pkg/lightning: fix chunk restore may ignore error if ctx is canceled (#873)

[ti-srebot]

cherry-pick #873 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In br repo:
git pr https://github.com/pingcap/br/pull/874

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/br.git pr/874:release-4.0-1a00477da3b3

---

What problem does this PR solve?

Fix the bug that lightning may ignore context cancel error and lose data.
Close #855.

What is changed and how it works?

In the failed lightning_checkpoint_chunks integrations tests, there are logs as follows:

[2021-03-13T13:05:59.989Z] [2021/03/13 21:05:39.395 +08:00] [INFO] [restore.go:2553] ["restore file completed"] [table=`cpch_tsr`.`tbl`] [engineNumber=0] [fileIndex=3] [path=cpch_tsr.tbl.4.sql:0] [readDur=1.161606ms] [encodeDur=2.307684ms] [deliverDur=0s] [checksum="{cksum=0,size=0,kvs=0}"] [takeTime=4.021432ms] []

Note the deliverDuris 0, this means thedeliverResult` is an object with the default parameter which is caused by reading from a closed channel.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Note

• Fix a bug that chunk restore task may ignore context cancel error and causes data loss.

[kennytm]

/lgtm

[overvenus]

LGTM

[glorv]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@ti-srebot merge failed.

[glorv]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@ti-srebot merge failed.

[glorv]

/merge

[ti-srebot]

/run-all-tests