Add `auth_external_user_exists_check` option

This makes `user_check` hook work better with authentication methods
that don't have a way to determine if user exists (like is the case for
jwt and cert based authentication), and as result will improve mod_offline
and mod_mam handling of offline messages to those users. This reuses
information stored by `mod_last` for this purpose.

Should fix issue #3377.

src/ejabberd_auth.erl

@@ -398,15 +398,29 @@ user_exists(_User, <<"">>) ->
 user_exists(User, Server) ->
     case validate_credentials(User, Server) of
 	{ok, LUser, LServer} ->
-	    lists:any(
-	      fun(M) ->
-		      case db_user_exists(LUser, LServer, M) of
-			  {error, _} ->
-			      false;
-			  Else ->
-			      Else
-		      end
-	      end, auth_modules(LServer));
+	    {Exists, PerformExternalUserCheck} =
+	    lists:foldl(
+		fun(M, {Exists0, PerformExternalUserCheck0}) ->
+		    case db_user_exists(LUser, LServer, M) of
+			{{error, _}, Check} ->
+			    {Exists0, PerformExternalUserCheck0 orelse Check};
+			{Else, Check2} ->
+			    {Exists0 orelse Else, PerformExternalUserCheck0 orelse Check2}
+		    end
+		end, {false, false}, auth_modules(LServer)),
+	    case (not Exists) andalso PerformExternalUserCheck andalso
+		 ejabberd_option:auth_external_user_exists_check(Server) andalso
+		 gen_mod:is_loaded(Server, mod_last) of
+		true ->
+		    case mod_last:get_last_info(User, Server) of
+			not_found ->
+			    false;
+			_ ->
+			    true
+		    end;
+		_ ->
+		    Exists
+	    end;
 	_ ->
 	    false
     end.
@@ -420,11 +434,11 @@ user_exists_in_other_modules_loop([], _User, _Server) ->
     false;
 user_exists_in_other_modules_loop([AuthModule | AuthModules], User, Server) ->
     case db_user_exists(User, Server, AuthModule) of
-	true ->
+	{true, _} ->
 	    true;
-	false ->
+	{false, _} ->
 	    user_exists_in_other_modules_loop(AuthModules, User, Server);
-	{error, _} ->
+	{{error, _}, _} ->
 	    maybe
     end.
 
@@ -628,9 +642,9 @@ db_get_password(User, Server, Mod) ->
 db_user_exists(User, Server, Mod) ->
     case db_get_password(User, Server, Mod) of
 	{ok, _} ->
-	    true;
+	    {true, false};
 	not_found ->
-	    false;
+	    {false, false};
 	error ->
 	    case {Mod:store_type(Server), use_cache(Mod, Server)} of
 		{external, true} ->
@@ -649,18 +663,18 @@ db_user_exists(User, Server, Mod) ->
 			  end,
 		    case Val of
 			{ok, _} ->
-			    true;
+			    {true, Mod /= ejabberd_auth_anonymous} ;
 			not_found ->
-			    false;
+			    {false, Mod /= ejabberd_auth_anonymous};
 			error ->
-			    false;
+			    {false, Mod /= ejabberd_auth_anonymous};
 			{error, _} = Err ->
-			    Err
+			    {Err, Mod /= ejabberd_auth_anonymous}
 		    end;
 		{external, false} ->
-		    ets_cache:untag(Mod:user_exists(User, Server));
+		    {ets_cache:untag(Mod:user_exists(User, Server)), Mod /= ejabberd_auth_anonymous};
 		_ ->
-		    false
+		    {false, false}
 	    end
     end.
 

src/ejabberd_option.erl

@@ -14,6 +14,7 @@
 -export([auth_cache_life_time/0]).
 -export([auth_cache_missed/0]).
 -export([auth_cache_size/0]).
+-export([auth_external_user_exists_check/0, auth_external_user_exists_check/1]).
 -export([auth_method/0, auth_method/1]).
 -export([auth_opts/0, auth_opts/1]).
 -export([auth_password_format/0, auth_password_format/1]).
@@ -224,6 +225,13 @@ auth_cache_missed() ->
 auth_cache_size() ->
     ejabberd_config:get_option({auth_cache_size, global}).
 
+-spec auth_external_user_exists_check() -> boolean().
+auth_external_user_exists_check() ->
+    auth_external_user_exists_check(global).
+-spec auth_external_user_exists_check(global | binary()) -> boolean().
+auth_external_user_exists_check(Host) ->
+    ejabberd_config:get_option({auth_external_user_exists_check, Host}).
+
 -spec auth_method() -> [atom()].
 auth_method() ->
     auth_method(global).

src/ejabberd_options.erl

@@ -81,6 +81,8 @@ opt_type(auth_password_format) ->
     econf:enum([plain, scram]);
 opt_type(auth_scram_hash) ->
     econf:enum([sha, sha256, sha512]);
+opt_type(auth_external_user_exists_check) ->
+    econf:bool();
 opt_type(auth_use_cache) ->
     econf:bool();
 opt_type(c2s_cafile) ->
@@ -542,6 +544,7 @@ options() ->
      {auth_opts, []},
      {auth_password_format, plain},
      {auth_scram_hash, sha},
+     {auth_external_user_exists_check, true},
      {auth_use_cache,
       fun(Host) -> ejabberd_config:get_option({use_cache, Host}) end},
      {c2s_cafile, undefined},

src/ejabberd_options_doc.erl

@@ -395,6 +395,13 @@ doc() ->
            "You shouldn't change this if you already have passwords generated with "
            "a different algorithm - users that have such passwords will not be able "
            "to authenticate. The default value is 'sha'.")}},
+     {auth_external_user_exists_check,
+      #{value => "true | false",
+        desc =>
+        ?T("Supplement check for user existence based on 'mod_last' data, for authentication "
+           "methods that don't have a way to reliable tell if user exists (like is the case for "
+           "'jwt' and certificate based authentication). This helps with processing offline message "
+           "for those users. The default value is 'true'.")}},
      {auth_use_cache,
       #{value => "true | false",
         desc =>