PubSub: Support 'max_items=max' node configuration

Let clients request the maximum limit for the node configuration option
'max_items' by specifying the special value 'max' instead of an integer.
This was added to XEP-0060, revision 1.17.0 (and clarified in revision
1.20.0).

Thanks to Ammonit Measurement GmbH for sponsoring this work.

mix.exs

@@ -124,7 +124,7 @@ defmodule Ejabberd.MixProject do
      {:pkix, "~> 1.0"},
      {:stringprep, ">= 1.0.26"},
      {:stun, "~> 1.0"},
-     {:xmpp, "~> 1.5"},
+     {:xmpp, git: "https://github.com/processone/xmpp", ref: "e943c0285aa85e3cbd4bfb9259f6b7de32b00395", override: true},
      {:yconf, "~> 1.0"}]
     ++ cond_deps()
   end

rebar.config

@@ -59,7 +59,7 @@
         {stringprep, ".*", {git, "https://github.com/processone/stringprep", {tag, "1.0.27"}}},
         {if_var_true, stun,
          {stun, ".*", {git, "https://github.com/processone/stun", {tag, "1.0.44"}}}},
-        {xmpp, ".*", {git, "https://github.com/processone/xmpp", {tag, "1.5.4"}}},
+        {xmpp, ".*", {git, "https://github.com/processone/xmpp", "e943c0285aa85e3cbd4bfb9259f6b7de32b00395"}},
         {yconf, ".*", {git, "https://github.com/processone/yconf", {tag, "1.0.12"}}}
        ]}.
 

src/mod_pubsub.erl

@@ -3553,8 +3553,10 @@ decode_get_pending(#xdata{fields = Fs}, Lang) ->
 check_opt_range(_Opt, _Opts, undefined) ->
     true;
 check_opt_range(Opt, Opts, Max) ->
-    Val = proplists:get_value(Opt, Opts, Max),
-    Val =< Max.
+    case proplists:get_value(Opt, Opts, Max) of
+	max -> true;
+	Val -> Val =< Max
+    end.
 
 -spec get_max_items_node(host()) -> undefined | non_neg_integer().
 get_max_items_node(Host) ->
@@ -3708,6 +3710,7 @@ features() ->
      <<"access-whitelist">>,   % OPTIONAL
      <<"collections">>,   % RECOMMENDED
      <<"config-node">>,   % RECOMMENDED
+     <<"config-node-max">>,
      <<"create-and-configure">>,   % RECOMMENDED
      <<"item-ids">>,   % RECOMMENDED
      <<"last-published">>,   % RECOMMENDED