Error 500 creating diff note (comment) - hide whitespace mode

Scenario 1 (review): Steps:

1. Go to jay_mccure/bug_triage!1 (diffs)
2. Change settings to be Side by side and do not show whitespace (bug can also occur in inline mode also.
3. Start a review with a comment on line 54 (right hand side)
4. Finish Review -> Submit review

Actual outcome:

• 500 error occurs from /publish
• Comment is actually submitted, but it is also pending and the review is still open
• Other comments in the review are lost
• Deleting the comment that was submitted requires the user to delete the comment AND refresh the page to remove it.

Scenario 2 (add comment now):

1. Go to jay_mccure/bug_triage!1 (diffs)
2. Change settings to be Side by side and do not show whitespace
3. Add an immediate comment on line 54 (right hand side)

Actual outcome:

• 500 error occurs from /notes
• Overview page fails to show diff for comment (see screenshot below)
• Deleting the comment that was submitted requires the user to delete the comment AND refresh the page to remove it.

500comment480

Other symptoms

1. The diff for the comment fails to load in the overview page. Try Again does nothing.

2. Comments on whitespace can get hidden see: #800 (closed)

Request to `/drafts`

{
    "view": "parallel",
    "line_type": "old",
    "merge_request_diff_head_sha": "1e617cd264d281b92eb93e97cc015572a0c6135a",
    "in_reply_to_discussion_id": "",
    "note_project_id": "",
    "target_type": "merge_request",
    "target_id": 176052042,
    "return_discussion": true,
    "draft_note":
    {
        "note": "fd",
        "position": "{\"base_sha\":\"1b35335879ed1f25044ebcde2b50c9fb451a1976\",\"start_sha\":\"1b35335879ed1f25044ebcde2b50c9fb451a1976\",\"head_sha\":\"1e617cd264d281b92eb93e97cc015572a0c6135a\",\"old_path\":\"500_comment_test.rb\",\"new_path\":\"500_comment_test.rb\",\"position_type\":\"text\",\"old_line\":null,\"new_line\":54,\"line_range\":{\"start\":{\"line_code\":\"28827ed01984e87b49208380e411ccab758513f8_37_54\",\"type\":\"new\",\"old_line\":null,\"new_line\":54},\"end\":{\"line_code\":\"28827ed01984e87b49208380e411ccab758513f8_37_54\",\"type\":\"new\",\"old_line\":null,\"new_line\":54}}}",
        "noteable_type": "MergeRequest",
        "noteable_id": 176052042,
        "commit_id": null,
        "type": "DiffNote",
        "line_code": "28827ed01984e87b49208380e411ccab758513f8_37_54"
    }
}

Response from `/drafts`

{
    "id": 19738888,
    "author":
    {
        "id": 11809982,
        "username": "jay_mccure",
        "name": "Jay McCure",
        "state": "active",
        "avatar_url": "https://gitlab.com/uploads/-/system/user/avatar/11809982/avatar.png",
        "status_tooltip_html": null,
        "show_status": false,
        "availability": null,
        "path": "/jay_mccure"
    },
    "merge_request_id": 176052042,
    "position":
    {
        "base_sha": "1b35335879ed1f25044ebcde2b50c9fb451a1976",
        "start_sha": "1b35335879ed1f25044ebcde2b50c9fb451a1976",
        "head_sha": "3b17b67d51dfbb19dc27886bb037194739e98c23",
        "old_path": "500_comment_test.rb",
        "new_path": "500_comment_test.rb",
        "position_type": "text",
        "old_line": 84,
        "new_line": 54,
        "line_range":
        {
            "start":
            {
                "line_code": "28827ed01984e87b49208380e411ccab758513f8_37_54",
                "type": "new",
                "old_line": null,
                "new_line": 54
            },
            "end":
            {
                "line_code": "28827ed01984e87b49208380e411ccab758513f8_37_54",
                "type": "new",
                "old_line": null,
                "new_line": 54
            }
        }
    },
    "line_code": "28827ed01984e87b49208380e411ccab758513f8_37_54",
    "file_identifier_hash": "6b80226da38a9a0ecf6ac5004b967aa864e1d117",
    "file_hash": "28827ed01984e87b49208380e411ccab758513f8",
    "file_path": "500_comment_test.rb",
    "note": "fd",
    "note_html": "<p data-sourcepos=\"1:1-1:2\" dir=\"auto\">fd</p>",
    "references":
    {
        "users":
        [],
        "commands": ""
    },
    "discussion_id": null,
    "resolve_discussion": false,
    "noteable_type": "MergeRequest",
    "current_user":
    {
        "can_edit": true,
        "can_award_emoji": false,
        "can_resolve": false
    }
}

3. When using /submit_review quick action a confusing error is displayed and the /submit_review comment also fails

Sentry suspects !68045 (merged).

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2765545/?referrer=gitlab_plugin

DiffNote::NoteDiffFileCreationError: Failed to find diff line for: spec/lib/release_tools/promotion/status_note_spec.rb, old_line: , new_line: 42
  app/models/diff_note.rb:46:in `create_diff_file'
    raise NoteDiffFileCreationError, DIFF_LINE_NOT_FOUND_MESSAGE % {
  lib/gitlab/metrics/instrumentation.rb:160:in `block in create_diff_file'
    .measure { super }
  lib/gitlab/metrics/method_call.rb:27:in `measure'
    retval = yield
  lib/gitlab/metrics/instrumentation.rb:160:in `create_diff_file'
    .measure { super }
  config/initializers/forbid_sidekiq_in_transactions.rb:52:in `block in committed!'
    Sidekiq::Worker.skipping_transaction_check { super }
...
(179 additional frame(s) were not displayed)

Proposal

This issue tracks the mitigation of the data loss caused in this scenario.

We'll go ahead with:

• Disabling adding comments when seeing the diff with Show whitespace changes turned off.

We'll have a follow-up issue to add support for commenting in this diff mode: #398186 (closed)