Bugfix: Set the cursor via the Xlib connection if XCursor is supported

This fixes a race where we created cursors on the Xlib connection, flushed,
then used the cursor on the XCB connection. Even though we flushed, the X
server did not process the requests yet and therefore returned a BadCursor
error.

This bugfix uses the Xlib connection for setting the root window cursor which
will ensure that the requests are properly serialized.

An easy test for this (on my machine) is the following ~/.xsession:
    xsetroot -cursor_name cross
    exec i3
If you see a cross cursor instead of the pointer, the race happens. You’ll see
a error_code=6 error in your ~/.xsession-errors.

include/xcursor.h

@@ -17,4 +17,17 @@ extern void xcursor_load_cursors();
 extern Cursor xcursor_get_cursor(enum xcursor_cursor_t c);
 extern int xcursor_get_xcb_cursor(enum xcursor_cursor_t c);
 
+/**
+ * Sets the cursor of the root window to the 'pointer' cursor.
+ *
+ * This function is called when i3 is initialized, because with some login
+ * managers, the root window will not have a cursor otherwise.
+ *
+ * We have a separate xcursor function to use the same X11 connection as the
+ * xcursor_load_cursors() function. If we mix the Xlib and the XCB connection,
+ * races might occur (even though we flush the Xlib connection).
+ *
+ */
+void xcursor_set_root_cursor();
+
 #endif

src/main.c

@@ -338,8 +338,7 @@ int main(int argc, char *argv[]) {
     /* Set a cursor for the root window (otherwise the root window will show no
        cursor until the first client is launched). */
     if (xcursor_supported) {
-        uint32_t values[1] = { xcursor_get_cursor(XCURSOR_CURSOR_POINTER) };
-        xcb_change_window_attributes(conn, root, XCB_CW_CURSOR, values);
+        xcursor_set_root_cursor();
     } else {
         xcb_cursor_t cursor_id = xcb_generate_id(conn);
         i3Font cursor_font = load_font("cursor", false);

src/xcursor.c

@@ -28,7 +28,23 @@ void xcursor_load_cursors() {
     cursors[XCURSOR_CURSOR_POINTER] = load_cursor("left_ptr");
     cursors[XCURSOR_CURSOR_RESIZE_HORIZONTAL] = load_cursor("sb_h_double_arrow");
     cursors[XCURSOR_CURSOR_RESIZE_VERTICAL] = load_cursor("sb_v_double_arrow");
+}
 
+/*
+ * Sets the cursor of the root window to the 'pointer' cursor.
+ *
+ * This function is called when i3 is initialized, because with some login
+ * managers, the root window will not have a cursor otherwise.
+ *
+ * We have a separate xcursor function to use the same X11 connection as the
+ * xcursor_load_cursors() function. If we mix the Xlib and the XCB connection,
+ * races might occur (even though we flush the Xlib connection).
+ *
+ */
+void xcursor_set_root_cursor() {
+    XSetWindowAttributes attributes;
+    attributes.cursor = xcursor_get_cursor(XCURSOR_CURSOR_POINTER);
+    XChangeWindowAttributes(xlibdpy, DefaultRootWindow(xlibdpy), CWCursor, &attributes);
     XFlush(xlibdpy);
 }