ui: Implement module for thread-safe communication with U

The ui_bridge.c module implements a surrogate UI that forwards calls to another
thread.

src/nvim/ui_bridge.h

@@ -0,0 +1,40 @@
+// Bridge used for communication between a builtin UI thread and nvim core
+#ifndef NVIM_UI_BRIDGE_H
+#define NVIM_UI_BRIDGE_H
+
+#include <uv.h>
+
+#include "nvim/ui.h"
+#include "nvim/event/defs.h"
+
+typedef struct ui_bridge_data UIBridgeData;
+typedef void(*ui_main_fn)(UIBridgeData *bridge, UI *ui);
+struct ui_bridge_data {
+  UI bridge;  // actual UI passed to ui_attach
+  UI *ui;     // UI pointer that will have it's callback called in
+              // another thread
+  event_scheduler scheduler;
+  uv_thread_t ui_thread;
+  ui_main_fn ui_main;
+  uv_mutex_t mutex;
+  uv_cond_t cond;
+  // When the UI thread is called, the main thread will suspend until
+  // the call returns. This flag is used as a condition for the main
+  // thread to continue.
+  bool ready;
+};
+
+#define CONTINUE(b)                                                    \
+  do {                                                                 \
+    UIBridgeData *d = (UIBridgeData *)b;                               \
+    uv_mutex_lock(&d->mutex);                                          \
+    d->ready = true;                                                   \
+    uv_cond_signal(&d->cond);                                          \
+    uv_mutex_unlock(&d->mutex);                                        \
+  } while (0)
+
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "ui_bridge.h.generated.h"
+#endif
+#endif  // NVIM_UI_BRIDGE_H