Align waitforexternalevent waiter picking order to LIFO

[YunchuWang]

Fix #508

This pull request refactors the handling of external events in the orchestration worker to use a stack (LIFO) approach instead of a queue (FIFO). This change improves the delivery of events to the most recent waiter, making it easier for users to abandon events they no longer care about and avoids issues with cancelled waiters. The update is thoroughly validated by new and updated integration tests that confirm correct LIFO event delivery and proper handling of cancellation and buffering scenarios.

Core event handling changes:

• Refactored the externalEventSources data structure in TaskOrchestrationContextWrapper from a queue to a stack using a single-linked list, so the most recent waiter receives events first.
• Updated the logic in WaitForExternalEvent and CompleteExternalEvent to add new waiters to the top of the stack and deliver events in LIFO order, ensuring correct handling when waiters are cancelled or abandoned [1] [2] [3].
• Extended the IEventSource interface and its implementation to support stack-based linking via a new Next property [1] [2].

Testing and validation:

• Added a comprehensive test suite (ExternalEventStackTests.cs) that covers LIFO event delivery, cancellation scenarios, multiple waiters, and event buffering to ensure correctness of the new stack-based behavior.
• Updated existing tests (e.g., OrchestrationPatterns.cs) to expect LIFO delivery order, reflecting the new semantics.

[Copilot]

Pull Request Overview

This PR changes the external event waiter picking order from FIFO (First In First Out) to LIFO (Last In First Out) for the isolated worker implementation, aligning it with the in-process model behavior. This change makes it easier for users to abandon external events they no longer care about, particularly in scenarios using Task.WhenAny in loops.

• Replaced Queue-based event waiter storage with a custom Stack implementation using a single-linked list
• Added comprehensive integration tests validating LIFO behavior including cancellation scenarios, multiple waiters/events, and event buffering
• Updated existing test expectations to reflect LIFO ordering

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/Worker/Core/Shims/TaskOrchestrationContextWrapper.cs	Refactored external event handling from Queue (FIFO) to Stack (LIFO) by changing the data structure from Dictionary<string, Queue<IEventSource>> to Dictionary<string, IEventSource> with linked list implementation, and updated event completion logic to pop from the top of the stack
src/Worker/Core/Shims/TaskOrchestrationContextWrapper.EventSource.cs	Added Next property to IEventSource interface and its implementation to support single-linked list structure for LIFO ordering
test/Grpc.IntegrationTests/OrchestrationPatterns.cs	Updated ExternalEventsInParallel test expectations to reflect LIFO behavior with detailed explanatory comments
test/Grpc.IntegrationTests/ExternalEventStackTests.cs	Added new comprehensive integration test file with four test cases covering LIFO behavior, cancellation handling (Issue #508), multiple events/waiters, and event buffering scenarios

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (1)

test/Grpc.IntegrationTests/ExternalEventStackTests.cs:179

• This assignment to metadata is useless, since its value is never read.

        await server.Client.WaitForInstanceStartAsync(
            instanceId, this.TimeoutToken);

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.