cherry-pick #2338 to release-5.1
You can switch your code base to this Pull Request by using git-extras:

# In tics repo:
git pr https://github.com/pingcap/tics/pull/2341

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

git push git@github.com:ti-srebot/tics.git pr/2341:release-5.1-f0733d9677ee

What problem does this PR solve?

Issue Number: close #2322

Problem Summary:
As the issue decribed.

What is changed and how it works?

Proposal: xxx

What's Changed:

1. catch exception for each tunnel during MPPTask::writeErrToAllTunnel
2. make sure finished flag is set in MPPTunnel::close()
3. Do not hold sender task in EstablishMPPConnection

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch:

Check List

Tests

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

Side effects

Release note

• fix potential mpp task leak