X11: Fix empty event race condition with a pipe

There is a seemingly unavoidable race condition when waiting for data on
the X11 display connection, as long as any other thread is also making
Xlib calls.  The event data we are waiting for could be read by the
other thread as part of looking for the reply to its request, before our
poll has begun.

This commit replaces the X11 event sent by glfwPostEmptyEvent with
writing to an unnamed pipe.  The race condition remains if other Xlib
calls are made on other threads, but glfwPostEmptyEvent should now be
race-free.

This commit is based on work by pcwalton, OlivierSohn, kovidgoyal and
joaodasilva.

Closes #2033
Related to #379
Related to #1281
Related to #1285

src/x11_init.c

@@ -35,6 +35,8 @@
 #include <stdio.h>
 #include <locale.h>
 #include <unistd.h>
+#include <fcntl.h>
+#include <errno.h>
 
 
 // Translate the X11 KeySyms for a key to a GLFW key code
@@ -1042,6 +1044,37 @@ static Window createHelperWindow(void)
                          CWEventMask, &wa);
 }
 
+// Create the pipe for empty events without assumuing the OS has pipe2(2)
+//
+static GLFWbool createEmptyEventPipe(void)
+{
+    if (pipe(_glfw.x11.emptyEventPipe) != 0)
+    {
+        _glfwInputError(GLFW_PLATFORM_ERROR,
+                        "X11: Failed to create empty event pipe: %s",
+                        strerror(errno));
+        return GLFW_FALSE;
+    }
+
+    for (int i = 0; i < 2; i++)
+    {
+        const int sf = fcntl(_glfw.x11.emptyEventPipe[i], F_GETFL, 0);
+        const int df = fcntl(_glfw.x11.emptyEventPipe[i], F_GETFD, 0);
+
+        if (sf == -1 || df == -1 ||
+            fcntl(_glfw.x11.emptyEventPipe[i], F_SETFL, sf | O_NONBLOCK) == -1 ||
+            fcntl(_glfw.x11.emptyEventPipe[i], F_SETFD, df | FD_CLOEXEC) == -1)
+        {
+            _glfwInputError(GLFW_PLATFORM_ERROR,
+                            "X11: Failed to set flags for empty event pipe: %s",
+                            strerror(errno));
+            return GLFW_FALSE;
+        }
+    }
+
+    return GLFW_TRUE;
+}
+
 // X error handler
 //
 static int errorHandler(Display *display, XErrorEvent* event)
@@ -1491,6 +1524,9 @@ int _glfwInitX11(void)
 
     getSystemContentScale(&_glfw.x11.contentScaleX, &_glfw.x11.contentScaleY);
 
+    if (!createEmptyEventPipe())
+        return GLFW_FALSE;
+
     if (!initExtensions())
         return GLFW_FALSE;
 
@@ -1604,5 +1640,11 @@ void _glfwTerminateX11(void)
         _glfwPlatformFreeModule(_glfw.x11.xlib.handle);
         _glfw.x11.xlib.handle = NULL;
     }
+
+    if (_glfw.x11.emptyEventPipe[0] || _glfw.x11.emptyEventPipe[1])
+    {
+        close(_glfw.x11.emptyEventPipe[0]);
+        close(_glfw.x11.emptyEventPipe[1]);
+    }
 }