Missing logo file causes CSFR error in debug mode

[millipedia]

Here's a fun one this morning.

I normally set a custom logo in the admin masthead using the settings in the AdminThemeUikit module (mostly so I know which site I'm in when I've got serveral open at the same time). Today I mistyped the logo name when I was setting this for a site so the logo didn't load. But then when I logged out and tried to log in again I got the dreaded red screen of death.

After checking all the usual suspects (file permissions, disc space etc) I renamed the logo file to match my misspelled setting and Bob's your uncle, I can log in again. If I delete the logo file the CSFR error comes back, replace it and it's all good.

This only happens in debug mode so my guess is that there's some PHP warning message that's mucking up some javascript but I've not managed to find it yet.

I'll log it as a bug on GitHub but thought I'd stick it up here cos it's the kind of bug that could have ages to find and I'm feeling smug that I got it quickly (so no one tell me it's a known bug and I could have just Googled it).

 

[millipedia]

After having logged this on GitHub and having wiser people than me investigate, we managed to figure out that this only happens when using Varnish cache (in particular using Varnish on a Cloudways server). Disabling Varnish was enough to fix the problem.

I'll leave it here though because hey, it's a nice bug, and maybe it'll help someone else.