[glib/2-58-339-key-file-free-fix] gkeyfile: remain usable after g_key_file_free()

[Philip Withnall <pwithnall src gnome org>]

commit e9058850ccde9cfeb9d065cf1f74478c59ebc4d4
Author: Will Thompson <will willthompson co uk>
Date:   Thu Sep 20 16:56:26 2018 +0100

    gkeyfile: remain usable after g_key_file_free()
    
    Previously, in the case where 'kf' has more than one ref, calling
    g_key_file_free(kf) would break it. For example, calling
    g_key_file_has_key(kf, ...) would hit the following assertion:
    
        g_hash_table_lookup: assertion 'hash_table != NULL' failed
    
    This is because g_key_file_free() calls g_key_file_clear() which sets
    self->groups and other fields to NULL; most lookup functions assume
    these fields are non-NULL.
    
    One fix would be to call g_key_file_init() right after
    g_key_file_clear() in g_key_file_free(). However, in the case where
    there are no other refs to the keyfile, this would mean allocating
    many new hash tables which will be immediately destroyed when
    g_key_file_unref() removes the last ref. Instead, inline the unref, and
    re-initialize the internal state when the keyfile is still alive.

 glib/gkeyfile.c      |  6 +++++-
 glib/tests/keyfile.c | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 1 deletion(-)
---
diff --git a/glib/gkeyfile.c b/glib/gkeyfile.c
index ae3bbbc1d..4e9b53541 100644
--- a/glib/gkeyfile.c
+++ b/glib/gkeyfile.c
@@ -1191,7 +1191,11 @@ g_key_file_free (GKeyFile *key_file)
   g_return_if_fail (key_file != NULL);
 
   g_key_file_clear (key_file);
-  g_key_file_unref (key_file);
+
+  if (g_atomic_int_dec_and_test (&key_file->ref_count))
+    g_slice_free (GKeyFile, key_file);
+  else
+    g_key_file_init (key_file);
 }
 
 /**
diff --git a/glib/tests/keyfile.c b/glib/tests/keyfile.c
index 16f3b788b..9ee7d02c4 100644
--- a/glib/tests/keyfile.c
+++ b/glib/tests/keyfile.c
@@ -1726,6 +1726,42 @@ test_get_locale (void)
   g_key_file_free (kf);
 }
 
+static void
+test_free_when_not_last_ref (void)
+{
+  GKeyFile *kf;
+  GError *error = NULL;
+  const gchar *data =
+    "[Group]\n"
+    "Key=Value\n";
+
+  kf = load_data (data, G_KEY_FILE_NONE);
+  /* Add a second ref */
+  g_key_file_ref (kf);
+
+  /* Quick coherence check */
+  g_assert_true (g_key_file_has_group (kf, "Group"));
+  g_assert_true (g_key_file_has_key (kf, "Group", "Key", &error));
+  g_assert_no_error (error);
+
+  /* Should clear all keys and groups, and remove one ref */
+  g_key_file_free (kf);
+
+  /* kf should still work */
+  g_assert_false (g_key_file_has_group (kf, "Group"));
+  g_assert_false (g_key_file_has_key (kf, "Group", "Key", &error));
+  check_error (&error, G_KEY_FILE_ERROR, G_KEY_FILE_ERROR_GROUP_NOT_FOUND);
+  g_clear_error (&error);
+
+  g_key_file_load_from_data (kf, data, -1, G_KEY_FILE_NONE, &error);
+  g_assert_no_error (error);
+
+  g_assert_true (g_key_file_has_group (kf, "Group"));
+  g_assert_true (g_key_file_has_key (kf, "Group", "Key", &error));
+
+  g_key_file_unref (kf);
+}
+
 int
 main (int argc, char *argv[])
 {
@@ -1771,6 +1807,7 @@ main (int argc, char *argv[])
   g_test_add_func ("/keyfile/roundtrip", test_roundtrip);
   g_test_add_func ("/keyfile/bytes", test_bytes);
   g_test_add_func ("/keyfile/get-locale", test_get_locale);
+  g_test_add_func ("/keyfile/free-when-not-last-ref", test_free_when_not_last_ref);
 
   return g_test_run ();
 }