The contribution row is saved as part of the job, which happens (long) after the initial validation. For the same reason, the same edit can be associated with different events, if the requests are sent at the same time. We don't have a mechanism to allow inserting (sort of) a placeholder row, so we can't be entirely accurate in the validation. However, it's still possible to resolve or at least mitigate the issue by:

• Adding a locking system via object cache to register write intentions when a job is enqueued, and checking that first when validating a contribution
• Deduplicating jobs for the same revision (should be unnecessary when object cache locking is available, but there should never be duplicates anyway)
• Adding a unique index on the relevant DB columns (as a last safety net)

Change #1271023 had a related patch set uploaded (by Daimona Eaytoy; author: Daimona Eaytoy):

[mediawiki/extensions/CampaignEvents@master] EventContributionJob: set ignoreDuplicates to true

https://gerrit.wikimedia.org/r/1271023

Change #1274282 had a related patch set uploaded (by Daimona Eaytoy; author: Daimona Eaytoy):

[mediawiki/extensions/CampaignEvents@master] [WIP] Implement locking of event contribution insertion

https://gerrit.wikimedia.org/r/1274282

In T422844#11821175, @Daimona wrote:

• Adding a unique index on the relevant DB columns (as a last safety net)

I'm dense, that index is already there. It was added with r1196679, but because it was a backwards-INcompatible change, it had to be applied manually, and I forgot to do that after the patch was merged.

This is actually good news though, because it means we don't have duplicates in production or in beta (verified via: select cec_wiki,cec_revision_id,count(*) as n from ce_event_contributions group by cec_wiki,cec_revision_id having n > 1; for paranoia) and we don't need to apply another schema change.

Also, the failure mode is different than the one originally reported, so I'll update the task description.

Tests made:

• Add a revision ID, and click on add multiple times
• Add a revision ID, and click on add, than close the modal, open it again and try adding the same previous id.

All looks good!

Change #1271023 merged by jenkins-bot:

[mediawiki/extensions/CampaignEvents@master] EventContributionJob: set ignoreDuplicates to true

https://gerrit.wikimedia.org/r/1271023

Change #1274282 merged by jenkins-bot:

[mediawiki/extensions/CampaignEvents@master] Implement locking of event contribution insertion

https://gerrit.wikimedia.org/r/1274282