Batch Changes and Bitbucket: 409 error when updating batch changes after they were published

Created by: northyg

• Sourcegraph version: 3.34.2
• Platform information: docker-container

Steps to reproduce:

1. Attempted to update some batch changes after they have been published
2. Most of those changes are now in an error state and failing with errors like:

Updating changeset: Bitbucket API HTTP error: code=409 url="<url example here>" body="{"errors":[{"context":null,"message":"You are attempting to modify a pull request based on out-of-date information.","exceptionName":"com.atlassian.bitbucket.pull.PullRequestOutOfDateException","currentVersion":2,"expectedVersion":1,"pullRequest":{"id":89,"version":2,...

3. Updated step: This looks like it could have been caused by changing the PR title and/or description in the batch change. Reverting the title/description changes appears to get sourcegraph back in a better state.

Expected behavior:

Batch change should succeed without error or not get stuck in a failed state.

Actual behavior:

Stuck in a failed state.

If would like immediate help on this, please email [email protected] (you can still create the issue, but there are not SLAs on issues like there are for support requests).