[glib] gutils: Fix deadlock if g_get_home_dir() fails when called twice

[Philip Withnall <pwithnall src gnome org>]

commit 9cbfb560614a251edf6d965eb3724c6294bab3f0
Author: Philip Withnall <withnall endlessm com>
Date:   Thu Apr 26 11:14:48 2018 +0100

    gutils: Fix deadlock if g_get_home_dir() fails when called twice
    
    If g_get_home_dir() calculated a NULL home directory (due to $HOME being
    unset and /etc/passwd being inaccessible, for example due to an
    overly-zealous LSM), it would call g_once_init_leave (&home_dir, NULL),
    which would emit a critical and fail to leave the GOnce critical
    section. That meant that the following call to g_get_home_dir() would
    deadlock in g_once_init_enter().
    
    Fix that by setting the home directory to a made-up value in such cases
    (which the documentation handily already explicitly allows).
    
    Thanks to Simon McVittie for the analysis leading to an easy patch.
    
    Signed-off-by: Philip Withnall <withnall endlessm com>
    
    https://bugzilla.gnome.org/show_bug.cgi?id=773435

 glib/gutils.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)
---
diff --git a/glib/gutils.c b/glib/gutils.c
index de4401ad3..5813b2281 100644
--- a/glib/gutils.c
+++ b/glib/gutils.c
@@ -883,6 +883,18 @@ g_get_home_dir (void)
           tmp = entry->home_dir;
         }
 
+      /* If we have been denied access to /etc/passwd (for example, by an
+       * overly-zealous LSM), make up a junk value. The return value at this
+       * point is explicitly documented as ‘undefined’. Memory management is as
+       * immediately above: strictly this should be copied, but we know not
+       * copying it is OK. */
+      if (tmp == NULL)
+        {
+          g_warning ("Could not find home directory: $HOME is not set, and "
+                     "user database could not be read.");
+          tmp = "/";
+        }
+
       g_once_init_leave (&home_dir, tmp);
     }