bugfix: thread unsafe issue of MySQLKeywordChecker initialization

[chenxi-null]

Performing additional initialization on an object after assignment to a shared variable guarded by double-checked locking is not thread-safe, and could result in unexpected behavior.

Potential race condition: this instance of keywordChecker is available to other threads before keywordSet are assign, which may causes NPE if other threads call '#check' method.

Below is code segment (the code comment is added by me):

private static volatile KeywordChecker keywordChecker = null;
private static volatile Set<String> keywordSet = null;

public static KeywordChecker getInstance() {
    if (keywordChecker == null) {
        synchronized (MySQLKeywordChecker.class) {
            if (keywordChecker == null) {
                keywordChecker = new MySQLKeywordChecker();
                // additional initialization inside double-checked locking is not thread-safe!
                keywordSet = Arrays.stream(MySQLKeyword.values()).map(MySQLKeyword::name).collect(Collectors.toSet());
            }
        }
    }
    return keywordChecker;
}


@Override
public boolean check(String fieldOrTableName) {
    if (keywordSet.contains(fieldOrTableName)) {
        return true;
    }
    if (null != fieldOrTableName) {
        fieldOrTableName = fieldOrTableName.toUpperCase();
    }
    return keywordSet.contains(fieldOrTableName);

}

Since MySQLKeywordChecker is singleton, the field keywordSet doesn't need to be static,
the initialization of keywordSet can be moved to constructor, removing the additional initialization inside locking, which ensures thread safely.

[codecov-io]

Codecov Report

Merging #1599 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@              Coverage Diff              @@
##             develop    #1599      +/-   ##
=============================================
+ Coverage      46.43%   46.43%   +<.01%     
- Complexity      1713     1714       +1     
=============================================
  Files            350      350              
  Lines          12827    12828       +1     
  Branches        1615     1615              
=============================================
+ Hits            5956     5957       +1     
  Misses          6224     6224              
  Partials         647      647

Impacted Files	Coverage Δ	Complexity Δ
...source/undo/mysql/keyword/MySQLKeywordChecker.java	98.58% <100%> (ø)	7 <0> (+1)	⬆️

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[xingfudeshi]

Good!LGTM.

[zjinlei]

Nice! LGTM