core: Add --union mode to checkout

This is another step towards ostbuild using this instead of the
"compose" builtin.

src/libostree/ostree-core.c

@@ -915,6 +915,9 @@ ostree_create_temp_file_from_input (GFile            *dir,
   /* 128 attempts seems reasonable... */
   for (i = 0; i < 128; i++)
     {
+      if (g_cancellable_set_error_if_cancelled (cancellable, error))
+        goto out;
+
       g_free (possible_name);
       possible_name = subst_xxxxxx (tmp_name->str);
       g_clear_object (&possible_file);
@@ -941,7 +944,7 @@ ostree_create_temp_file_from_input (GFile            *dir,
           break;
         }
     }
-  if (i == 128)
+  if (i >= 128)
     {
       g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
                    "Exhausted 128 attempts to create a temporary file");
@@ -990,3 +993,64 @@ ostree_create_temp_regular_file (GFile            *dir,
   g_clear_object (&ret_stream);
   return ret;
 }
+
+gboolean
+ostree_create_temp_hardlink (GFile            *dir,
+                             GFile            *src,
+                             const char       *prefix,
+                             const char       *suffix,
+                             GFile           **out_file,
+                             GCancellable     *cancellable,
+                             GError          **error)
+{
+  gboolean ret = FALSE;
+  GString *tmp_name = NULL;
+  char *possible_name = NULL;
+  GFile *possible_file = NULL;
+  int i = 0;
+
+  tmp_name = create_tmp_string (ot_gfile_get_path_cached (dir),
+                                prefix, suffix);
+  
+  /* 128 attempts seems reasonable... */
+  for (i = 0; i < 128; i++)
+    {
+      if (g_cancellable_set_error_if_cancelled (cancellable, error))
+        goto out;
+
+      g_free (possible_name);
+      possible_name = subst_xxxxxx (tmp_name->str);
+      g_clear_object (&possible_file);
+      possible_file = g_file_get_child (dir, possible_name);
+
+      if (link (ot_gfile_get_path_cached (src), ot_gfile_get_path_cached (possible_file)) < 0)
+        {
+          if (errno == EEXIST)
+            continue;
+          else
+            {
+              ot_util_set_error_from_errno (error, errno);
+              goto out;
+            }
+        }
+      else
+        {
+          break;
+        }
+    }
+  if (i >= 128)
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                   "Exhausted 128 attempts to create a temporary file");
+      goto out;
+    }
+
+  ret = TRUE;
+  ot_transfer_out_value(out_file, &possible_file);
+ out:
+  if (tmp_name)
+    g_string_free (tmp_name, TRUE);
+  g_free (possible_name);
+  g_clear_object (&possible_file);
+  return ret;
+}