captcha_url option now accepts 'auto' value, and it's the default

src/ejabberd_captcha.erl

@@ -441,18 +441,37 @@ do_create_image(Key, FileName) when is_binary(FileName) ->
 get_prog_name() ->
     case ejabberd_option:captcha_cmd() of
         undefined ->
-            ?DEBUG("The option captcha_cmd is not configured, "
+            ?WARNING_MSG("The option captcha_cmd is not configured, "
                    "but some module wants to use the CAPTCHA "
                    "feature.",
                    []),
             false;
         FileName ->
+            maybe_warning_norequesthandler(),
             FileName
     end.
 
+maybe_warning_norequesthandler() ->
+    Host = hd(ejabberd_option:hosts()),
+    URL = get_auto_url(any, ?MODULE, Host),
+    case URL of
+        undefined ->
+            ?WARNING_MSG("The option captcha_cmd is configured, "
+                   "but there is NO request_handler in listen option "
+                   "configured with ejabberd_captcha. Please check "
+                   "https://docs.ejabberd.im/admin/configuration/basic/#captcha",
+                   []);
+        _ ->
+            ok
+    end.
+
 -spec get_url(binary()) -> binary().
 get_url(Str) ->
     case ejabberd_option:captcha_url() of
+	auto ->
+            Host =  ejabberd_config:get_myname(),
+            URL = get_auto_url(any, ?MODULE, Host),
+            <<URL/binary, $/, Str/binary>>;
 	undefined ->
 	    URL = parse_captcha_host(),
 	    <<URL/binary, "/captcha/", Str/binary>>;
@@ -477,6 +496,40 @@ parse_captcha_host() ->
 	    <<"http://", (ejabberd_config:get_myname())/binary>>
     end.
 
+get_auto_url(Tls, Module, Host) ->
+    case find_handler_port_path(Tls, Module) of
+        [] -> undefined;
+        TPPs ->
+            {ThisTls, Port, Path} = case lists:keyfind(true, 1, TPPs) of
+                                        false ->
+                                            lists:keyfind(false, 1, TPPs);
+                                        TPP ->
+                                            TPP
+                                    end,
+            Protocol = case ThisTls of
+                           false -> <<"http">>;
+                           true -> <<"https">>
+                       end,
+            <<Protocol/binary,
+              "://", Host/binary, ":",
+              (integer_to_binary(Port))/binary,
+              "/",
+              (str:join(Path, <<"/">>))/binary>>
+    end.
+
+find_handler_port_path(Tls, Module) ->
+    lists:filtermap(
+      fun({{Port, _, _},
+           ejabberd_http,
+           #{tls := ThisTls, request_handlers := Handlers}})
+            when (Tls == any) or (Tls == ThisTls) ->
+              case lists:keyfind(Module, 2, Handlers) of
+                  false -> false;
+                  {Path, Module} -> {true, {ThisTls, Port, Path}}
+              end;
+         (_) -> false
+      end, ets:tab2list(ejabberd_listener)).
+
 get_transfer_protocol(PortString) ->
     PortNumber = binary_to_integer(PortString),
     PortListeners = get_port_listeners(PortNumber),

src/ejabberd_option.erl

@@ -329,7 +329,7 @@ captcha_host() ->
 captcha_limit() ->
     ejabberd_config:get_option({captcha_limit, global}).
 
--spec captcha_url() -> 'undefined' | binary().
+-spec captcha_url() -> 'auto' | 'undefined' | binary().
 captcha_url() ->
     ejabberd_config:get_option({captcha_url, global}).
 

src/ejabberd_options.erl

@@ -123,7 +123,9 @@ opt_type(captcha_host) ->
 opt_type(captcha_limit) ->
     econf:pos_int(infinity);
 opt_type(captcha_url) ->
-    econf:url();
+    econf:either(
+      econf:url(),
+      econf:enum([auto, undefined]));
 opt_type(certfiles) ->
     econf:list(econf:binary());
 opt_type(cluster_backend) ->
@@ -546,7 +548,7 @@ options() ->
      {captcha_cmd, undefined},
      {captcha_host, <<"">>},
      {captcha_limit, infinity},
-     {captcha_url, undefined},
+     {captcha_url, auto},
      {certfiles, undefined},
      {cluster_backend, mnesia},
      {cluster_nodes, []},

src/ejabberd_options_doc.erl

@@ -458,8 +458,8 @@ doc() ->
         note => "improved in 23.01",
         desc =>
             ?T("Full path to a script that generates http://../basic/#captcha[CAPTCHA] images. "
-               "@VERSION@ is replaced with ejabberd version number in XX.YY format. "
-               "@SEMVER@ is replaced with ejabberd version number in semver format "
+               "'@VERSION@' is replaced with ejabberd version number in 'XX.YY' format. "
+               "'@SEMVER@' is replaced with ejabberd version number in semver format "
                "when compiled with Elixir's mix, or XX.YY format otherwise. "
                "Alternatively, it can be the name of a module that implements ejabberd CAPTCHA support. "
                "There is no default value: when this option is not "
@@ -477,11 +477,17 @@ doc() ->
       #{value => "String",
         desc => ?T("Deprecated. Use _`captcha_url`_ instead.")}},
      {captcha_url,
-      #{value => ?T("URL"),
+      #{value => ?T("URL | auto | undefined"),
+        note => "improved in 23.xx",
         desc =>
             ?T("An URL where http://../basic/#captcha[CAPTCHA] requests should be sent. NOTE: you need "
                "to configure 'request_handlers' for 'ejabberd_http' listener "
-               "as well. There is no default value.")}},
+               "as well. "
+               "If set to 'auto', it builds the URL using a 'request_handler' "
+               "already enabled, with encryption if available. "
+               "If set to 'undefined', it builds the URL using "
+               "the deprecated _`captcha_host`_ + /captcha. "
+               "The default value is 'auto'.")}},
      {certfiles,
       #{value => "[Path, ...]",
         desc =>