[ISSUE #651] fix an issue where a map read operation was not protected by the lock

[czah]

What is the purpose of the change

see #651

Brief changelog

Aquire lock before accessing the indexer.

Verifying this change

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[codecov-commenter]

Codecov Report

Merging #652 (64cbc7f) into master (dfa26d1) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #652      +/-   ##
==========================================
+ Coverage   24.36%   24.39%   +0.02%     
==========================================
  Files          48       48              
  Lines        5204     5202       -2     
==========================================
+ Hits         1268     1269       +1     
+ Misses       3726     3724       -2     
+ Partials      210      209       -1     

Impacted Files	Coverage Δ
producer/selector.go	73.68% <100.00%> (+6.18%)	⬆️

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[maixiaohai]

lgtm