fix X11 crash if DISPLAY is not valid

CMakeLists.txt

@@ -212,7 +212,7 @@ if (UNIX)
         if (HAVE_Xtst)
 
             # Xtxt depends on X11.
-            set (HAVE_X11)
+            set (HAVE_X11 1)
             list (APPEND libs Xtst X11)
 
         else()

src/gui/CMakeLists.txt

@@ -39,8 +39,10 @@ elseif (APPLE)
 elseif (${CMAKE_SYSTEM_NAME} MATCHES "Linux" OR
         ${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD")
     target_link_libraries (barrier dns_sd)
-else()
+endif()
-    target_link_libraries (barrier)
+
+if (HAVE_X11)
+    target_link_libraries (barrier X11)
 endif()
 
 if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")

src/gui/src/DisplayIsValid.cpp

@@ -0,0 +1,14 @@
+#ifdef WINAPI_XWINDOWS
+
+#include "DisplayIsValid.h"
+#include <X11/Xlib.h>
+
+bool display_is_valid()
+{
+    auto dsp = XOpenDisplay(NULL);
+    if (dsp != NULL)
+        XCloseDisplay(dsp);
+    return dsp != NULL;
+}
+
+#endif

src/gui/src/DisplayIsValid.h

@@ -0,0 +1,5 @@
+#pragma once
+
+#ifdef WINAPI_XWINDOWS
+bool display_is_valid();
+#endif

src/gui/src/main.cpp

@@ -23,6 +23,7 @@
 #include "MainWindow.h"
 #include "AppConfig.h"
 #include "SetupWizard.h"
+#include "DisplayIsValid.h"
 
 #include <QtCore>
 #include <QtGui>
@@ -54,6 +55,14 @@ bool checkMacAssistiveDevices();
 
 int main(int argc, char* argv[])
 {
+#ifdef WINAPI_XWINDOWS
+    // QApplication's constructor will call a fscking abort() if
+    // DISPLAY is bad. Let's check it first and handle it gracefully
+    if (!display_is_valid()) {
+        fprintf(stderr, "The Barrier GUI requires a display. Quitting...\n");
+        return 1;
+    }
+#endif
 #ifdef Q_OS_DARWIN
     /* Workaround for QTBUG-40332 - "High ping when QNetworkAccessManager is instantiated" */
     ::setenv ("QT_BEARER_POLL_TIMEOUT", "-1", 1);