use exml library for parsing XML

[goj]

No description provided.

[erszcz]

The main rationale behind switching to exml is the proper support for XML Namespaces.

The current state of XML handling in ejabberd is that it doesn't store the NS (namespace) of every element, but in general assumes that the NS is jabber:client and stores it as pseudo-attribute xmlns only when it knows it's different, i.e. declared by that element itself as an attribute.

What it means in practice is that we deal just right with the following example of XMPP stream:

<stream:stream xmlns:stream="..." xmlns="...">
  <iq>
    <query xmlns="some_query_namespace">
  </iq>
...

but wouldn't be able to cope with equally legal in terms of the standard example:

<stream:stream xmlns:stream="..." xmlns="..." xmlns:q="some_query_namespace">
  <iq>
    <q:query>
  </iq>
...

Firstly, some XMPP client might prefer the latter formulation and current ejabberd just won't understand it.

Secondly, it might prove to be a source of network usage optimization to declare often used namespaces only once and then use just the short prefixes for specific elements.

[michalwski]

Further work is done in #183