404 Helper

[johnbillion]

I've been working on some functionality for QM which aims to help you out when you encounter a 404 error. It will help pinpoint the reason for the 404 and hopefully make it easier to identify the reason you're seeing a 404 error page instead of the nice new thing that you're building.

I've started this but it needs more work. Some of the ideas are worth exposing even when you're not encountering a 404.

Current ideas:

• Identify whether the rewrite rules need flushing.
• Display a stack trace for the call to set_404().
• Identify whether any rewrite rule matched or whether the fallback rule for Pages matched.
• Identify other matching rewrite rules with lower priority (already exists in the Request panel, could be improved).
• Display information relating to rewrite endpoints (see also Feature request: current EP mask for endpoint #318) including identifying whether one was matched or not.
• Expose the main WP_Query properties (allow user to identify mismatch between it and their expectation).
• List the filters and actions that have callbacks hooked into hooks that affect the request (already exists in the Request panel).
• Identify whether the main query triggered the 404 due to an empty result set, or something else called set_404() manually.
• Identify whether the current request is the result of a redirect from somewhere else (which may not be expected).
• Identify query variable clashes and use of reserved terms in query vars.
• Identify unmatched custom rewrite tags (%foo%), not sure if this is even possible.
• Identify messed up queried objects (eg. empty or null).
• Is WP looking for a child page or child category when you're not expecting it?
• Permalinks are set to the default (no rewriting)
• Some Multisite stuff (eg. are you on the correct site?).

Other ideas very welcome.