumer(sock, /*count=*/1);
+  consumer.start();
+
+  UnixIpcBridge bridge(sock);
+  bridge.send(42);
+
+  consumer.join();
+
+  ASSERT_EQ(consumer.received().size(), 1u);
+  EXPECT_EQ(consumer.received()[0], 42);
+}
+
+/// Sends zero and a negative value — makes sure sign bits survive.
+TEST(UnixIpcBridgeTest, SendsZeroAndNegativeValues)
+{
+  // Zero
+  {
+    const std::string sock = "/tmp/test_ipc_zero.sock";
+    FakeConsumer consumer(sock, 1);
+    consumer.start();
+
+    UnixIpcBridge bridge(sock);
+    bridge.send(0);
+
+    consumer.join();
+    ASSERT_EQ(consumer.received().size(), 1u);
+    EXPECT_EQ(consumer.received()[0], 0);
+  }
+
+  // Negative
+  {
+    const std::string sock = "/tmp/test_ipc_neg.sock";
+    FakeConsumer consumer(sock, 1);
+    consumer.start();
+
+    UnixIpcBridge bridge(sock);
+    bridge.send(-1);
+
+    consumer.join();
+    ASSERT_EQ(consumer.received().size(), 1u);
+    EXPECT_EQ(consumer.received()[0], -1);
+  }
+}
+
+/// Sends INT_MAX / INT_MIN to check for truncation or overflow.
+TEST(UnixIpcBridgeTest, SendsExtremeBoundaryValues)
+{
+  {
+    const std::string sock = "/tmp/test_ipc_max.sock";
+    FakeConsumer consumer(sock, 1);
+    consumer.start();
+
+    UnixIpcBridge bridge(sock);
+    bridge.send(std::numeric_limits<int>::max());
+
+    consumer.join();
+    ASSERT_EQ(consumer.received().size(), 1u);
+    EXPECT_EQ(consumer.received()[0], std::numeric_limits<int>::max());
+  }
+
+  {
+    const std::string sock = "/tmp/test_ipc_min.sock";
+    FakeConsumer consumer(sock, 1);
+    consumer.start();
+
+    UnixIpcBridge bridge(sock);
+    bridge.send(std::numeric_limits<int>::min());
+
+    consumer.join();
+    ASSERT_EQ(consumer.received().size(), 1u);
+    EXPECT_EQ(consumer.received()[0], std::numeric_limits<int>::min());
+  }
+}
+
+/// Connecting to a non-existent socket must throw, not silently fail.
+TEST(UnixIpcBridgeTest, ThrowsWhenNoConsumerListening)
+{
+  const std::string sock = "/tmp/test_ipc_noserver.sock";
+  unlink(sock.c_str());  // make sure nothing is there
+
+  UnixIpcBridge bridge(sock);
+  EXPECT_THROW(bridge.send(99), std::runtime_error);
+}
+
+/// Multiple sequential sends (each reopens the connection).
+TEST(UnixIpcBridgeTest, MultipleSendsSequentially)
+{
+  const std::string sock = "/tmp/test_ipc_multi.sock";
+  constexpr int kMessages = 5;
+
+  // Server expects exactly kMessages ints from kMessages connections.
+  // Because the bridge reconnects every send(), we run kMessages
+  // single-message consumers sequentially.
+  std::vector<int> all_received;
+  for (int i = 0; i < kMessages; ++i)
+  {
+    FakeConsumer consumer(sock, 1);
+    consumer.start();
+
+    UnixIpcBridge bridge(sock);
+    bridge.send(i * 10);
+
+    consumer.join();
+    ASSERT_EQ(consumer.received().size(), 1u);
+    all_received.push_back(consumer.received()[0]);
+  }
+
+  ASSERT_EQ(all_received.size(), static_cast<size_t>(kMessages));
+  for (int i = 0; i < kMessages; ++i)
+  {
+    EXPECT_EQ(all_received[i], i * 10);
+  }
+}