v16 backport: Fix closed channel panic in Online DDL cutover

[shlomi-noach]

Manual backport of #13729

For some reason the backport bot did not run.

This backport is not strictly required, since the bug does not happen in v16 - the channel wasn't being closed in this version. However, we do the backport so as to close the channel appropriately and release resources.

Description

Closing a channel should be owned by the writer to the channel. This PR fixes a closed channel panic, described in #13728.

Related discussion in golang google group: https://groups.google.com/g/golang-nuts/c/_Q6FCjWIr18/m/Wb_NlyusRL8J?pli=1

Related Issue(s)

#13728

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes