[glib/wip/lantw/dont-limit-host-name-to-99-bytes] gutils: Don't limit the length of the host name to 99

[Ting-Wei Lan <lantw src gnome org>]

commit a178be9c35dbfce05c083d5470cb82ba020b041c
Author: Ting-Wei Lan <lantw src gnome org>
Date:   Mon Jun 24 01:47:26 2019 +0800

    gutils: Don't limit the length of the host name to 99
    
    It is unclear that why the size of the buffer was chosen to be 100
    because the commit introduced the code didn't mention the reason.
    POSIX defines _POSIX_HOST_NAME_MAX to be 255 and provides a way to
    determine the suitable value with sysconf, so we should use it instead
    of hard-coding a small value.

 glib/gutils.c | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)
---
diff --git a/glib/gutils.c b/glib/gutils.c
index 87db6a1f3..749badc24 100644
--- a/glib/gutils.c
+++ b/glib/gutils.c
@@ -1003,8 +1003,32 @@ g_get_host_name (void)
       gchar *utmp;
 
 #ifndef G_OS_WIN32
-      gchar *tmp = g_malloc (sizeof (gchar) * 100);
-      failed = (gethostname (tmp, sizeof (gchar) * 100) == -1);
+      glong max;
+      gsize size;
+      const gsize size_large = (gsize) 256 * 256;
+      gchar *tmp;
+
+      max = sysconf (_SC_HOST_NAME_MAX);
+      if (max > 0)
+        size = (gsize) max + 1;
+      else
+#ifdef HOST_NAME_MAX
+        size = HOST_NAME_MAX + 1;
+#else
+        /* _POSIX_HOST_NAME_MAX is defined to be 255, so 'size' is 256 here. */
+        size = _POSIX_HOST_NAME_MAX + 1;
+#endif
+
+      tmp = g_malloc (size);
+      failed = (gethostname (tmp, size) == -1);
+      if (failed && size < size_large)
+        {
+          /* Try again with a larger buffer if 'size' may be too small. */
+          g_free (tmp);
+          tmp = g_malloc (size_large);
+          failed = (gethostname (tmp, size_large) == -1);
+        }
+
       if (failed)
         g_clear_pointer (&tmp, g_free);
       utmp = tmp;