Switch between fallback and authority when auth cert fetch fails

changes/bug18816

@@ -1,4 +1,6 @@
   o Minor bugfix (bootstrap):
     - Consistently use the consensus download schedule for
       authority certificates.
+    - When downloading authority certificates fails, switch from
+      using a fallback to using an authority.
       Resolves ticket 18816; fix on fddb814fe in 0.2.4.13-alpha.

src/or/routerlist.c

@@ -912,11 +912,14 @@ authority_certs_fetch_missing(networkstatus_t *status, time_t now)
     } SMARTLIST_FOREACH_END(d);
 
     if (smartlist_len(fps) > 1) {
+      static int want_auth = 0;
       resource = smartlist_join_strings(fps, "", 0, NULL);
-      /* XXX - do we want certs from authorities or mirrors? - teor */
       directory_get_from_dirserver(DIR_PURPOSE_FETCH_CERTIFICATE, 0,
                                    resource, PDS_RETRY_IF_NO_SERVERS,
-                                   DL_WANT_ANY_DIRSERVER);
+                                   want_auth ? DL_WANT_AUTHORITY
+                                             : DL_WANT_ANY_DIRSERVER);
+      /* on failure, swap between using fallbacks and authorities */
+      want_auth = !want_auth;
       tor_free(resource);
     }
     /* else we didn't add any: they were all pending */
@@ -958,11 +961,14 @@ authority_certs_fetch_missing(networkstatus_t *status, time_t now)
     } SMARTLIST_FOREACH_END(d);
 
     if (smartlist_len(fp_pairs) > 1) {
+      static int want_auth = 0;
       resource = smartlist_join_strings(fp_pairs, "", 0, NULL);
-      /* XXX - do we want certs from authorities or mirrors? - teor */
       directory_get_from_dirserver(DIR_PURPOSE_FETCH_CERTIFICATE, 0,
                                    resource, PDS_RETRY_IF_NO_SERVERS,
-                                   DL_WANT_ANY_DIRSERVER);
+                                   want_auth ? DL_WANT_AUTHORITY
+                                             : DL_WANT_ANY_DIRSERVER);
+      /* on failure, swap between using fallbacks and authorities */
+      want_auth = !want_auth;
       tor_free(resource);
     }
     /* else they were all pending */