Using callbacks in an API page throws "API resolved without sending a response"

[mattcarlotta]

Bug report

When using middleware callbacks in an API page, the warning: API resolved without sending a response for /api/signedin, this may result in stalled requests is thrown despite a response always being returned.

To Reproduce

Within an API page, add middleware callback functions -- which take some time to resolve -- that wrap the final destination function.

Reproducible example (codesandbox is a bit wonky and not show the warning, so it'll require an additional refresh within the Browser tab and waiting a few seconds before it displays -- check Terminal for warning):

In case of the codesandbox not working, here's a screenshot (this example somewhat mimics a local repo I'm currently working on, which throws the same warnings):
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Expected behavior

Not to throw a warning.

System information

• OS: Ubuntu 18.04 LTS
• Browser: Chrome
• Version of Next.js: 9.3.2

Additional context

Wrapping each callback function within a Promise, as mentioned here, doesn't seem to suppress the warning.

[lfades]

Known issue, we currently thinking on a solution for this. Closing as duplicated of #10589 and #10777

[balazsorban44]

This issue has been automatically locked due to no recent activity. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.