Backend: Process Slack chat unfurl events (public links only for issues + MRs)

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This issue builds on the events API prepared in #358676 (closed) to process Slack link unfurl events &6389.

To iterate on link unfurling, this issue stops short on a few aspects:

Only links to public things will be unfurled. Links to private and confidential things will be done in #358683.
Only Issues and Merge Request links will be unfurled (this is being clarified in #337481 (comment 907440946)).

Configure our development Slack app according to these steps.
Build on the existing Slack events API endpoint to process the chat unfurl event on a queue.
Identify the object from the URL (we may be able to use Gitlab::ReferenceExtractor to do this, see !105655 (merged) as an example of identifying objects from their URIs)
Only unfurl links to public things. Private and confidential things will be unfurled later in #358683.
Post back to the Slack API chat.unfurl method using Slack::API.post.

An example of the Slack Block Kit code to make a merge request view is in this comment #337481[Unfurling_states_-_issues.png] (comment 730095948). See also the Design UX issue #337481.

Suggestion:

add higher level unit or request specs that assert the correct response content is being sent to the slack hook for requests from slack. (issue and merge requests)
Future state: write e2e tests against Slack app.

Considerations that may already be considered:

Edge cases and empty states - Example: (20 assignees on an issue, or no assignees, long issue or MR titles)
Consistency between Slack app / Slash commands
For slash commands, consider how a self-hosted instance with relative urls will generate links.

Edited Jul 29, 2025 by 🤖 GitLab Bot 🤖