bugfix: unsafe double-checked locking in SofaRegistryServiceImpl

[chenxi-null]

Ⅰ. Describe what this PR did

This usage of double check locking is not thread-safe, the reason is present in this PR #1599 which I submitted before.

How to fix:

Because DefaultRegistryClient is in third-part jar, the init method invocation can't be moved to the constructor.

So the local variable is used to correct DCL and minimize the number of field reads.

See also: https://lgtm.com/rules/1507076816054/

Quote from the article:

// a local variable can be used to avoid reading the field more times than neccessary.

private Object lock = new Object();
private volatile MyObject f = null;

public MyObject getMyObject() {
  MyObject result = f;
  if (result == null) {
    synchronized(lock) {
      result = f;
      if (result == null) {
        result = new MyObject();
        result.init();
        f = result; // GOOD
      }
    }
  }
  return result;
}

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #2642 into develop will decrease coverage by 0.01%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop    #2642      +/-   ##
=============================================
- Coverage      51.25%   51.24%   -0.02%     
+ Complexity      2819     2817       -2     
=============================================
  Files            556      556              
  Lines          17803    17803              
  Branches        2101     2101              
=============================================
- Hits            9125     9123       -2     
  Misses          7818     7818              
- Partials         860      862       +2     

Impacted Files	Coverage Δ	Complexity Δ
...o/seata/server/coordinator/DefaultCoordinator.java	54.63% <0.00%> (-0.52%)	28.00% <0.00%> (-1.00%)
...in/java/io/seata/server/session/GlobalSession.java	83.71% <0.00%> (-0.46%)	71.00% <0.00%> (-1.00%)

[xingfudeshi]

LGTM.

[xingfudeshi]

Please associate your GitHub account with your email.@chenxi-null

[chenxi-null]

Please associate your GitHub account with your email.@chenxi-null

@xingfudeshi Is there anything wrong with my email ?
I have already associated with my GitHub account with the email chenxiCoder@gmail.com, and it can receive notification from Github.

[zjinlei]

Please associate your GitHub account with your email.@chenxi-null

@xingfudeshi Is there anything wrong with my email ?
I have already associated with my GitHub account with the email chenxiCoder@gmail.com, and it can receive notification from Github.

https://blog.csdn.net/idealcitier/article/details/82697268

[zjinlei]

LGTM