[glib: 1/2] socket: Fix get_available_bytes on systems other than Linux and Windows

[Philip Withnall <pwithnall src gnome org>]

commit 293c103a7de9075b24e2510998a6407cf4f00d58
Author: Ting-Wei Lan <lantw src gnome org>
Date:   Sat May 26 11:56:25 2018 +0800

    socket: Fix get_available_bytes on systems other than Linux and Windows
    
    FIONREAD ioctl on Linux reports the size of payload on UDP sockets.
    However, other systems usually add internal header size to the reported
    size, which vary between different operating systems and socket types.
    To make it work on more systems, we should follow what we do on Windows
    instead of using this unreliable FIONREAD ioctl.
    
    This fixes socket test on FreeBSD.

 gio/gsocket.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)
---
diff --git a/gio/gsocket.c b/gio/gsocket.c
index 11be2e738..859e807cb 100644
--- a/gio/gsocket.c
+++ b/gio/gsocket.c
@@ -2957,9 +2957,11 @@ g_socket_check_connect_result (GSocket  *socket,
 gssize
 g_socket_get_available_bytes (GSocket *socket)
 {
-#ifdef G_OS_WIN32
+#ifndef SO_NREAD
   const gint bufsize = 64 * 1024;
   static guchar *buf = NULL;
+#endif
+#ifdef G_OS_WIN32
   u_long avail;
 #else
   gint avail;
@@ -2967,25 +2969,37 @@ g_socket_get_available_bytes (GSocket *socket)
 
   g_return_val_if_fail (G_IS_SOCKET (socket), -1);
 
-#if defined (SO_NREAD)
+#ifdef SO_NREAD
   if (!g_socket_get_option (socket, SOL_SOCKET, SO_NREAD, &avail, NULL))
       return -1;
-#elif !defined (G_OS_WIN32)
-  if (ioctl (socket->priv->fd, FIONREAD, &avail) < 0)
-    avail = -1;
 #else
   if (socket->priv->type == G_SOCKET_TYPE_DATAGRAM)
     {
       if (G_UNLIKELY (g_once_init_enter (&buf)))
         g_once_init_leave (&buf, g_malloc (bufsize));
 
+      /* On datagram sockets, FIONREAD ioctl is not reliable because many
+       * systems add internal header size to the reported size, making it
+       * unusable for this function. */
       avail = recv (socket->priv->fd, buf, bufsize, MSG_PEEK);
-      if (avail == -1 && get_socket_errno () == WSAEWOULDBLOCK)
-        avail = 0;
+      if (avail == -1)
+        {
+          int errsv = get_socket_errno ();
+#ifdef G_OS_WIN32
+          if (errsv == WSAEWOULDBLOCK)
+#else
+          if (errsv == EWOULDBLOCK || errsv == EAGAIN)
+#endif
+            avail = 0;
+        }
     }
   else
     {
+#ifdef G_OS_WIN32
       if (ioctlsocket (socket->priv->fd, FIONREAD, &avail) < 0)
+#else
+      if (ioctl (socket->priv->fd, FIONREAD, &avail) < 0)
+#endif
         avail = -1;
     }
 #endif