Pagination fails silently when mysql.trace_mode is on

[ManuelRazzari]

A sysadmin left enabled mysql.trace_mode on php.ini without me knowing about it, and I spent hours trying to find out why the next_posts_link wasn't showing up.

This is very hard to debug, because

• it fails silently, with no error to look for
• it's hard to repro on a dev server, which may not have the same php.ini settings as in staging/prod (ideally should, but rare for 99% of devs)

Other people had this same issue:

• "​No error, it's just not working"
• "​Am I going crazy"?
• Trace_mode defaulted to "on" in ​this guy's install (!).

I've read through #18536, #10469 and related issues, hoping that usage of SQL_CALC_FOUND_ROWS would be removed, but it seems to be still there, in trunk.

So I guess a "fix" for this would be to check for trace_mode and, if enabled, notify admin users on the wp-admin dashboard.

Meanwhile, I've added a note about this on the ​next_posts_link page.