Improve reliability of clipboard X->Windows pastes

Sometimes, particularly with large clipboard pastes to Windows, we could end up
waiting for the timeout to expire, rather than pasting the data.

Various changes to improve reliability:

1. Use XFlush() not XSync() in winProcessXEventsTimeout().

It makes no sense to ensure we have received replies to outstanding requests if
we are going to wait for them using select()

2. Add XFlush() to winClipboardProc()

Make sure we have sent any requests before we wait using select()

3. Don't use FD_ISSET() to check which fd is ready

This looks like a select() bug in that it sometimes returns 0 with an empty fd
set before the timeout expires, but a fd appears to be ready.

Add select() return value to debug output when we are warning that this has
happened.

4. Drain event queues before entering select()

Unconditionally drain event queues before entering select().  This is the
recommended way of writing select() and X event processing loops.

winClipboardFlushXEvents() checks using XPending(), and
winClipboardFlushWindowsMessageQueue() checks using PeekMessage() so this is
safe against blocking, but means that may not need to enter select() at all
sometimes.

2 files changed, 31 insertions, 48 deletions

diff --git a/hw/xwin/winclipboard/thread.c b/hw/xwin/winclipboard/thread.c
index 2b9a6f34f..1653747ea 100644
--- a/hw/xwin/winclipboard/thread.c
+++ b/hw/xwin/winclipboard/thread.c
@@ -244,24 +244,26 @@ winClipboardProc(Bool fUseUnicode, char *szDisplay)
         }
     }
 
-    /* Pre-flush X events */
-    /*
-     * NOTE: Apparently you'll freeze if you don't do this,
-     *       because there may be events in local data structures
-     *       already.
-     */
     ClipboardConversionData data;
     data.fUseUnicode = fUseUnicode;
-    winClipboardFlushXEvents(hwnd, iWindow, pDisplay, &data, &atoms);
-
-    /* Pre-flush Windows messages */
-    if (!winClipboardFlushWindowsMessageQueue(hwnd)) {
-        ErrorF("winClipboardProc - winClipboardFlushWindowsMessageQueue failed\n");
-        pthread_exit(NULL);
-    }
 
-    /* Loop for X events */
+    /* Loop for events */
     while (1) {
+
+        /* Process X events */
+        winClipboardFlushXEvents(hwnd,
+                                 iWindow, pDisplay, &data, &atoms);
+
+        /* Process Windows messages */
+        if (!winClipboardFlushWindowsMessageQueue(hwnd)) {
+          ErrorF("winClipboardProc - winClipboardFlushWindowsMessageQueue trapped "
+                       "WM_QUIT message, exiting main loop.\n");
+          break;
+        }
+
+        /* We need to ensure that all pending requests are sent */
+        XFlush(pDisplay);
+
         /* Setup the file descriptor set */
         /*
          * NOTE: You have to do this before every call to select
@@ -312,14 +314,9 @@ winClipboardProc(Bool fUseUnicode, char *szDisplay)
 
         winDebug("winClipboardProc - select returned %d\n", iReturn);
 
-        /* Branch on which descriptor became active */
         if (FD_ISSET(iConnectionNumber, &fdsRead)) {
             winDebug
                 ("winClipboardProc - X connection ready, pumping X event queue\n");
-
-            /* Process X events */
-            winClipboardFlushXEvents(hwnd,
-                                     iWindow, pDisplay, &data, &atoms);
         }
 
 #ifdef HAS_DEVWINDOWS
@@ -331,20 +328,12 @@ winClipboardProc(Bool fUseUnicode, char *szDisplay)
         {
             winDebug
                 ("winClipboardProc - /dev/windows ready, pumping Windows message queue\n");
-
-            /* Process Windows messages */
-            if (!winClipboardFlushWindowsMessageQueue(hwnd)) {
-                ErrorF("winClipboardProc - "
-                       "winClipboardFlushWindowsMessageQueue trapped "
-                       "WM_QUIT message, exiting main loop.\n");
-                break;
-            }
         }
 
 #ifdef HAS_DEVWINDOWS
         if (!(FD_ISSET(iConnectionNumber, &fdsRead)) &&
             !(FD_ISSET(fdMessageQueue, &fdsRead))) {
-            winDebug("winClipboardProc - Spurious wake\n");
+            winDebug("winClipboardProc - Spurious wake, select() returned %d\n", iReturn);
         }
 #endif
     }
diff --git a/hw/xwin/winclipboard/wndproc.c b/hw/xwin/winclipboard/wndproc.c
index 83f11417d..b5566fa5a 100644
--- a/hw/xwin/winclipboard/wndproc.c
+++ b/hw/xwin/winclipboard/wndproc.c
@@ -83,8 +83,18 @@ winProcessXEventsTimeout(HWND hwnd, Window iWindow, Display * pDisplay,
         fd_set fdsRead;
         long remainingTime;
 
-        /* We need to ensure that all pending events are processed */
-        XSync(pDisplay, FALSE);
+        /* Process X events */
+        iReturn = winClipboardFlushXEvents(hwnd, iWindow, pDisplay, data, atoms);
+
+        winDebug("winProcessXEventsTimeout () - winClipboardFlushXEvents returned %d\n", iReturn);
+
+        if ((WIN_XEVENTS_NOTIFY_DATA == iReturn) || (WIN_XEVENTS_NOTIFY_TARGETS == iReturn) || (WIN_XEVENTS_FAILED == iReturn)) {
+          /* Bail out */
+          return iReturn;
+        }
+
+        /* We need to ensure that all pending requests are sent */
+        XFlush(pDisplay);
 
         /* Setup the file descriptor set */
         FD_ZERO(&fdsRead);
@@ -113,24 +123,8 @@ winProcessXEventsTimeout(HWND hwnd, Window iWindow, Display * pDisplay,
             break;
         }
 
-        /* Branch on which descriptor became active */
-        if (FD_ISSET(iConnNumber, &fdsRead)) {
-            /* Process X events */
-            /* Exit when we see that server is shutting down */
-            iReturn = winClipboardFlushXEvents(hwnd,
-                                               iWindow, pDisplay, data, atoms);
-
-            winDebug
-                ("winProcessXEventsTimeout () - winClipboardFlushXEvents returned %d\n",
-                 iReturn);
-
-            if ((WIN_XEVENTS_NOTIFY_DATA == iReturn) || (WIN_XEVENTS_NOTIFY_TARGETS == iReturn) || (WIN_XEVENTS_FAILED == iReturn)) {
-                /* Bail out */
-                return iReturn;
-            }
-        }
-        else {
-            winDebug("winProcessXEventsTimeout - Spurious wake\n");
+        if (!FD_ISSET(iConnNumber, &fdsRead)) {
+            winDebug("winProcessXEventsTimeout - Spurious wake, select() returned %d\n", iReturn);
         }
     }