ddl: reimplement the backfill scheduler

[tangenta]

What problem does this PR solve?

Problem Summary:

This is a refactoring PR, let's skip the issue check.

What is changed and how it works?

This PR extracts backfillScheduler to an interface:

// backfillScheduler is used to manage the lifetime of backfill workers.
type backfillScheduler interface {
	setupWorkers() error
	close(force bool)

	sendTask(task *reorgBackfillTask) error
	drainTasks()
	receiveResult() (*backfillResult, bool)

	setMaxWorkerSize(maxSize int)
	currentWorkerSize() int
	adjustWorkerSize() error
}

There are two implementations:

• txnBackfillScheduler: the transaction way to backfill data.
• ingestBackfillScheduler: the ingest way to backfill data.

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Benjamin2037
• wjhuang2016

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[Benjamin2037]

LGTM

[Benjamin2037]

/merge

[ti-chi-bot]

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

Commit hash: c48e80b

[Benjamin2037]

Why does here need an engine flush lock？

[Benjamin2037]

if using a select instead only for loop better?
func (b *ingestBackfillScheduler) drainTasks() {
for {
select {
case <-b.taskCh:
// Discard message from channel
default:
// Channel is empty, exit loop
return
}
}
}