fix data conflict when prepending preloads from document meta

[youpy]

Fix data race due to the concurrent access of shared slice by multiple goroutines.

rel/repository.go

Line 1204 in 6c3495d

query.PreloadQuery = append(meta.preload, query.PreloadQuery...)

The above code prepends meta.preload to query.PreloadQuery. However:

• If the underlying array (meta.preload's capacity) has enough space, the new slice still references the same underlying array.
• When multiple goroutines prepend to query.PreloadQuery concurrently, they might end up using the same underlying array, leading to:
  • Overwriting of values by different goroutines.
  • Inconsistent results in query.PreloadQuery, as one goroutine's operation interferes with another's.
  • https://go.dev/play/p/ulUQgeS6EBa

To fix this, clone meta.preload and use it for prepending.

Reproduction example

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7ac79bf) to head (32bf325).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #379   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           35        35           
  Lines         4085      4100   +15     
=========================================
+ Hits          4085      4100   +15     

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