Check for "the right descriptor", not just "any descriptor".

This patch adds a new node_has_preferred_descriptor() function, and
replaces most users of node_has_descriptor() with it.  That's an
important change, since as of d1874b4339 (our fix for #25213),
we are willing to say that a node has _some_ descriptor, but not the
_right_ descriptor for a particular use case.

Part of a fix for 25691 and 25692.

src/or/circuitbuild.c

@@ -417,7 +417,7 @@ onion_populate_cpath(origin_circuit_t *circ)
                                     circ->cpath->extend_info->identity_digest);
     /* If we don't know the node and its descriptor, we must be bootstrapping.
      */
-    if (!node || !node_has_descriptor(node)) {
+    if (!node || !node_has_preferred_descriptor(node, 1)) {
       return 0;
     }
   }
@@ -1869,7 +1869,7 @@ choose_good_exit_server_general(int need_uptime, int need_capacity)
        */
       continue;
     }
-    if (!node_has_descriptor(node)) {
+    if (!node_has_preferred_descriptor(node, 0)) {
       n_supported[i] = -1;
       continue;
     }
@@ -2443,6 +2443,10 @@ circuit_extend_to_new_exit(origin_circuit_t *circ, extend_info_t *exit_ei)
 
 /** Return the number of routers in <b>routers</b> that are currently up
  * and available for building circuits through.
+ *
+ * (Note that this function may overcount or undercount, if we have
+ * descriptors that are not the type we would prefer to use for some
+ * particular router. See bug #25885.)
  */
 MOCK_IMPL(STATIC int,
 count_acceptable_nodes, (smartlist_t *nodes))
@@ -2847,9 +2851,10 @@ extend_info_new(const char *nickname,
  * of the node (i.e. its IPv4 address) unless
  * <b>for_direct_connect</b> is true, in which case the preferred
  * address is used instead. May return NULL if there is not enough
- * info about <b>node</b> to extend to it--for example, if there is no
- * routerinfo_t or microdesc_t, or if for_direct_connect is true and none of
- * the node's addresses are allowed by tor's firewall and IP version config.
+ * info about <b>node</b> to extend to it--for example, if the preferred
+ * routerinfo_t or microdesc_t is missing, or if for_direct_connect is
+ * true and none of the node's addresses is allowed by tor's firewall
+ * and IP version config.
  **/
 extend_info_t *
 extend_info_from_node(const node_t *node, int for_direct_connect)
@@ -2857,17 +2862,8 @@ extend_info_from_node(const node_t *node, int for_direct_connect)
   tor_addr_port_t ap;
   int valid_addr = 0;
 
-  const int is_bridge = node_is_a_configured_bridge(node);
-  const int we_use_mds = we_use_microdescriptors_for_circuits(get_options());
-
-  if ((is_bridge && for_direct_connect) || !we_use_mds) {
-    /* We need an ri in this case. */
-    if (!node->ri)
-      return NULL;
-  } else {
-    /* Otherwise we need an md. */
-    if (node->rs == NULL || node->md == NULL)
-      return NULL;
+  if (!node_has_preferred_descriptor(node, for_direct_connect)) {
+    return NULL;
   }
 
   /* Choose a preferred address first, but fall back to an allowed address.

src/or/circuituse.c

@@ -2384,7 +2384,7 @@ circuit_get_open_circ_or_launch(entry_connection_t *conn,
         const node_t *r;
         int opt = conn->chosen_exit_optional;
         r = node_get_by_nickname(conn->chosen_exit_name, 0);
-        if (r && node_has_descriptor(r)) {
+        if (r && node_has_preferred_descriptor(r, conn->want_onehop ? 1 : 0)) {
           /* We might want to connect to an IPv6 bridge for loading
              descriptors so we use the preferred address rather than
              the primary. */
@@ -2394,7 +2394,7 @@ circuit_get_open_circ_or_launch(entry_connection_t *conn,
                      "Discarding this circuit.", conn->chosen_exit_name);
             return -1;
           }
-        } else  { /* ! (r && node_has_descriptor(r)) */
+        } else  { /* ! (r && node_has_preferred_descriptor(...)) */
           log_debug(LD_DIR, "considering %d, %s",
                     want_onehop, conn->chosen_exit_name);
           if (want_onehop && conn->chosen_exit_name[0] == '$') {

src/or/entrynodes.c

@@ -185,14 +185,14 @@ should_apply_guardfraction(const networkstatus_t *ns)
   return options->UseGuardFraction;
 }
 
-/** Return true iff we know a descriptor for <b>guard</b> */
+/** Return true iff we know a preferred descriptor for <b>guard</b> */
 static int
 guard_has_descriptor(const entry_guard_t *guard)
 {
   const node_t *node = node_get_by_id(guard->identity);
   if (!node)
     return 0;
-  return node_has_descriptor(node);
+  return node_has_preferred_descriptor(node, 1);
 }
 
