Avoid excluding guards from path building in minimal test networks

choose_good_entry_server() now excludes current entry
guards and their families, unless we're in a test network,
and excluding guards would exclude all nodes.

This typically occurs in incredibly small tor networks,
and those using TestingAuthVoteGuard *

This is an incomplete fix, but is no worse than the previous
behaviour, and only applies to minimal, testing tor networks
(so it's no less secure).

Discovered as part of #13718.

changes/bug13718-avoid-excluding-guards

@@ -0,0 +1,8 @@
+  o Minor bugfixes:
+    - Avoid excluding guards from path building in minimal test networks,
+      when we're in a test network, and excluding guards would exclude
+      all nodes. This typically occurs in incredibly small tor networks,
+      and those using TestingAuthVoteGuard *
+      This fix only applies to minimal, testing tor networks,
+      so it's no less secure.
+      Discovered as part of #13718.

src/or/circuitbuild.c

@@ -2053,9 +2053,18 @@ choose_good_entry_server(uint8_t purpose, cpath_build_state_t *state)
         smartlist_add(excluded, (void*)node);
     });
   }
-  /* and exclude current entry guards and their families, if applicable */
+  /* and exclude current entry guards and their families,
+   * unless we're in a test network, and excluding guards
+   * would exclude all nodes (i.e. we're in an incredibly small tor network,
+   * or we're using TestingAuthVoteGuard *).
+   * This is an incomplete fix, but is no worse than the previous behaviour,
+   * and only applies to minimal, testing tor networks
+   * (so it's no less secure) */
   /*XXXX025 use the using_as_guard flag to accomplish this.*/
-  if (options->UseEntryGuards) {
+  if (options->UseEntryGuards
+      && (!options->TestingTorNetwork ||
+          smartlist_len(nodelist_get_list()) > smartlist_len(get_entry_guards())
+     )) {
     SMARTLIST_FOREACH(get_entry_guards(), const entry_guard_t *, entry,
       {
         if ((node = node_get_by_id(entry->identity))) {