5 files changed, 40 insertions, 22 deletions
diff --git a/src/or/config.c b/src/or/config.c
index 7b42c9fdb3..148fa9fe04 100644
--- a/src/or/config.c
+++ b/src/or/config.c
@@ -916,6 +916,7 @@ add_default_fallback_dir_servers,(void))
 {
   int i;
   const char *fallback[] = {
+#include "fallback_dirs.inc"
     NULL
   };
   for (i=0; fallback[i]; i++) {
diff --git a/src/or/fallback_dirs.inc b/src/or/fallback_dirs.inc
new file mode 100644
index 0000000000..d9214cb884
--- /dev/null
+++ b/src/or/fallback_dirs.inc
@@ -0,0 +1 @@
+/* This list will be empty until opt-ins are finalised. */
diff --git a/src/or/include.am b/src/or/include.am
index 264c4ae802..7371e55324 100644
--- a/src/or/include.am
+++ b/src/or/include.am
@@ -158,6 +158,7 @@ ORHEADERS = \
 	src/or/dnsserv.h				\
 	src/or/eventdns_tor.h				\
 	src/or/ext_orport.h				\
+	src/or/fallback_dirs.inc			\
 	src/or/fp_pair.h				\
 	src/or/geoip.h					\
 	src/or/entrynodes.h				\
diff --git a/src/or/routerlist.c b/src/or/routerlist.c
index 5e7906475f..638c6c551d 100644
--- a/src/or/routerlist.c
+++ b/src/or/routerlist.c
@@ -1295,8 +1295,8 @@ router_get_fallback_dir_servers(void)
 /** Try to find a running dirserver that supports operations of <b>type</b>.
  *
  * If there are no running dirservers in our routerlist and the
- * <b>PDS_RETRY_IF_NO_SERVERS</b> flag is set, set all the authoritative ones
- * as running again, and pick one.
+ * <b>PDS_RETRY_IF_NO_SERVERS</b> flag is set, set all the fallback ones
+ * (including authorities) as running again, and pick one.
  *
  * If the <b>PDS_IGNORE_FASCISTFIREWALL</b> flag is set, then include
  * dirservers that we can't reach.
@@ -1304,8 +1304,9 @@ router_get_fallback_dir_servers(void)
  * If the <b>PDS_ALLOW_SELF</b> flag is not set, then don't include ourself
  * (if we're a dirserver).
  *
- * Don't pick an authority if any non-authority is viable; try to avoid using
- * servers that have returned 503 recently.
+ * Don't pick a fallback directory mirror if any non-fallback is viable;
+ * (the fallback directory mirrors include the authorities)
+ * try to avoid using servers that have returned 503 recently.
  */
 const routerstatus_t *
 router_pick_directory_server(dirinfo_type_t type, int flags)
@@ -1332,7 +1333,7 @@ router_pick_directory_server(dirinfo_type_t type, int flags)
   log_info(LD_DIR,
            "No reachable router entries for dirservers. "
            "Trying them all again.");
-  /* mark all authdirservers as up again */
+  /* mark all fallback directory mirrors as up again */
   mark_all_dirservers_up(fallback_dir_servers);
   /* try again */
   choice = router_pick_directory_server_impl(type, flags, NULL);
diff --git a/src/test/test_config.c b/src/test/test_config.c
index 28e9fa0f32..00489d1d1a 100644
--- a/src/test/test_config.c
+++ b/src/test/test_config.c
@@ -1521,9 +1521,9 @@ test_config_adding_dir_servers(void *arg)
     );
 
   /* We need to know if add_default_fallback_dir_servers is called,
+   * whatever the size of the list in fallback_dirs.inc,
    * so we use a version of add_default_fallback_dir_servers that adds
-   * one known default fallback directory.
-   * There doesn't appear to be any need to test it unmocked. */
+   * one known default fallback directory. */
   MOCK(add_default_fallback_dir_servers,
        add_default_fallback_dir_servers_known_default);
 
@@ -1581,20 +1581,6 @@ test_config_adding_dir_servers(void *arg)
    *  Cases expected to yield this outcome:
    *    0 (DirAuthorities, AlternateBridgeAuthority, AlternateDirAuthority
    *       and FallbackDir are all NULL)
-   *
-   * Before #15642 but after #13163 - Stop using default authorities when both
-   * Alternate Dir and Bridge Authority are set
-   * (#13163 was committed in 0.2.6 as c1dd43d823c7)
-   *
-   * The behaviour is different in the following cases
-   * where FallbackDir is NULL:
-   *  2, 6, 8
-   *
-   * In these cases, the Default Fallback Directories are applied, even when
-   * DirAuthorities or AlternateDirAuthority are set.
-   *
-   * However, as the list of default fallback directories is currently empty,
-   * this change doesn't modify any user-visible behaviour.
    */
 
   /*
@@ -3209,11 +3195,39 @@ test_config_adding_dir_servers(void *arg)
   UNMOCK(add_default_fallback_dir_servers);
 }
 
+static void
+test_config_default_fallback_dirs(void *arg)
+{
+  const char *fallback[] = {
+#include "../or/fallback_dirs.inc"
+    NULL
+  };
+
+  int n_included_fallback_dirs = 0;
+  int n_added_fallback_dirs = 0;
+
+  (void)arg;
+  clear_dir_servers();
+
+  while (fallback[n_included_fallback_dirs])
+    n_included_fallback_dirs++;
+
+  add_default_fallback_dir_servers();
+
+  n_added_fallback_dirs = smartlist_len(router_get_fallback_dir_servers());
+
+  tt_assert(n_included_fallback_dirs == n_added_fallback_dirs);
+
+  done:
+  clear_dir_servers();
+}
+
 #define CONFIG_TEST(name, flags)                          \
   { #name, test_config_ ## name, flags, NULL, NULL }
 
 struct testcase_t config_tests[] = {
-  CONFIG_TEST(adding_dir_servers, TT_FORK),
+  CONFIG_TEST(adding_dir_servers, 0),
+  CONFIG_TEST(default_fallback_dirs, 0),
   CONFIG_TEST(resolve_my_address, TT_FORK),
   CONFIG_TEST(addressmap, 0),
   CONFIG_TEST(parse_bridge_line, 0),