Enforce directionality in connection_or_set_identity_digest().

This function has never gotten testing for the case where an
identity had been set, and then got set to something else.  Rather
than make it handle those cases, we forbid them.

src/or/connection_or.c

@@ -105,20 +105,51 @@ connection_or_clear_identity_map(void)
 }
 
 /** Change conn->identity_digest to digest, and add conn into
- * orconn_digest_map. */
+ * the appropriate digest maps.
+ *
+ * NOTE that this function only allows two kinds of transitions: from
+ * unset identity to set identity, and from idempotent re-settings
+ * of the same identity.  It's not allowed to clear an identity or to
+ * change an identity.  Return 0 on success, and -1 if the transition
+ * is not allowed.
+ **/
 static void
 connection_or_set_identity_digest(or_connection_t *conn,
                                   const char *rsa_digest,
                                   const ed25519_public_key_t *ed_id)
 {
+  channel_t *chan = NULL;
   tor_assert(conn);
   tor_assert(rsa_digest);
 
-  if (tor_memeq(conn->identity_digest, rsa_digest, DIGEST_LEN))
+  if (conn->chan)
+    chan = TLS_CHAN_TO_BASE(conn->chan);
+
+  log_info(LD_HANDSHAKE, "Set identity digest for %p (%s): %s %s.",
+           conn,
+           escaped_safe_str(conn->base_.address),
+           hex_str(rsa_digest, DIGEST_LEN),
+           ed25519_fmt(ed_id));
+  log_info(LD_HANDSHAKE, "   (Previously: %s %s)",
+           hex_str(conn->identity_digest, DIGEST_LEN),
+           chan ? ed25519_fmt(&chan->ed25519_identity) : "<null>");
+
+  const int rsa_id_was_set = ! tor_digest_is_zero(conn->identity_digest);
+  const int ed_id_was_set =
+    chan && !ed25519_public_key_is_zero(&chan->ed25519_identity);
+  const int rsa_changed =
+    tor_memneq(conn->identity_digest, rsa_digest, DIGEST_LEN);
+  const int ed_changed = ed_id_was_set &&
+    (!ed_id || !ed25519_pubkey_eq(ed_id, &chan->ed25519_identity));
+
+  tor_assert(!rsa_changed || !rsa_id_was_set);
+  tor_assert(!ed_changed || !ed_id_was_set);
+
+  if (!rsa_changed && !ed_changed)
     return;
 
   /* If the identity was set previously, remove the old mapping. */
-  if (! tor_digest_is_zero(conn->identity_digest)) {
+  if (rsa_id_was_set) {
     connection_or_clear_identity(conn);
     if (conn->chan)
       channel_clear_identity_digest(TLS_CHAN_TO_BASE(conn->chan));
@@ -126,8 +157,9 @@ connection_or_set_identity_digest(or_connection_t *conn,
 
   memcpy(conn->identity_digest, rsa_digest, DIGEST_LEN);
 
-  /* If we're setting the ID to zero, don't add a mapping. */
-  if (tor_digest_is_zero(rsa_digest))
+  /* If we're initializing the IDs to zero, don't add a mapping yet. */
+  if (tor_digest_is_zero(rsa_digest) &&
+      (!ed_id || ed25519_public_key_is_zero(ed_id)))
     return;
 
   /* Deal with channels */