ddl: ddl job will hang on loop when it encounts panic in cancelling logic

[AilinKid]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

The case is that when add index takes a long time, the user manually canceled the DDL job, so the add index logic will be converted to drop index logic.

Once there is a panic in the path of canceling, the goroutine will be pulled up and set its state as canceling again, which is a dead loop.

We should count the error count when the goroutine being pulled up from a panic.

Reproduce step:

mock a panic in function `dropIndexColumnFlag`
alter table t drop index  
loop...

2. What did you expect to see? (Required)

it can break the loop when it has been pulled up from panic for 512 times.

3. What did you see instead (Required)

dead loop

4. What is your TiDB version? (Required)

master, 5.0, 4.0 ....

[ti-srebot]

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Bug

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1. Root Cause Analysis (RCA) (optional)

2. Symptom (optional)

3. All Trigger Conditions (optional)

4. Workaround (optional)

5. Affected versions

[v4.0.0:v4.0.11],[v5.0.0-rc]

6. Fixed versions

master