Thunks: Make xcb's callback more robust.

Fixes a crash in xcb thunks since more stuff on FEX side has moved over
to jemalloc.

Destructors don't actually get called when a shared library is removed.
It's some weirdo quirk that we can't work around. Instead refcount
Display connections being created and disconnected. Creating the thread
on the first display creation, and tearing down on final display
teardown.

Must be merged before #2564 otherwise that PR will break thunks.

ThunkLibs/libxcb/libxcb_Guest.cpp

@@ -30,6 +30,8 @@ $end_info$
 
 static std::thread CBThread{};
 static std::atomic<bool> CBDone{false};
+static std::mutex CBThreadMutex;
+static uint32_t ScreenRefCount;
 
 static CrossArchEvent WaitForWork{};
 static CrossArchEvent WorkDone{};
@@ -72,18 +74,28 @@ static void CallbackThreadFunc() {
 }
 
 extern "C" {
-  static void init_lib() {
+  void CreateCallback() {
+    std::unique_lock lk{CBThreadMutex};
+    ++ScreenRefCount;
+
     // Start a guest side thread that allows us to do callbacks from xcb safely
     if (!CBThread.joinable()) {
       CBThread = std::thread(CallbackThreadFunc);
     }
   }
-  __attribute__((destructor)) static void close_lib() {
-    if (CBThread.joinable()) {
-      CBDone = true;
-      NotifyWorkFunc(&WaitForWork);
-      NotifyWorkFunc(&WorkDone);
-      CBThread.join();
+
+  void RemoveCallback() {
+    std::unique_lock lk{CBThreadMutex};
+    --ScreenRefCount;
+
+    // If the new value is 0 then shutdown the work thread.
+    if (ScreenRefCount == 0) {
+      if (CBThread.joinable()) {
+        CBDone = true;
+        NotifyWorkFunc(&WaitForWork);
+        NotifyWorkFunc(&WorkDone);
+        CBThread.join();
+      }
     }
   }
 
@@ -117,17 +129,33 @@ extern "C" {
 
   xcb_connection_t * xcb_connect(const char * a_0,int * a_1){
     auto ret = fexfn_pack_xcb_connect(a_0, a_1);
+
+    if (xcb_get_file_descriptor(ret) != -1) {
+      // Only create callback on valid xcb connections.
+      // Checking for FD is the easiest way to do this.
+      CreateCallback();
+    }
     InitializeExtensions(ret);
     return ret;
   }
 
   xcb_connection_t * xcb_connect_to_display_with_auth_info(const char * a_0,xcb_auth_info_t * a_1,int * a_2){
     auto ret = fexfn_pack_xcb_connect_to_display_with_auth_info(a_0, a_1, a_2);
+    if (xcb_get_file_descriptor(ret) != -1) {
+      // Only create callback on valid xcb connections.
+      // Checking for FD is the easiest way to do this.
+      CreateCallback();
+    }
     InitializeExtensions(ret);
     return ret;
   }
 
   void xcb_disconnect(xcb_connection_t * a_0){
+    if (a_0 != nullptr && xcb_get_file_descriptor(a_0) != -1) {
+      // Only decrement callback reference on valid displays.
+      // Checking for FD and nullptr is the easiest way to do this.
+      RemoveCallback();
+    }
     fexfn_pack_xcb_disconnect(a_0);
   }
 
@@ -1407,4 +1435,4 @@ extern "C" {
   }
 }
 
-LOAD_LIB_INIT(libxcb, init_lib)
+LOAD_LIB(libxcb)