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Adding a diff comment to an image will sometimes result in a 422 response

Summary

Adding a comment to an image in the diff view will sometimes result in a 422 Unprocessable Entity response, with the response body containing the error must be a valid json schema.

If you view the request that got a 422 response in your browsers developer console, right click and go to Copy >> Copy as cURL you can easily reproduce this.

The issue lies with the x and y positional coordinates we pass. On the requests that fail, one of these values will be a decimal. For example, here's the data that was passed to one of my failing requests:

'{"line_type":"old","merge_request_diff_head_sha":"716f5292cbcc68e144e50fbc4114c9a7a1488176","in_reply_to_discussion_id":"","note_project_id":"","target_type":"merge_request","target_id":133095735,"return_discussion":true,"note":{"note":"test","position":"{\"base_sha\":\"fcf0efd9fb4d950a78de10111587925c02c1b233\",\"start_sha\":\"fcf0efd9fb4d950a78de10111587925c02c1b233\",\"head_sha\":\"716f5292cbcc68e144e50fbc4114c9a7a1488176\",\"old_path\":\"nice.jpg\",\"new_path\":\"nice.jpg\",\"position_type\":\"image\",\"old_line\":null,\"new_line\":null,\"x\":219,\"y\":218.00000000000003,\"width\":400,\"height\":400}","noteable_type":"MergeRequest","noteable_id":133095735,"commit_id":null,"type":"DiffNote","line_code":null}}'

Note the value of y above - it is 218.00000000000003. I did some testing by editing this value and resending the request, and it seems this occurs anytime the decimal is a non-repeating number. For example, 218.00000000000003 and 218.01 fail, but 218.0000000000000 and 218.00 result in a successful request.

Steps to reproduce

  1. Create new project with just README.md
  2. Web IDE -> upload image -> commit & create new branch & MR
  3. View changes (/diffs) in MR
  4. Click anywhere on image, type something into comment box
  5. Click "add comment now" not "Start a review"
  6. Sometimes doesn't work, message in UI says "Saving the comment failed", 422 error in browser console

What is the current bug behavior?

Adding a comment to an image in the diff view will sometimes result in a 422 Unprocessable Entity response, with the response body containing the error must be a valid json schema.

What is the expected correct behavior?

Comment is added to the image diff as expected.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com.

Possible fixes

It looks like this is occurring here. Perhaps we should just be rounding these numbers down/not grabbing decimal values at all.