Concurrency checks in UnitValueAbbreviationLookup no longer working correctly

[oysteinkrog]

Describe the bug
The concurrency checks in UnitAbbreviationsCache are no longer working correctly.
Specificially, we sometimes see this error in our tests:

System.InvalidOperationException: Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct.
	   at System.Collections.Generic.Dictionary`2.FindValue(TKey key)
	   at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
	   at UnitsNet.UnitAbbreviationsCache.GetAbbreviations(UnitInfo unitInfo, IFormatProvider formatProvider)
	   at UnitsNet.UnitAbbreviationsCache.TryGetUnitAbbreviations(Type unitType, Int32 unitValue, IFormatProvider formatProvider, String[]& abbreviations)
	   at UnitsNet.UnitAbbreviationsCache.GetUnitAbbreviations(Type unitType, Int32 unitValue, IFormatProvider formatProvider)
	   at UnitsNet.UnitAbbreviationsCache.GetDefaultAbbreviation(Type unitType, Int32 unitValue, IFormatProvider formatProvider)
	   at UnitsNet.UnitFormatter.GetFormatArgs[TUnitType](TUnitType unit, Double value, IFormatProvider culture, IEnumerable`1 args)
	   at UnitsNet.QuantityFormatter.ToStringWithSignificantDigitsAfterRadix[TUnitType](IQuantity`1 quantity, IFormatProvider formatProvider, Int32 number)
	   at UnitsNet.QuantityFormatter.FormatUntrimmed[TUnitType](IQuantity`1 quantity, String format, IFormatProvider formatProvider)
	   at UnitsNet.QuantityFormatter.Format[TUnitType](IQuantity`1 quantity, String format, IFormatProvider formatProvider)
	   at UnitsNet.Mass.ToString(String format, IFormatProvider provider)
	   at UnitsNet.Mass.ToString(String format)
	   at UnitsNet.Mass.ToString()

To Reproduce
Set up a concurrency situation (multiple threads) with e.g. UnitsNetSetup.Default.UnitAbbreviations.GetDefaultAbbreviation(unit, culture)

Expected behavior
Should never throw Exception.

Additional context
I see from the history in this file that at one point a ConcurrencyDictionary was used, but that it was later removed.
Perhaps re-introduce this, or simply add correct locking around access to the AbbreviationsMap object.

[angularsen]

I was just checking what dev team would first catch this subtle racing condition 🧐 Well done! 🥳

Just kidding, nice catch. This part of the code has had quite a bit of refactoring lately.
I wasn't able to write a unit test that reproduced this case, could you give the fix a try?

#1305

Nuget should be out shortly.

Release UnitsNet/5.31.0 · angularsen/UnitsNet