If there are unresolved references in grade calculations, accessing grades in any activity with grades triggers infinite loop. Moodle site becomes unresponsive until the timeout of http server or fpm-php kills the resulting process.

The same behavior in appears in moodle versions 3.5 and 3.6. It is not limited to qradebook, but affects all activities, that can be graded. The consequences are grave. The gradebook and any activity that can be graded become inaccessible in the affected course.

In version 3.1 this seems to be fixed as the gradebook reports an error, and do not try to calculate all the grades and possibly exaust all the resources.

The behavior is realy nasty, since any activity that can be graded in the affected course spawns a php proccess that uses 100% cpu and runs until the timeout and consequently few such request can exaust the CPU resources and can make the whole moodle site inaccessible.

The affected versions are 3.5 and 3.6. This was tested on Moodle 3.6.2+ (Build: 20190118) and Moodle 3.5.4 (Build: 20190114).

Steps to reproduce:

create an activity say Quiz1
label Quiz1 with say quiz1 in gradebook
add calculation that contains [[quiz1]] eg =[[quiz1]]
delete Quiz1
wait for cli to clean up Quiz1
visit gradebook or another quiz attemts

Possible workaround:

Course administration (wheel icon) -> More... -> Reset -> Gradebook -> Delete all items and categories

Related to MDL-51360