Bug fixes: add synchronized block to map operation in OfflineStore

[hermanliang]

No description provided.

[codecov]

Codecov Report

Merging #659 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #659      +/-   ##
============================================
+ Coverage     52.85%   52.86%   +<.01%     
  Complexity     1676     1676              
============================================
  Files           131      131              
  Lines         10148    10150       +2     
  Branches       1408     1408              
============================================
+ Hits           5364     5366       +2     
  Misses         4341     4341              
  Partials        443      443

Impacted Files	Coverage Δ	Complexity Δ
Parse/src/main/java/com/parse/OfflineStore.java	4.61% <100%> (+0.34%)	5 <0> (ø)	⬇️

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[rogerhu]

Where did you see the bug?

[hermanliang]

It's my fault. I not noticed this point when dealt with PR #607. It could be a potential problem with concurrent operations if map not synchronized. I think it's not serious because this logic is not often called.