Wrap _listeners access with _realtimeLockQueue in fetchLatestConfig completionHandler

[yanz-safe]

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Issue: #13773

I did not copy the listeners nor use DispatchQueue.sync to keep the same logic and style of code.

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This change solves data-racing issue when listener can be added at same time when it is enumerated in completion handler.

Please suggest me If I need to add test for this and what would be the best approach for unit testing this?

I can try to add a 1s-2s loop that will add listeners and, in parallel, call config update.

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[yanz-safe]

This change solves data-racing issue when listener can be added at same time when it is enumerated in completion handler.

Please suggest me how If I need to add test for this and what would be the best approach for unit testing this?

I can try to add a 1s-2s loop that will add listeners and, in parallel, call config update.

[paulb777]

The fix looks good to me. Thanks for the bug report and fix!

[karenyz]

Nice catch! Makes me wonder if this block could also be a method (to match the propogateErrors call above that also accesses listeners), but that can be a future refactor :)