Remove buggy IPv6 and ed25519 handling from get_lspecs_from_extend_info()

The previous version of this function had the following issues:
* it didn't check if the extend_info contained an IPv6 address,
* it didn't check if the ed25519 identity key was valid.
But we can't add IPv6 support in a bugfix release.

Instead, BUG() if the address is an IPv6 address, so we always put IPv4
addresses in link specifiers. And ignore missing ed25519 identifiers,
rather than generating an all-zero link specifier.

This supports v3 hidden services on IPv4, dual-stack, and IPv6, and
v3 single onion services on IPv4 only.

Part of 23820, bugfix on 0.3.2.1-alpha.

src/or/hs_circuit.c

@@ -533,7 +533,10 @@ retry_service_rendezvous_point(const origin_circuit_t *circ)
 }
 
 /* Using an extend info object ei, set all possible link specifiers in lspecs.
- * IPv4, legacy ID and ed25519 ID are mandatory thus MUST be present in ei. */
+ * legacy ID is mandatory thus MUST be present in ei. If IPv4 is not present,
+ * logs a BUG() warning, and returns an empty smartlist. Clients never make
+ * direct connections to rendezvous points, so they should always have an
+ * IPv4 address in ei. */
 static void
 get_lspecs_from_extend_info(const extend_info_t *ei, smartlist_t *lspecs)
 {
@@ -542,7 +545,11 @@ get_lspecs_from_extend_info(const extend_info_t *ei, smartlist_t *lspecs)
   tor_assert(ei);
   tor_assert(lspecs);
 
-  /* IPv4 is mandatory. */
+  /* We require IPv4, we will add IPv6 support in a later tor version */
+  if (BUG(!tor_addr_is_v4(&ei->addr))) {
+    return;
+  }
+
   ls = link_specifier_new();
   link_specifier_set_ls_type(ls, LS_IPV4);
   link_specifier_set_un_ipv4_addr(ls, tor_addr_to_ipv4h(&ei->addr));
@@ -560,15 +567,15 @@ get_lspecs_from_extend_info(const extend_info_t *ei, smartlist_t *lspecs)
   link_specifier_set_ls_len(ls, link_specifier_getlen_un_legacy_id(ls));
   smartlist_add(lspecs, ls);
 
-  /* ed25519 ID is mandatory. */
-  ls = link_specifier_new();
-  link_specifier_set_ls_type(ls, LS_ED25519_ID);
-  memcpy(link_specifier_getarray_un_ed25519_id(ls), &ei->ed_identity,
-         link_specifier_getlen_un_ed25519_id(ls));
-  link_specifier_set_ls_len(ls, link_specifier_getlen_un_ed25519_id(ls));
-  smartlist_add(lspecs, ls);
-
-  /* XXX: IPv6 is not clearly a thing in extend_info_t? */
+  /* ed25519 ID is only included if the node has it. */
+  if (!ed25519_public_key_is_zero(&ei->ed_identity)) {
+    ls = link_specifier_new();
+    link_specifier_set_ls_type(ls, LS_ED25519_ID);
+    memcpy(link_specifier_getarray_un_ed25519_id(ls), &ei->ed_identity,
+           link_specifier_getlen_un_ed25519_id(ls));
+    link_specifier_set_ls_len(ls, link_specifier_getlen_un_ed25519_id(ls));
+    smartlist_add(lspecs, ls);
+  }
 }
 
 /* Using the given descriptor intro point ip, the extend information of the
@@ -1053,6 +1060,14 @@ hs_circ_send_introduce1(origin_circuit_t *intro_circ,
    * object which is used to build the content of the cell. */
   setup_introduce1_data(ip, rend_circ->build_state->chosen_exit,
                         subcredential, &intro1_data);
+  /* If we didn't get any link specifiers, it's because our extend info was
+   * bad. */
+  if (BUG(!intro1_data.link_specifiers) ||
+      !smartlist_len(intro1_data.link_specifiers)) {
+    log_warn(LD_REND, "Unable to get link specifiers for INTRODUCE1 cell on "
+             "circuit %u.", TO_CIRCUIT(intro_circ)->n_circ_id);
+    goto done;
+  }
 
   /* Final step before we encode a cell, we setup the circuit identifier which
    * will generate both the rendezvous cookie and client keypair for this