Use self-pipe trick to implement signal handlers

This allows the rest of the Gazebo codebase to use callbacks that can
use non async-signal-safe functions

Signed-off-by: Addisu Z. Taddese <addisu@openrobotics.org>

src/SignalHandler.cc

@@ -19,8 +19,11 @@
 // comments when upgrading to gz-cmake's "make codecheck"
 #include "gz/common/SignalHandler.hh" // NOLINT(*)
 #include <atomic>
+#include <cstring>
 #include <csignal> // NOLINT(*)
 #include <functional> // NOLINT(*)
+#include <fcntl.h>
+#include <unistd.h>
 #include <map> // NOLINT(*)
 #include <mutex> // NOLINT(*)
 #include <utility> // NOLINT(*)
@@ -31,13 +34,52 @@ using namespace gz;
 using namespace common;
 
 // A wrapper for the sigaction sa_handler.
+// TODO(azeey) We should avoid using objects with non-trivial destructors as globals.
 GZ_COMMON_VISIBLE std::map<int, std::function<void(int)>> gOnSignalWrappers;
 std::mutex gWrapperMutex;
 
+namespace
+{
+
+class SelfPipe
+{
+  public:
+  static int pipeFd[2];
+
+  public:
+  static void Initialize();
+
+  public:
+  ~SelfPipe();
+
+  private:
+  SelfPipe();
+
+  private:
+  void CheckPipe();
+
+  private:
+  std::thread checkPipeThread;
+
+  private:
+  std::atomic<bool> running{false};
+};
+
+int SelfPipe::pipeFd[2];
+
+void onSignalTopHalf(int _value)
+{
+  auto valueByte = static_cast<std::uint8_t>(_value);
+  if (write(SelfPipe::pipeFd[1], &valueByte, 1) == -1)
+  {
+    // TODO (azeey) Not clear what to do here.
+  }
+}
+
 /////////////////////////////////////////////////
 /// \brief Callback to execute when a signal is received.
 /// \param[in] _value Signal number.
-void onSignal(int _value)
+void onSignalBottomHalf(int _value)
 {
   std::lock_guard<std::mutex> lock(gWrapperMutex);
   // Send the signal to each wrapper
@@ -47,6 +89,55 @@ void onSignal(int _value)
   }
 }
 
+SelfPipe::SelfPipe()
+{
+  if (pipe(this->pipeFd))
+  {
+    gzerr << "Unable to create pipe\n";
+  }
+
+  int flags = fcntl(this->pipeFd[1], F_GETFL);
+  fcntl(this->pipeFd[1], F_SETFL, flags | O_NONBLOCK);
+  // TODO(azeey) Handle errors
+  this->running = true;
+  std::cout << "Initialized self pipe " << running << std::endl;
+  this->checkPipeThread = std::thread(&SelfPipe::CheckPipe, this);
+}
+
+SelfPipe::~SelfPipe()
+{
+  this->running = false;
+  onSignalTopHalf(127);
+  this->checkPipeThread.join();
+}
+void SelfPipe::Initialize()
+{
+  // We actually need this object to be destructed in order to join the thread,
+  // so we can't use gz::utils::NeverDestroyed here.
+  static SelfPipe selfPipe;
+}
+
+void SelfPipe::CheckPipe()
+{
+  while (this->running)
+  {
+    std::uint8_t signal;
+    if (read(SelfPipe::pipeFd[0], &signal, 1) != -1)
+    {
+      if (this->running)
+      {
+        onSignalBottomHalf(signal);
+      }
+    }
+    else
+    {
+      gzerr << errno << " " << std::strerror(errno) << std::endl;
+    }
+  }
+}
+
+}  // namespace
+
 /////////////////////////////////////////////////
 class common::SignalHandlerPrivate
 {
@@ -74,14 +165,15 @@ SignalHandler::SignalHandler()
   static int counter = 0;
   std::lock_guard<std::mutex> lock(gWrapperMutex);
 
-  if (std::signal(SIGINT, onSignal) == SIG_ERR)
+  SelfPipe::Initialize();
+  if (std::signal(SIGINT, onSignalTopHalf) == SIG_ERR)
   {
     gzerr << "Unable to catch SIGINT.\n"
            << " Please visit http://community.gazebosim.org for help.\n";
     return;
   }
 
-  if (std::signal(SIGTERM, onSignal) == SIG_ERR)
+  if (std::signal(SIGTERM, onSignalTopHalf) == SIG_ERR)
   {
     gzerr << "Unable to catch SIGTERM.\n"
            << " Please visit http://community.gazebosim.org for help.\n";