Fix SingleConsumerUnboundedChannel WaitToReadAsync returning exception instead of false

[Copilot]

Description

SingleConsumerUnboundedChannel.WaitToReadAsync throws ChannelClosedException instead of returning false when a prior ReadAsync was cancelled and the channel is then completed normally.

In TryComplete, the error variable is mutated by CreateInvalidCompletionException when completing the blocked reader, then the now-non-null error is incorrectly propagated to the waiting reader:

// Before: mutates shared `error` variable
error = ChannelUtilities.CreateInvalidCompletionException(error); // null → ChannelClosedException
blockedReader.TrySetException(error);

// Later, `error` is no longer null, so waitingReader gets an exception instead of `false`
if (error is not null)
    waitingReader.TrySetException(error); // Bug: should have taken the else branch

Fix: inline the call so error is never reassigned.

blockedReader.TrySetException(ChannelUtilities.CreateInvalidCompletionException(error));

Adds a regression test covering the exact sequence: ReadAsync cancelled → WaitToReadAsync → Complete() → expect false.

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[Copilot]

Pull request overview

Fixes SingleConsumerUnboundedChannel.WaitToReadAsync incorrectly faulting with ChannelClosedException (instead of returning false) after a prior cancellable ReadAsync was canceled and the channel is then completed successfully.

Changes:

• Prevent TryComplete from mutating the shared error variable when completing a blocked reader, so waiting readers observe the correct completion state.
• Add a regression test covering the canceled-ReadAsync → WaitToReadAsync → normal Complete() sequence.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Threading.Channels/src/System/Threading/Channels/SingleConsumerUnboundedChannel.cs	Avoids reassigning error when completing a blocked reader, preventing incorrect exception propagation to a waiting reader.
src/libraries/System.Threading.Channels/tests/UnboundedChannelTests.cs	Adds regression coverage for WaitToReadAsync returning false after normal completion following a canceled ReadAsync.

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