Robustness checks, thread safety

[MikeStall]

Robustness checks (#981) - Several potential races. Writing is single threaded. Reading is multi-threaded.

We have some existing tests (https://github.com/microsoft/Power-Fx/blob/main/src/tests/Microsoft.PowerFx.Core.Tests/ImmutabilityTests.cs) . Expanding that test shows a few suspicious things:

1. DType.AssociatedDataSources returns mutable value
2. DType.ExpandInfo returns mutable value (IExpandInfo.UpdateEntityInfo)
3. DType.LazyTypeProvider returns mutable value (fixed in #1511)
4. WrappedDerivedRecordType._renameDriver returns mutable value

We should also ensure these eval/runtime types are ThreadSafe. ParsedExpression Core.IR.Nodes.IntermediateNode ReadOnlySymbolValues ComposedReadOnlySymbolValues

[MikeStall]

See #1520 for new test to scan these failures.

Currently, there are about 100 failures, mostly false-positives. We should manually review determine if it is safe?

• Yes, because it's protected under a lock --> adding ThreadSafeProtectedByLock?
• Yes, because the field's type is immutable --> add ThreadSafeImmutable to the type, or add to known immutable list.
• Yes, because the member type could use a safer type --> update , eg Dict --> IReadOnlyDict

• Yes, manually confirmed --> can we refactor to make it easy to verify? Eg, make the field readonly?

  ... if we can't do any of the above ... is it actually safe?