When an external tool returns a grade for a student, completion status that depends on that grade is not indicated until the student logs out and logs back in again.

To reproduce:

Create a new course.
Add an external tool activity that returns a grade.
Set activity to show as complete when student receives a grade.
Launch the external tool activity as a student and achieve a grade.
Return to the course.

Expected result: The activity should be marked as completed.

Actual result: The activity is marked as not completed.

It appears that this is happening because $SESSION->completioncache isn't being updated when the external tool submits a result back to Moodle. The tool provider submits a request directly to the Moodle server, so it doesn't have access to the user's $SESSION.