Fix race condition in WaitForInstanceAsync causing intermittent test failures

[Copilot]

RestartAsync_EndToEnd(restartWithNewInstanceId: True) was failing intermittently due to a TOCTOU race condition in WaitForInstanceAsync.

Problem

The original code checked completion status before adding a waiter. If orchestration completed between these two steps, the completion notification was sent to no registered waiter, causing the caller to wait indefinitely (30s timeout).

Fix

Reorder operations to add waiter first, then check status:

// Before: check then register (race window exists)
if (IsCompleted(state)) return state;
var tcs = waiters.GetOrAdd(instanceId, ...);

// After: register then check (no race window)
var tcs = waiters.GetOrAdd(instanceId, ...);
if (IsCompleted(state)) tcs.TrySetResult(state);

This ensures we either:

• Catch already-completed orchestrations in the status check, or
• Receive the notification if completion happens after registration

Original prompt

investigate and fix https://github.com/microsoft/durabletask-dotnet/actions/runs/20318223840/job/58367426650?pr=572 the failing testcase Microsoft.DurableTask.Grpc.Tests.DurableTaskGrpcClientIntegrationTests.RestartAsync_EndToEnd(restartWithNewInstanceId: True) [FAIL]

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[YunchuWang]

LGTM

[Copilot]

Pull request overview

This PR fixes an intermittent race condition in WaitForInstanceAsync that was causing the RestartAsync_EndToEnd test to fail. The issue occurred when an orchestration completed between checking its status and registering a waiter, resulting in missed completion notifications and 30-second timeouts.

Key Changes:

• Reordered operations in WaitForInstanceAsync to register the waiter before checking completion status, eliminating the race condition window
• Added clear comments explaining the race condition fix and the two-step registration-then-check pattern

[YunchuWang]

@copilot update pr description to follow .github/PULL_REQUEST_TEMPLATE.md