Enable async-over-sync Task in Dns to be canceled

[stephentoub]

While we can't currently cancel the synchronous call, we can at least cancel the Task prior to its being executed.

[ghost]

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Issue Details

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While we can't currently cancel the synchronous call, we can at least cancel the Task prior to its being executed.

Author:	stephentoub
Assignees:	-
Labels:	area-System.Net
Milestone:	-

[geoffkizer]

Do we have test for this? At least for pre-cancellation?

[gfoidl]

Do we have test for this? At least for pre-cancellation?

We have

runtime/src/libraries/System.Net.NameResolution/tests/FunctionalTests/GetHostEntryTest.cs

Line 256 in 87a7159

public async Task DnsGetHostEntry_PreCancelledToken_Throws()

[stephentoub]

Do we have test for this? At least for pre-cancellation?

This is hard to test well. We have a test for pre-cancellation, but we also have an upfront check for cancellation. So a test would need to trigger cancellation after the upfront check and before the thread pool processes the task. We could try issuing a bunch of requests and canceling them and hoping one gets canceled, e.g.
https://github.com/dotnet/runtime/pull/47036/files#diff-7c86b2d75d9c87e8aa5eb156215b9eb6eade4faef633a6e7d0d015f5459a3962R268
but I'm not sure it's worth it.