Old log entry parameters were stored with integer keys. In the new style, they are stored under keys such as "4::foo", which identifies that the parameter is "foo" and should be "$4" when passed into Mediawiki messages.

For core log entry types, we remap either of these to more normal names. But for non-core types, we just output the given names directly. This works fine for the old style, and still works for the new style for most formats except for the part where clients now have to check for both "4::foo" and "0". But bug 43221 points out that format=xml winds up generating invalid XML, something like <item 4::foo="value" />.

At the moment, I'm leaning towards the following to fix this: 1. Numeric keys will continue to be output as-is, since there's really nothing else we can do. 2. Keys like "4::foo" will be output as "foo". 3. Add a hook to allow extensions to override all of the above, like we already do for core modules. 4. (probably) Patch WMF-deployed extensions that ever generated old-style log entries to use the hook.

For new-style non-core log entries, and for types provided by extensions that start taking advantage of the new hook, this will break backwards compatibility but will result in the cleanest output.

Any comments or alternative suggestions?