Add a mode to ensure a server is not already running.

tools/xdisplaycheck/xdisplaycheck.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // This is a small program that tries to connect to the X server.  It
 // continually retries until it connects or 30 seconds pass.  If it fails
 // to connect to the X server or fails to find needed functiona, it returns
 // an error code of -1.
 //
 // This is to help verify that a useful X server is available before we start
 // start running tests on the build bots.
 
 #include <errno.h>
 #include <stdio.h>
+#include <string.h>
 #include <time.h>
 #include <X11/Xlib.h>
 
 #if defined(USE_AURA)
 #include <X11/extensions/XInput2.h>
 #endif
 
 void Sleep(int duration_ms) {
   struct timespec sleep_time, remaining;
 
   // Contains the portion of duration_ms >= 1 sec.
   sleep_time.tv_sec = duration_ms / 1000;
   duration_ms -= sleep_time.tv_sec * 1000;
 
   // Contains the portion of duration_ms < 1 sec.
   sleep_time.tv_nsec = duration_ms * 1000 * 1000;  // nanoseconds.
 
   while (nanosleep(&sleep_time, &remaining) == -1 && errno == EINTR)
     sleep_time = remaining;
 }
 
 int main(int argc, char* argv[]) {
+  Display* display = NULL;
+  if (argv[1] && strcmp(argv[1], "--noserver") == 0) {
+    display = XOpenDisplay(NULL);
+    if (display) {
+      fprintf(stderr, "Found unexpected connectable display %s\n",
+              XDisplayName(NULL));
+    }
+    // Return success when we got an unexpected display so that the code
+    // without the --noserver is the same, but slow, rather than inverted.
+    return !display;
+  }
+
   int kNumTries = 78;  // 78*77/2 * 10 = 30s of waiting
-  Display* display = NULL;
   int tries;
   for (tries = 0; tries < kNumTries; ++tries) {
     display = XOpenDisplay(NULL);
     if (display)
       break;
     Sleep(10 * tries);
   }
 
   if (!display) {
     fprintf(stderr, "Failed to connect to %s\n", XDisplayName(NULL));
@@ skipping 23 matching lines @@
   XIDeviceInfo* devices = XIQueryDevice(display, XIAllDevices, &count);
   if (devices)
     XIFreeDeviceInfo(devices);
 
   fprintf(stderr,
       "XInput2 verified initially sane on %s.\n", XDisplayName(NULL));
 #endif
 
   return 0;
 }