_XDefaultIOError: Do better at detecting explicit shutdown

Currently, when the X server crashes or a client is disconnected with
XKillClient, you get a somewhat confusing error message from libX11
along the lines of:

XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0"
      after 98 requests (40 known processed) with 0 events remaining.

What's happening here is the previous recvmsg has thrown EAGAIN, since
the socket is non-blocking. In this case, check whether the socket has
any more data to read, and if not treat it like EPIPE.

Signed-off-by: Adam Jackson <ajax@redhat.com>

src/XlibInt.c

@@ -50,6 +50,8 @@ from The Open Group.
 #ifdef XTHREADS
 #include "locking.h"
 
+#include <sys/ioctl.h>
+
 /* these pointers get initialized by XInitThreads */
 LockInfoPtr _Xglobal_lock = NULL;
 void (*_XCreateMutex_fn)(LockInfoPtr) = NULL;
@@ -1234,6 +1236,21 @@ _XWireToEvent(
 	return(True);
 }
 
+static int
+SocketBytesReadable(Display *dpy)
+{
+    int bytes = 0, last_error;
+#ifdef WIN32
+    last_error = WSAGetLastError();
+    ioctlsocket(ConnectionNumber(dpy), FIONREAD, &bytes);
+    WSASetLastError(last_error);
+#else
+    last_error = errno;
+    ioctl(ConnectionNumber(dpy), FIONREAD, &bytes);
+    errno = last_error;
+#endif
+    return bytes;
+}
 
 /*
  * _XDefaultIOError - Default fatal system error reporting routine.  Called
@@ -1242,7 +1259,17 @@ _XWireToEvent(
 _X_NORETURN int _XDefaultIOError(
 	Display *dpy)
 {
-	if (ECHECK(EPIPE)) {
+	int killed = ECHECK(EPIPE);
+
+	/*
+	 * If the socket was closed on the far end, the final recvmsg in
+	 * xcb will have thrown EAGAIN because we're non-blocking. Detect
+	 * this to get the more informative error message.
+	 */
+	if (ECHECK(EAGAIN) && SocketBytesReadable(dpy) <= 0)
+	    killed = True;
+
+	if (killed) {
 	    fprintf (stderr,
                      "X connection to %s broken (explicit kill or server shutdown).\r\n",
                      DisplayString (dpy));