vdagent-x11: ensure no events are pending before returning to the main loop

Note: Our event loop is only called when there is data to be read from the
X11 socket. If events have arrived and have already been read by libX11 from
the socket triggered by other libX11 calls from this file, the select for
read in the main loop, won't see these and our event loop won't get called!

Thus we must make sure that all queued events have been consumed, whenever
we return to the main loop. IOW all (externally callable) functions in this
file must end with calling XPending and consuming all queued events.

Calling XPending when-ever we return to the mainloop also ensures any
pending writes are flushed.

1 files changed, 31 insertions, 12 deletions

diff --git a/vdagent-x11.c b/vdagent-x11.c
index ebc2ad4..e704b66 100644
--- a/vdagent-x11.c
+++ b/vdagent-x11.c
@@ -19,10 +19,17 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-/* Note *all* X11 calls in this file which do not wait for a result must be
-   followed by an XFlush, given that the X11 code pumping the event loop
-   (and thus flushing queued writes) is only called when there is data to be
-   read from the X11 socket. */
+/* Note: Our event loop is only called when there is data to be read from the
+   X11 socket. If events have arrived and have already been read by libX11 from
+   the socket triggered by other libX11 calls from this file, the select for
+   read in the main loop, won't see these and our event loop won't get called!
+   
+   Thus we must make sure that all queued events have been consumed, whenever
+   we return to the main loop. IOW all (externally callable) functions in this
+   file must end with calling XPending and consuming all queued events.
+   
+   Calling XPending when-ever we return to the mainloop also ensures any
+   pending writes are flushed. */
 
 #include <stdlib.h>
 #include <limits.h>
@@ -183,7 +190,8 @@ struct vdagent_x11 *vdagent_x11_create(struct udscs_connection *vdagentd,
     x11->height = attrib.height;
     vdagent_x11_send_daemon_guest_xorg_res(x11);
 
-    /* No need for XFlush as XGetWindowAttributes does an implicit XFlush */
+    /* Flush output buffers and consume any pending events */
+    vdagent_x11_do_read(x11);
 
     return x11;
 }
@@ -292,7 +300,6 @@ static void vdagent_x11_handle_event(struct vdagent_x11 *x11, XEvent event)
         XConvertSelection(x11->display, x11->clipboard_atom, x11->targets_atom,
                           x11->targets_atom, x11->selection_window,
                           CurrentTime);
-        XFlush(x11->display);
         x11->expected_targets_notifies++;
         return;
     }
@@ -467,12 +474,10 @@ static int vdagent_x11_get_selection(struct vdagent_x11 *x11, XEvent *event,
             XSelectInput(x11->display, x11->selection_window,
                          PropertyChangeMask);
             XDeleteProperty(x11->display, x11->selection_window, prop);
-            XFlush(x11->display);
             XFree(data);
             return 0; /* Wait for more data */
         }
         XDeleteProperty(x11->display, x11->selection_window, prop);
-        XFlush(x11->display);
     }
 
     if (type_ret != type) {
@@ -720,7 +725,6 @@ static void vdagent_x11_send_selection_notify(struct vdagent_x11 *x11,
     res.xselection.target = event->xselectionrequest.target;
     res.xselection.time = event->xselectionrequest.time;
     XSendEvent(x11->display, event->xselectionrequest.requestor, 0, 0, &res);
-    XFlush(x11->display);
 
     if (process_next_req) {
         vdagent_x11_next_selection_request(x11);
@@ -854,10 +858,12 @@ void vdagent_x11_set_monitor_config(struct vdagent_x11 *x11,
     XRRSetScreenConfig(x11->display, config, x11->root_window, best,
                        rotation, CurrentTime);
     XRRFreeScreenConfigInfo(config);
-    XFlush(x11->display);
     x11->width = sizes[best].width;
     x11->height = sizes[best].height;
     vdagent_x11_send_daemon_guest_xorg_res(x11);
+
+    /* Flush output buffers and consume any pending events */
+    vdagent_x11_do_read(x11);
 }
 
 void vdagent_x11_clipboard_request(struct vdagent_x11 *x11, uint32_t type)
@@ -889,7 +895,9 @@ void vdagent_x11_clipboard_request(struct vdagent_x11 *x11, uint32_t type)
     x11->clipboard_request_target = target;
     XConvertSelection(x11->display, x11->clipboard_atom, target,
                       x11->clipboard_atom, x11->selection_window, CurrentTime);
-    XFlush(x11->display);
+
+    /* Flush output buffers and consume any pending events */
+    vdagent_x11_do_read(x11);
 }
 
 void vdagent_x11_clipboard_grab(struct vdagent_x11 *x11, uint32_t *types,
@@ -905,8 +913,10 @@ void vdagent_x11_clipboard_grab(struct vdagent_x11 *x11, uint32_t *types,
 
     XSetSelectionOwner(x11->display, x11->clipboard_atom,
                        x11->selection_window, CurrentTime);
-    XFlush(x11->display);
     vdagent_x11_set_clipboard_owner(x11, owner_client);
+
+    /* Flush output buffers and consume any pending events */
+    vdagent_x11_do_read(x11);
 }
 
 void vdagent_x11_clipboard_data(struct vdagent_x11 *x11, uint32_t type,
@@ -931,6 +941,9 @@ void vdagent_x11_clipboard_data(struct vdagent_x11 *x11, uint32_t type,
                 type_from_event, type);
         vdagent_x11_send_selection_notify(x11, None, 1);
         free(data);
+
+        /* Flush output buffers and consume any pending events */
+        vdagent_x11_do_read(x11);
         return;
     }
 
@@ -944,6 +957,9 @@ void vdagent_x11_clipboard_data(struct vdagent_x11 *x11, uint32_t type,
                     data, size);
     vdagent_x11_send_selection_notify(x11, prop, 1);
     free(data);
+
+    /* Flush output buffers and consume any pending events */
+    vdagent_x11_do_read(x11);
 }
 
 void vdagent_x11_clipboard_release(struct vdagent_x11 *x11)
@@ -967,4 +983,7 @@ void vdagent_x11_clipboard_release(struct vdagent_x11 *x11)
 
     /* Note no need to do a set_clipboard_owner(owner_none) here, as that is
        already done by processing the XFixesSetSelectionOwnerNotify event. */
+
+    /* Flush output buffers and consume any pending events */
+    vdagent_x11_do_read(x11);
 }