Fix forever hanging when HashAgg is called by apply (#12760)

[SunRunAway]

Automated cherry pick of #12760 on release-2.1.

---

What problem does this PR solve?

Fix #12759

What is changed and how it works?

The parent executors(apply) of HashAgg may call HashAgg.Next once more to make sure the returned size of chunk is 0.
HashAgg.Next handle these calls incorrectly after its all finalWorks exits.

On each call of HashAgg.Next after the job done, it returns a chunk with 0 size, but put a chk into finalInputCh. Each apply executor will call a HashAgg.Next once more, when the number of parent executors exceeds the channel buffer, the query will hang forever.

Explain the example in #12759,

1. The size of the channel buffer is 4,
2. The first call of HashAgg.Next which returns a one-row chunk to its parent, sends the chunk into the buffer (This is valid).
3. Three times calls of HashAgg.Next called by each of the three apply, which returns a zero-row chunk to its parent, sends the chunk into the buffer (This should not happen).
4. Now the channel is full.
5. One more call of HashAgg.Next called by the session of tidb, which returns a zero-row chunk to its parent, sends the chunk into the buffer and hangs forever.

Check List

Tests

• Unit test
• Integration test

Code changes

• None

Side effects

• None

Related changes

• None

Release note

• Fix forever hanging when HashAgg is called by apply.

[SunRunAway]

I've dropped the unit test because it does not support sql hint for hashagg in this branch.

[zz-jason]

LGTM

[XuHuaiyu]

LGTM

[sre-bot]

Sorry @XuHuaiyu, you don't have permission to trigger auto merge event on this branch.

[sre-bot]

Your auto merge job has been accepted, waiting for 12765, 12766

[sre-bot]

/run-all-tests