 /**
@@ -2269,7 +2269,8 @@ entry_guard_pick_for_circuit(guard_selection_t *gs,
   // XXXX #20827 check Ed ID.
   if (! node)
     goto fail;
-  if (BUG(usage != GUARD_USAGE_DIRGUARD && !node_has_descriptor(node)))
+  if (BUG(usage != GUARD_USAGE_DIRGUARD &&
+          !node_has_preferred_descriptor(node, 1)))
     goto fail;
 
   *chosen_node_out = node;

src/or/hs_common.c

@@ -1280,8 +1280,10 @@ node_has_hsdir_index(const node_t *node)
   tor_assert(node_supports_v3_hsdir(node));
 
   /* A node can't have an HSDir index without a descriptor since we need desc
-   * to get its ed25519 key */
-  if (!node_has_descriptor(node)) {
+   * to get its ed25519 key.  for_direct_connect should be zero, since we
+   * always use the consensus-indexed node's keys to build the hash ring, even
+   * if some of the consensus-indexed nodes are also bridges. */
+  if (!node_has_preferred_descriptor(node, 0)) {
     return 0;
   }
 
@@ -1612,12 +1614,17 @@ hs_pick_hsdir(smartlist_t *responsible_dirs, const char *req_key_str)
   hs_clean_last_hid_serv_requests(now);
 
   /* Only select those hidden service directories to which we did not send a
-   * request recently and for which we have a router descriptor here. */
+   * request recently and for which we have a router descriptor here.
+   *
+   * Use for_direct_connect==0 even if we will be connecting to the node
+   * directly, since we always use the key information in the
+   * consensus-indexed node descriptors for building the index.
+   **/
   SMARTLIST_FOREACH_BEGIN(responsible_dirs, routerstatus_t *, dir) {
     time_t last = hs_lookup_last_hid_serv_request(dir, req_key_str, 0, 0);
     const node_t *node = node_get_by_id(dir->identity_digest);
     if (last + hs_hsdir_requery_period(options) >= now ||
-        !node || !node_has_descriptor(node)) {
+        !node || !node_has_preferred_descriptor(node, 0)) {
       SMARTLIST_DEL_CURRENT(responsible_dirs, dir);
       continue;
     }

src/or/nodelist.c

@@ -43,6 +43,7 @@
 #include "or.h"
 #include "address.h"
 #include "address_set.h"
+#include "bridges.h"
 #include "config.h"
 #include "control.h"
 #include "dirserv.h"
@@ -1139,6 +1140,32 @@ node_has_descriptor(const node_t *node)
           (node->rs && node->md));
 }
 
+/** Return true iff <b>node</b> has the kind of descriptor we would prefer to
+ * use for it, given our configuration and how we intend to use the node.
+ *
+ * If <b>for_direct_connect</b> is true, we intend to connect to the node
+ * directly, as the first hop of a circuit; otherwise, we intend to connect to
+ * it indirectly, or use it as if we were connecting to it indirectly. */
+int
+node_has_preferred_descriptor(const node_t *node,
+                              int for_direct_connect)
+{
+  const int is_bridge = node_is_a_configured_bridge(node);
+  const int we_use_mds = we_use_microdescriptors_for_circuits(get_options());
+
+  if ((is_bridge && for_direct_connect) || !we_use_mds) {
+    /* We need an ri in this case. */
+    if (!node->ri)
+      return 0;
+  } else {
+    /* Otherwise we need an rs and an md. */
+    if (node->rs == NULL || node->md == NULL)
+      return 0;
+  }
+
+  return 1;
+}
+
 /** Return the router_purpose of <b>node</b>. */
 int
 node_get_purpose(const node_t *node)
@@ -2221,7 +2248,8 @@ compute_frac_paths_available(const networkstatus_t *consensus,
               nu);
 
     SMARTLIST_FOREACH_BEGIN(myexits_unflagged, const node_t *, node) {
-      if (node_has_descriptor(node) && node_exit_policy_rejects_all(node)) {
+      if (node_has_preferred_descriptor(node, 0) &&
+          node_exit_policy_rejects_all(node)) {
         SMARTLIST_DEL_CURRENT(myexits_unflagged, node);
         /* this node is not actually an exit */
         np--;

src/or/nodelist.h

@@ -47,6 +47,8 @@ void node_get_verbose_nickname_by_id(const char *id_digest,
                                 char *verbose_name_out);
 int node_is_dir(const node_t *node);
 int node_has_descriptor(const node_t *node);
+int node_has_preferred_descriptor(const node_t *node,
+                                  int for_direct_connect);
 int node_get_purpose(const node_t *node);
 #define node_is_bridge(node) \
   (node_get_purpose((node)) == ROUTER_PURPOSE_BRIDGE)

src/or/rendservice.c

@@ -3596,7 +3596,7 @@ directory_post_to_hs_dir(rend_service_descriptor_t *renddesc,
         /* Don't upload descriptor if we succeeded in doing so last time. */
         continue;
       node = node_get_by_id(hs_dir->identity_digest);
-      if (!node || !node_has_descriptor(node)) {
+      if (!node || !node_has_preferred_descriptor(node,0)) {
         log_info(LD_REND, "Not launching upload for for v2 descriptor to "
                           "hidden service directory %s; we don't have its "
                           "router descriptor. Queuing for later upload.",