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]);
+    }
 }
 

src/x11_platform.h

@@ -582,6 +582,7 @@ typedef struct _GLFWlibraryX11
     double          restoreCursorPosX, restoreCursorPosY;
     // The window whose disabled cursor mode is active
     _GLFWwindow*    disabledCursorWindow;
+    int             emptyEventPipe[2];
 
     // Window manager atoms
     Atom            NET_SUPPORTED;

src/x11_window.c

@@ -114,8 +114,12 @@ static GLFWbool waitForX11Event(double* timeout)
 //
 static GLFWbool waitForAnyEvent(double* timeout)
 {
-    nfds_t count = 1;
-    struct pollfd fds[2] = { { ConnectionNumber(_glfw.x11.display), POLLIN } };
+    nfds_t count = 2;
+    struct pollfd fds[3] =
+    {
+        { ConnectionNumber(_glfw.x11.display), POLLIN },
+        { _glfw.x11.emptyEventPipe[0], POLLIN }
+    };
 
 #if defined(__linux__)
     if (_glfw.joysticksInitialized)
@@ -137,6 +141,32 @@ static GLFWbool waitForAnyEvent(double* timeout)
     return GLFW_TRUE;
 }
 
+// Writes a byte to the empty event pipe
+//
+static void writeEmptyEvent(void)
+{
+    for (;;)
+    {
+        const char byte = 0;
+        const int result = write(_glfw.x11.emptyEventPipe[1], &byte, 1);
+        if (result == 1 || (result == -1 && errno != EINTR))
+            break;
+    }
+}
+
+// Drains available data from the empty event pipe
+//
+static void drainEmptyEvents(void)
+{
+    for (;;)
+    {
+        char dummy[64];
+        const int result = read(_glfw.x11.emptyEventPipe[0], dummy, sizeof(dummy));
+        if (result == -1 && errno != EINTR)
+            break;
+    }
+}
+
 // Waits until a VisibilityNotify event arrives for the specified window or the
 // timeout period elapses (ICCCM section 4.2.2)
 //
@@ -2778,6 +2808,8 @@ GLFWbool _glfwRawMouseMotionSupportedX11(void)
 
 void _glfwPollEventsX11(void)
 {
+    drainEmptyEvents();
+
 #if defined(__linux__)
     if (_glfw.joysticksInitialized)
         _glfwDetectJoystickConnectionLinux();
@@ -2823,13 +2855,7 @@ void _glfwWaitEventsTimeoutX11(double timeout)
 
 void _glfwPostEmptyEventX11(void)
 {
-    XEvent event = { ClientMessage };
-    event.xclient.window = _glfw.x11.helperWindowHandle;
-    event.xclient.format = 32; // Data is 32-bit longs
-    event.xclient.message_type = _glfw.x11.NULL_;
-
-    XSendEvent(_glfw.x11.display, _glfw.x11.helperWindowHandle, False, 0, &event);
-    XFlush(_glfw.x11.display);
+    writeEmptyEvent();
 }
 
 void _glfwGetCursorPosX11(_GLFWwindow* window, double* xpos, double* ypos)