hs-v3: Build onion address before registering ephemeral service

With the work on #25500 (reducing CPU client usage), the HS service main loop
callback is enabled as soon as the HS service map changes which happens when
registering a new service.

Unfortunately, for an ephemeral service, we were building the onion address
*after* the registration leading to the "service->onion_address` to be an
empty string.

This broke the "HS_DESC CREATED" event which had no onion address in it. And
also, we were logging an empty onion address for that service.

Fixes #25939

Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/bug25939

@@ -0,0 +1,3 @@
+  o Minor bugfixes (onion services):
+    - Fix a bug that blocked the creation of ephemeral v3 onion services. Fixes
+      bug 25939; bugfix on 0.3.4.1-alpha.

src/or/hs_service.c

@@ -3062,6 +3062,12 @@ hs_service_add_ephemeral(ed25519_secret_key_t *sk, smartlist_t *ports,
     goto err;
   }
 
+  /* Build the onion address for logging purposes but also the control port
+   * uses it for the HS_DESC event. */
+  hs_build_address(&service->keys.identity_pk,
+                   (uint8_t) service->config.version,
+                   service->onion_address);
+
   /* The only way the registration can fail is if the service public key
    * already exists. */
   if (BUG(register_service(hs_service_map, service) < 0)) {
@@ -3071,14 +3077,10 @@ hs_service_add_ephemeral(ed25519_secret_key_t *sk, smartlist_t *ports,
     goto err;
   }
 
-  /* Last step is to build the onion address. */
-  hs_build_address(&service->keys.identity_pk,
-                   (uint8_t) service->config.version,
-                   service->onion_address);
-  *address_out = tor_strdup(service->onion_address);
-
   log_info(LD_CONFIG, "Added ephemeral v3 onion service: %s",
            safe_str_client(service->onion_address));
+
+  *address_out = tor_strdup(service->onion_address);
   ret = RSAE_OKAY;
   goto end;