fix: add barrier for thread safety

[vdkdamian]

New Pull Request Checklist

• [x] I am not disclosing a vulnerability.
• [x] I am creating this PR in reference to an issue.

Issue Description

This fixes an issue when downloading or uploading multiple items in ParseSwift that was caused by a thread-safety issue.

Related issue: #347

Approach

This PR adds a barrier to fix the related issue

TODOs before merging

• [ ] Add entry to changelog
• [x] Lint

[parse-github-assistant[bot]]

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[vdkdamian]

@cbaker6 Why are the checks failing?

[cbaker6]

@cbaker6 Why are the checks failing?

Not sure, maybe you can set break points on the failing tests and your changes to determine where the failures are coming from?

[vdkdamian]

Not sure, maybe you can set break points on the failing tests and your changes to determine where the failures are coming from?

I tested it in my app and it works perfectly, but the tests are failing.

It's coming from the test [ParseSwiftTests.ParseFileAsyncTests testFetchFileProgress].

[codecov[bot]]

Codecov Report

Merging #348 (a888b84) into main (675168e) will decrease coverage by 0.07%. The diff coverage is 62.10%.

@@            Coverage Diff             @@
##             main     #348      +/-   ##
==========================================
- Coverage   89.17%   89.09%   -0.08%     
==========================================
  Files         156      156              
  Lines       14553    14605      +52     
==========================================
+ Hits        12977    13013      +36     
- Misses       1576     1592      +16     

Impacted Files	Coverage Δ
...urces/ParseSwift/API/ParseURLSessionDelegate.swift	12.67% <0.00%> (-3.12%)	:arrow_down:
Sources/ParseSwift/LiveQuery/LiveQuerySocket.swift	69.49% <67.56%> (+4.78%)	:arrow_up:
Sources/ParseSwift/API/API+Command.swift	89.36% <100.00%> (+0.15%)	:arrow_up:
Sources/ParseSwift/Extensions/URLSession.swift	78.63% <100.00%> (+1.46%)	:arrow_up:
Sources/ParseSwift/Objects/ParseUser.swift	85.62% <0.00%> (-0.18%)	:arrow_down:

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[cbaker6]

@vdkdamian can you try the latest branch in your code to see if you still run into the crash in your app?

[vdkdamian]

@vdkdamian can you try the latest branch in your code to see if you still run into the crash in your app?

Yes, the crash is back.

[cbaker6]

Closing in favor of #394