Annotate PDF generates blank page for external files

Hi,

we experienced the following issue.

Using an Assign activity, configured as follow:

• Submission types: File submissions
• Feedback types: Annotate PDF

When you try to grade file submitted as Access controlled link with a repository that allows Access controlled links (ie. Google Drive, ...), you get a blank page.

NOTE: using the same Repository, if you submit the file as Internal, it works fine.

We analyzed the issue and it seems that external files are all stored with the same content hash, referencing a blank empty file in filedir.

In file /mod/assign/feedback/editpdf/classes/document_services.php the function list_compatible_submission_files_for_attempt iterates over submitted files in order to collect files to combine. For external files it uses the "stored blank file" instead of the "original external file".

 How to reproduce:

1. Configure a Google Drive repository, enabling Access controlled links (as described https://docs.moodle.org/311/en/Google_Drive_repository).
2. Create a "test course"
3. Create a test Assignment inside the "test course", configured as follow:
   • Submission types: File submissions
   • Feedback types: Annotate PDF
4. Enrol yourself as Student in "test course"
5. Switch role to Student (using feature Switch role to...)
6. Enter the newly created assignment and submit a PDF file using Google Drive repository (ensure you are submitting the file as link)
7. Switch back to normal role
8. Enter the assignment and view all the submissions
9. Grade your submission
10. It should display a blank page in Annotate PDF frame

Giorgio