fix: Handle X events before polling for IO (#2820)

* Use m_connection.poll_for_event

* fix: Handle X events before polling for IO

Polling the XCB file descriptor for X events doesn't detect events that
are already in XCB's event queue but not yet handled.

If this happens, the eventloop polls for IO and the queued events wait
until another event arrives, causing some desyncs in the bar.

This can easily happen if something (e.g. a click event) triggers some
xcb calls, which as a consequence buffer some incoming events.

We "fix" this by adding a libuv prepare handle (which runs right before
polling for IO) that processes pending X events.

include/components/eventloop.hpp

@@ -390,12 +390,27 @@ namespace eventloop {
     cb_connect connect_callback;
   };
 
+  class PrepareHandle : public Handle<PrepareHandle, uv_prepare_t> {
+   public:
+    using Handle::Handle;
+    using cb = cb_void;
+
+    void init();
+    void start(cb user_cb);
+
+   private:
+    static void connect_cb(uv_connect_t* req, int status);
+
+    cb callback;
+  };
+
   class loop : public non_copyable_mixin, public non_movable_mixin {
    public:
     loop();
     ~loop();
     void run();
     void stop();
+    uint64_t now() const;
 
     template <typename H, typename... Args>
     H& handle(Args... args) {
@@ -404,13 +419,12 @@ namespace eventloop {
       return ptr->leak(std::move(ptr));
     }
 
-   protected:
     uv_loop_t* get() const;
 
    private:
     std::unique_ptr<uv_loop_t> m_loop{nullptr};
   };
 
-}  // namespace eventloop
+} // namespace eventloop
 
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src/components/controller.cpp

@@ -171,7 +171,7 @@ void controller::conn_cb() {
   }
 
   shared_ptr<xcb_generic_event_t> evt{};
-  while ((evt = shared_ptr<xcb_generic_event_t>(xcb_poll_for_event(m_connection), free)) != nullptr) {
+  while ((evt = m_connection.poll_for_event()) != nullptr) {
     try {
       m_connection.dispatch_event(evt);
     } catch (xpp::connection_error& err) {
@@ -247,14 +247,25 @@ void controller::read_events(bool confwatch) {
   m_log.info("Entering event loop (thread-id=%lu)", this_thread::get_id());
 
   try {
-    auto& poll_handle = m_loop.handle<PollHandle>(m_connection.get_file_descriptor());
-    poll_handle.start(
+    auto& x_poll_handle = m_loop.handle<PollHandle>(m_connection.get_file_descriptor());
+    x_poll_handle.start(
         UV_READABLE, [this](const auto&) { conn_cb(); },
         [this](const auto& e) {
           m_log.err("libuv error while polling X connection: "s + uv_strerror(e.status));
           stop(false);
         });
 
+    auto& x_prepare_handle = m_loop.handle<PrepareHandle>();
+    x_prepare_handle.start([this]() {
+      /*
+       * We have to also handle events in the prepare handle (which runs right
+       * before polling for IO) to process any already queued X events which
+       * wouldn't trigger the uv_poll handle.
+       */
+      conn_cb();
+      m_connection.flush();
+    });
+
     for (auto s : {SIGINT, SIGQUIT, SIGTERM, SIGUSR1, SIGALRM}) {
       auto& signal_handle = m_loop.handle<SignalHandle>();
       signal_handle.start(s, [this](const auto& e) { signal_handler(e.signum); });

src/components/eventloop.cpp

@@ -38,6 +38,9 @@ namespace eventloop {
       case UV_NAMED_PIPE:
         static_cast<PipeHandle*>(handle->data)->close();
         break;
+      case UV_PREPARE:
+        static_cast<PrepareHandle*>(handle->data)->close();
+        break;
       default:
         assert(false);
     }
@@ -157,6 +160,17 @@ namespace eventloop {
   }
   // }}}
 
+  // PrepareHandle {{{
+  void PrepareHandle::init() {
+    UV(uv_prepare_init, loop(), get());
+  }
+
+  void PrepareHandle::start(cb user_cb) {
+    this->callback = user_cb;
+    UV(uv_prepare_start, get(), void_event_cb<&PrepareHandle::callback>);
+  }
+  // }}}
+
   // eventloop {{{
   static void close_walk_cb(uv_handle_t* handle, void*) {
     if (!uv_is_closing(handle)) {
@@ -201,11 +215,15 @@ namespace eventloop {
     uv_stop(m_loop.get());
   }
 
+  uint64_t loop::now() const {
+    return uv_now(m_loop.get());
+  }
+
   uv_loop_t* loop::get() const {
     return m_loop.get();
   }
   // }}}
 
-}  // namespace eventloop
+} // namespace eventloop
 
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