kv/client: use ephemeral goroutine to handle region range request (#1720)

[ti-chi-bot]

This is an automated cherry-pick of #1720

What problem does this PR solve?

Fix part bug found in #1599

• The region request is processed in a block way

https://github.com/pingcap/ticdc/blob/109257825f7805bfc0f87b68d9ef7e969aa19161/cdc/kv/client.go#L578-L591

• The divideAndSendEventFeedToRegions function scans range into several regions, and dispatch each region via scheduleRegionRequest. There could be stale regions, and these stale regions will be resent to s.requestRangeCh in L646. If the s.requestRangeCh is full, deadlock happens.

https://github.com/pingcap/ticdc/blob/109257825f7805bfc0f87b68d9ef7e969aa19161/cdc/kv/client.go#L627-L648

https://github.com/pingcap/ticdc/blob/109257825f7805bfc0f87b68d9ef7e969aa19161/cdc/kv/client.go#L616-L623

What is changed and how it works?

In order to consume region range request from requestRangeCh as soon as possible, we create a new goroutine to handle it. The sequence of region range we process is not matter, the region lock keeps the region access sequence. Besides the count or frequency of range request is limitted, we use ephemeral goroutine instead of permanent gourotine.

Check List

Tests

• Unit test
• Integration test

Release note

• Fix a deadlock bug in the kv client that deadlock happens when large stale regions exist.

[overvenus]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• amyangfei
• overvenus

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[overvenus]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 2c384d1

[codecov-commenter]

Codecov Report

Merging #1800 (2c384d1) into release-4.0 (753073a) will decrease coverage by 1.2098%.
The diff coverage is 53.9912%.

@@                 Coverage Diff                 @@
##           release-4.0      #1800        +/-   ##
===================================================
- Coverage      53.7661%   52.5563%   -1.2099%     
===================================================
  Files              153        153                
  Lines            15958      15941        -17     
===================================================
- Hits              8580       8378       -202     
- Misses            6475       6652       +177     
- Partials           903        911         +8     

[lonng]

/merge

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

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[amyangfei]

/run-unit-tests