gmain: use Linux eventfd() for main context wake up

The Linux eventfd() call is basically tailor made for the main loop
wake up pipe - all we want is a threadsafe way to write to a file
descriptor, and wake up the context on the other end; we don't care
about the content at all.

The eventfd manual page basically explains the benefits:

       Applications can use an eventfd file descriptor instead of a
       pipe (see pipe(2)) in all cases where a pipe is used simply to
       signal events.  The kernel overhead of an eventfd file
       descriptor is much lower than that of a pipe, and only one file
       descriptor is required (versus the two required for a pipe).

When writing my multithreaded spawn test case I actually hit the 1024
file descriptor limit quickly, because we used 2 fds per main context.
This brings that down to 1.

https://bugzilla.gnome.org/show_bug.cgi?id=653140

configure.ac

@@ -2604,6 +2604,26 @@ main (void)
   AC_MSG_RESULT(no)
 ])
 
+AC_MSG_CHECKING([for eventfd(2) system call])
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([
+#include <sys/eventfd.h>
+#include <unistd.h>
+],[
+int
+main (void)
+{
+  eventfd (0, EFD_CLOEXEC);
+  return 0;
+}
+])],
+[
+  AC_MSG_RESULT(yes)
+  AC_DEFINE(HAVE_EVENTFD, 1, [we have the eventfd(2) system call])
+],
+[
+  AC_MSG_RESULT(no)
+])
+
 dnl ****************************************
 dnl *** GLib POLL* compatibility defines ***
 dnl ****************************************

glib/gmain.c

@@ -49,6 +49,9 @@
 
 #ifdef G_OS_UNIX
 #include "glib-unix.h"
+#ifdef HAVE_EVENTFD
+#include <sys/eventfd.h>
+#endif
 #endif
 
 #include <signal.h>
@@ -397,6 +400,14 @@ static GSList *main_contexts_without_pipe = NULL;
 
 #ifndef G_OS_WIN32
 
+#ifdef HAVE_EVENTFD
+typedef struct {
+  guint checked_eventfd : 1;
+  guint have_eventfd : 1;
+} EventFdState;
+EventFdState event_fd_state = { 0, 0 };
+#endif
+
 /* The UNIX signal pipe contains a single byte specifying which
  * signal was received.
  */ 
@@ -529,7 +540,9 @@ g_main_context_unref (GMainContext *context)
   if (g_thread_supported())
     {
 #ifndef G_OS_WIN32
+      if (context->wake_up_pipe[0] != -1)
 	close (context->wake_up_pipe[0]);
+      if (context->wake_up_pipe[1] != -1)
 	close (context->wake_up_pipe[1]);
 #else
       CloseHandle (context->wake_up_semaphore);
@@ -556,8 +569,32 @@ g_main_context_init_pipe (GMainContext *context)
   if (context->wake_up_pipe[0] != -1)
     return;
 
+#ifdef HAVE_EVENTFD
+  if (!event_fd_state.checked_eventfd
+      || event_fd_state.have_eventfd)
+    {
+      int efd;
+
+      event_fd_state.checked_eventfd = TRUE;
+      efd = eventfd (0, EFD_CLOEXEC);
+      if (efd == -1 && errno == ENOSYS)
+	{
+	  event_fd_state.have_eventfd = FALSE;
 	  if (!g_unix_open_pipe (context->wake_up_pipe, FD_CLOEXEC, &error))
 	    g_error ("Cannot create pipe main loop wake-up: %s", error->message);
+	}
+      else if (efd >= 0)
+	{
+	  event_fd_state.have_eventfd = TRUE;
+	  context->wake_up_pipe[0] = efd;
+	}
+      else
+	g_error ("Cannot create eventfd for main loop wake-up: %s", g_strerror (errno));
+    }
+#else
+  if (!g_unix_open_pipe (context->wake_up_pipe, FD_CLOEXEC, &error))
+    g_error ("Cannot create pipe main loop wake-up: %s", error->message);
+#endif
 
   context->wake_up_rec.fd = context->wake_up_pipe[0];
   context->wake_up_rec.events = G_IO_IN;
@@ -580,7 +617,9 @@ g_main_context_init_pipe (GMainContext *context)
 void
 _g_main_thread_init (void)
 {
-  GSList *curr = main_contexts_without_pipe;
+  GSList *curr;
+  
+  curr = main_contexts_without_pipe;
   while (curr)
     {
       g_main_context_init_pipe ((GMainContext *)curr->data);
@@ -2942,8 +2981,18 @@ g_main_context_check (GMainContext *context,
   if (!context->poll_waiting)
     {
 #ifndef G_OS_WIN32
+#ifdef HAVE_EVENTFD
+      if (event_fd_state.have_eventfd)
+	{
+	  guint64 buf;
+	  read (context->wake_up_pipe[0], &buf, sizeof(guint64));
+        }
+      else
+#endif
+	{
 	  gchar a;
 	  read (context->wake_up_pipe[0], &a, 1);
+	}
 #endif
     }
   else
@@ -3794,6 +3843,14 @@ g_main_context_wakeup_unlocked (GMainContext *context)
     {
       context->poll_waiting = FALSE;
 #ifndef G_OS_WIN32
+#ifdef HAVE_EVENTFD
+      if (event_fd_state.have_eventfd)
+	{
+	  guint64 buf = 1;
+	  write (context->wake_up_pipe[0], &buf, sizeof(buf));
+	}
+      else
+#endif
 	write (context->wake_up_pipe[1], "A", 1);
 #else
       ReleaseSemaphore (context->wake_up_semaphore, 1, NULL);