types/json: fix an over-quoted bug in BinaryJSON.Unquote function (#11934)

[sre-bot]

cherry-pick #11934 to release-3.0

---

What problem does this PR solve?

This PR tries to fix the following issue:

create table a(a json);
insert into a values ("{\"low_keywords\":\"[{\\\"x\\\":\\\"{\\\\\\\"y\\\\\\\":12}\\\"}]\"}");

In current TiDB(master/3.0/2.1):

tidb> select a->>"$.low_keywords" from a;
+----------------------+
| a->>"$.low_keywords" |
+----------------------+
| [{"x":"{"y":12}"}] |
+----------------------+
1 rows in set (0.02 sec)

In MySQL 5.7/8.0:

tidb> select a->>"$.low_keywords" from a;
+----------------------+
| a->>"$.low_keywords" |
+----------------------+
| [{"x":"{\"y\":12}"}] |
+----------------------+
1 rows in set (0.02 sec)

What is changed and how it works?

The issue is caused by the over-unquote behaviors of BinaryJSON.Unquote: for a string literal of JSON, it should be unquoted only when enclosed by double quotes(")

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

tidb> select json_unquote("\"[{\\\"x\\\":\\\"{\\\\\\\"y\\\\\\\":12}\\\"}]\"");
+------------------------------------------------------------------+
| json_unquote("\"[{\\\"x\\\":\\\"{\\\\\\\"y\\\\\\\":12}\\\"}]\"") |
+------------------------------------------------------------------+
| [{"x":"{\"y\":12}"}]                                             |
+------------------------------------------------------------------+
1 row in set (0.00 sec)

tidb> select json_unquote("\\\\");
+----------------------+
| json_unquote("\\\\") |
+----------------------+
| \\                   |
+----------------------+
1 row in set (0.00 sec)

• No code

Code changes

• Has exported function/method change

Side effects

N/A

Related changes

• Need to cherry-pick to the release branch

Release note

Fix an over-quoted bug for JSON_UNQUOTE function: only values enclosed by the double quote marks(") should be unquoted. For example, result of SELECT JSON_UNQUOTE("\\\\") should be \\(not changed).

[sre-bot]

/run-all-tests

[lonng]

LGTM

[crazycs520]

LGTM

[sre-bot]

Sorry @bb7133, you don't have permission to trigger auto merge event on this branch.

[sre-bot]

/run-all-tests

[sre-bot]

@sre-bot merge failed.

[bb7133]

/run-common-test

[bb7133]

/run-all-tests

[zyxbest]

/run-common-test

[zz-jason]

/merge

[sre-bot]

/run-all-tests