add a NumDirectoryGuards consensus param too

changes/ticket12688

@@ -1,5 +1,6 @@
   Major features:
-    - Make the number of entry guards (and thus, by default, directory
-      guards too) configurable via a new NumEntryGuards consensus
+    - Make the number of entry guards configurable via a new
+      NumEntryGuards consensus parameter, and the number of directory
+      guards configurable via a new NumDirectoryGuards consensus
       parameter. Implements ticket 12688.
 

src/or/entrynodes.c

@@ -433,10 +433,18 @@ add_an_entry_guard(const node_t *chosen, int reset_status, int prepend,
 static int
 decide_num_guards(const or_options_t *options, int for_directory)
 {
-  if (for_directory && options->NumDirectoryGuards != 0)
-    return options->NumDirectoryGuards;
+  if (for_directory) {
+    int answer;
+    if (options->NumDirectoryGuards != 0)
+      return options->NumDirectoryGuards;
+    answer = networkstatus_get_param(NULL, "NumDirectoryGuards", 0, 0, 10);
+    if (answer) /* non-zero means use the consensus value */
+      return answer;
+  }
+
   if (options->NumEntryGuards)
     return options->NumEntryGuards;
+
   /* Use the value from the consensus, or 3 if no guidance. */
   return networkstatus_get_param(NULL, "NumEntryGuards", 3, 1, 10);
 }