[davidkelly]

GetAnyReplicaAsync and LookupInAnyReplicaAsync never returned if the caller's CancellationToken was cancelled before every replica read finished. TaskHelpers.WhenAnySuccessful completed its result only from continuations registered with that token, and a cancelled continuation never runs. LazyCancellation would not fix this: it only delays the cancellation, and the continuation still never runs.

Changes in WhenAnySuccessful

• Register the continuations with CancellationToken.None. The caller's token now cancels the result through a Register callback, which is disposed once the result completes.
• Run the continuations inline (ExecuteSynchronously), and create the result with RunContinuationsAsynchronously so the caller's own code never runs inside cts.Cancel() or a replica's completion.
• Remove the CancellationTokenSource, which was never disposed and whose token nothing read (NCBC-4249).
• Fix the fail-safe: observe the WhenAll fault, and return once the result is complete in any state. Before, every replica fault was reported as an unobserved task exception, and the fail-safe could throw from a discarded continuation.

Tests

• Rewrite AllCanceled and AllFailed. They discarded their assertions under #pragma warning disable xUnit2021, so they could never fail.
• Add tests for:
  • cancel, then every replica fails
  • cancel, then one replica succeeds
  • token already cancelled when called
  • one replica succeeds, then cancel

All of these use TaskCompletionSource and an explicit Cancel(), and check the state immediately, with no delays or timeouts.

Verification

• Against master, 5 of the new tests fail in under 1 ms each, with no hang. The success-then-cancel test passes on both.
• TaskHelpersTests passed in 25 of 25 runs on net8.0.
• The full net8.0 unit suite passes.
• The library builds with no warnings for every target framework.
• net48 was not run locally.

Behaviour change: callers now always resume on the thread pool after awaiting WhenAnySuccessful.

🤖 Generated with Claude Code

[github-actions]

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[Copilot]

🟢 Approval recommended

No active review issues remain for the changes addressing cancellation hangs in WhenAnySuccessful.

No open issues 🎉

What changed in this PR

This PR fixes cancellation hangs in WhenAnySuccessful, which supports GetAnyReplicaAsync and LookupInAnyReplicaAsync.

Changes:

• Cancel the result directly through the caller’s token while allowing replica continuations to run independently.
• Dispose the cancellation registration, observe aggregate faults, and safely handle already-completed results.
• Replace ineffective exception assertions and add deterministic tests for cancellation and replica completion ordering.

File	Description
tests/​Couchbase.UnitTests/​Utils/​TaskHelpersTests.cs	Repairs exception assertions and covers cancellation before and after replica completion.
src/​Couchbase/​Utils/​TaskHelpers.cs	Fixes cancellation handling, continuation scheduling, and fail-safe completion.

_{Review effort: Balanced}

[emilienbev]

LGTM