[v1.5] Fix SourceRef.Source and SinkRef.Sink non-idempotent property bug

[Aaronontheweb]

Summary

This PR backports #7905 from dev to v1.5.

This fixes issue #7895 by making both ISourceRef<T>.Source and ISinkRef<T>.Sink properties idempotent using Lazy<T>. Previously, these properties created new stage instances on every access, causing race conditions where multiple instances would compete for the same handshake, leading to intermittent subscription timeouts.

Changes

Files Modified:

1. SourceRefImpl.cs - Made .Source property idempotent using Lazy<Source<T, NotUsed>>
2. SinkRefImpl.cs - Made .Sink property idempotent using Lazy<Sink<T, NotUsed>>
3. StreamRefsSpec.cs - Added 2 reproduction tests

Implementation:

// Before (created new instance every time)
public Source<T, NotUsed> Source =>
    Dsl.Source.FromGraph(new SourceRefStageImpl<T>(InitialPartnerRef))
        .MapMaterializedValue(_ => NotUsed.Instance);

// After (cached with Lazy<T>)
private readonly Lazy<Source<T, NotUsed>> _source;

public SourceRefImpl(IActorRef initialPartnerRef) : base(initialPartnerRef)
{
    _source = new Lazy<Source<T, NotUsed>>(() =>
        Dsl.Source.FromGraph(new SourceRefStageImpl<T>(InitialPartnerRef))
            .MapMaterializedValue(_ => NotUsed.Instance));
}

public Source<T, NotUsed> Source => _source.Value;

Tests Added

1. SourceRef_Source_property_should_be_idempotent_issue_7895

• Verifies that multiple .Source property accesses return the same instance
• Before fix: Failed 25/25 times (100% failure rate)
• After fix: Passed 10/10 times (100% success rate)

2. SourceRef_multiple_materializations_cause_timeout_issue_7895

• Demonstrates that the fix eliminates race conditions from multiple property accesses
• Before fix: Failed consistently with timeouts
• After fix: Passes consistently

Impact

What This Fixes:

• ✅ Eliminates race conditions from accidental property accesses
  • Debugger hovering over variables
  • Logging frameworks inspecting properties
  • Serialization/framework reflection
  • IDE property viewers
• ✅ Prevents subscription timeouts caused by multiple SourceRefStageImpl instances
• ✅ Fixes intermittent ~30% failure rate in production workloads (see issue StreamRef: Multiple .Source property accesses cause intermittent subscription timeouts #7895)
• ✅ Thread-safe using Lazy<T> default ExecutionAndPublication mode

What Still Fails (As Designed):

• ❌ Intentional double materialization (user error)
  • Still fails gracefully at actor protocol level via ObserveAndValidateSender
  • Second materialization gets rejected with clear error message

Test Results

• ✅ Reproduction tests: 2/2 passed
• ✅ Serialization test: 1/1 passed
• ✅ Stability: 10/10 consecutive runs passed

Technical Notes

Why Lazy<T>?

• Simpler and cleaner than double-checked locking
• Built-in thread safety with ExecutionAndPublication mode
• No manual lock management required
• Well-tested by .NET framework

Serialization Compatibility:

• Lazy<T> fields are not serialized (transient)
• StreamRefs use surrogate pattern - only InitialPartnerRef is serialized
• On deserialization, new SourceRefImpl is created with fresh Lazy<T>
• Verified by existing serialization tests

Fixes #7895
Backport of #7905