Error when trying to merge a PR that's already merged

[Mariatta]

The error can be caught.

Log:
Aug 09 15:03:52 miss-islington app/web.1: GH delivery ID 19e9ae34-9c20-11e8-8119-f84026ce9ae3
Aug 09 15:03:53 miss-islington heroku/router: at=info method=POST path="/" host=miss-islington.herokuapp.com request_id=61fe7df2-985b-4549-bbaf-8a0d34964361 fwd="192.30.252.34" dyno=web.1 connect=1ms service=2394ms status=500 bytes=187 protocol=https
Aug 09 15:03:54 miss-islington app/web.1: Traceback (most recent call last):
Aug 09 15:03:54 miss-islington app/web.1: File "/app/miss_islington/main.py", line 40, in main
Aug 09 15:03:54 miss-islington app/web.1: await router.dispatch(event, gh)
Aug 09 15:03:54 miss-islington app/web.1: File "/app/.heroku/python/lib/python3.6/site-packages/gidgethub/routing.py", line 80, in dispatch
Aug 09 15:03:54 miss-islington app/web.1: await callback(event, *args, **kwargs)
Aug 09 15:03:54 miss-islington app/web.1: File "/app/miss_islington/status_change.py", line 22, in check_status
Aug 09 15:03:54 miss-islington app/web.1: await check_ci_status_and_approval(gh, sha, leave_comment=True)
Aug 09 15:03:54 miss-islington app/web.1: File "/app/miss_islington/status_change.py", line 80, in check_ci_status_and_approval
Aug 09 15:03:54 miss-islington app/web.1: await merge_pr(gh, pr_number, sha)
Aug 09 15:03:54 miss-islington app/web.1: File "/app/miss_islington/status_change.py", line 96, in merge_pr
Aug 09 15:03:54 miss-islington app/web.1: "merge_method": "squash",
Aug 09 15:03:54 miss-islington app/web.1: File "/app/.heroku/python/lib/python3.6/site-packages/gidgethub/abc.py", line 104, in put
Aug 09 15:03:54 miss-islington app/web.1: data, _ = await self._make_request("PUT", url, url_vars, data, accept)
Aug 09 15:03:54 miss-islington app/web.1: File "/app/.heroku/python/lib/python3.6/site-packages/gidgethub/abc.py", line 66, in _make_request
Aug 09 15:03:54 miss-islington app/web.1: data, self.rate_limit, more = sansio.decipher_response(*response)
Aug 09 15:03:54 miss-islington app/web.1: File "/app/.heroku/python/lib/python3.6/site-packages/gidgethub/sansio.py", line 310, in decipher_response
Aug 09 15:03:54 miss-islington app/web.1: raise exc_type(*args)
Aug 09 15:03:54 miss-islington app/web.1: gidgethub.BadRequest: Base branch was modified. Review and try the merge again.
Aug 09 15:03:54 miss-islington app/web.1: Traceback (most recent call last):
Aug 09 15:03:54 miss-islington app/web.1: File "/app/miss_islington/main.py", line 40, in main
Aug 09 15:03:54 miss-islington app/web.1: await router.dispatch(event, gh)
Aug 09 15:03:54 miss-islington app/web.1: File "/app/.heroku/python/lib/python3.6/site-packages/gidgethub/routing.py", line 80, in dispatch
Aug 09 15:03:54 miss-islington app/web.1: await callback(event, *args, **kwargs)
Aug 09 15:03:54 miss-islington app/web.1: File "/app/miss_islington/status_change.py", line 30, in pr_reviewed
Aug 09 15:03:54 miss-islington app/web.1: await check_ci_status_and_approval(gh, sha)
Aug 09 15:03:54 miss-islington app/web.1: File "/app/miss_islington/status_change.py", line 80, in check_ci_status_and_approval
Aug 09 15:03:54 miss-islington heroku/router: at=info method=POST path="/" host=miss-islington.herokuapp.com request_id=e469ce6a-3e94-46d5-8a34-8431731e8dfd fwd="192.30.252.34" dyno=web.1 connect=0ms service=2719ms status=500 bytes=187 protocol=https

[Mariatta]

GitMate.io thinks possibly related issues are python/core-workflow#96 (Add a bot to notify people when their PR has a merge conflict), python/core-workflow#243 (Assign the failed PR to be backported to the core dev who merged it), #88 (Assign the failed PR to be backported to the core dev who merged it), python/core-workflow#27 (Merge commit messages), and #44 (Merge the PR once CI pass and backport PR approved by core dev).

[Mariatta]

Different errror message:

Aug 31 15:52:39 miss-islington app/web.1: Traceback (most recent call last):
Aug 31 15:52:39 miss-islington app/web.1: File "/app/miss_islington/main.py", line 40, in main
Aug 31 15:52:39 miss-islington app/web.1: await router.dispatch(event, gh)
Aug 31 15:52:39 miss-islington app/web.1: File "/app/.heroku/python/lib/python3.6/site-packages/gidgethub/routing.py", line 80, in dispatch
Aug 31 15:52:39 miss-islington app/web.1: await callback(event, *args, **kwargs)
Aug 31 15:52:39 miss-islington app/web.1: File "/app/miss_islington/delete_branch.py", line 14, in delete_branch
Aug 31 15:52:39 miss-islington app/web.1: await gh.delete(url)
Aug 31 15:52:39 miss-islington app/web.1: File "/app/.heroku/python/lib/python3.6/site-packages/gidgethub/abc.py", line 109, in delete
Aug 31 15:52:39 miss-islington app/web.1: await self._make_request("DELETE", url, url_vars, data, accept)
Aug 31 15:52:39 miss-islington app/web.1: File "/app/.heroku/python/lib/python3.6/site-packages/gidgethub/abc.py", line 66, in _make_request
Aug 31 15:52:39 miss-islington app/web.1: data, self.rate_limit, more = sansio.decipher_response(*response)
Aug 31 15:52:39 miss-islington app/web.1: File "/app/.heroku/python/lib/python3.6/site-packages/gidgethub/sansio.py", line 299, in decipher_response
Aug 31 15:52:39 miss-islington app/web.1: raise InvalidField(errors, message)
Aug 31 15:52:39 miss-islington app/web.1: gidgethub.InvalidField: Reference does not exist