[release/3.0] Don't dispose StreamPipeWriter CancellationToken until after Flush

[jkotalik]

Hit this in ASP.NET Core. If you call FlushAsync and then CompleteAsync, you'll hit an ODE in CompleteAsync. Previous tests didn't call FlushAsync and then CompleteAsync.

[jkotalik]

@stephentoub I think we need to get this into 3.0. What is the process of doing that.

[davidfowl]

cc @danmosemsft We need this for 3.0

[danmoseley]

@stephentoub I think we need to get this into 3.0. What is the process of doing that.

@jkotalik presumably this is something you think customers will encounter? Is it a regression?

[davidfowl]

@jkotalik presumably this is something you think customers will encounter? Is it a regression?

Late change made in preview8 exploding in some scenarios in ASP.NET Core when completing the http response body.

[danmoseley]

OK, please merge into release/3.0 when ready. Put the "tell-mode" label on the PR. You can paste this text into it.

[jkotalik]

Seems to be a hanging test on linux...

[danmoseley]

@jkotalik note that we do not mirror from release/3.0 to master. So this needs a separate PR into master. (In general, folks merge into master first (with no process), then port to release/3.0.)

[jkotalik]

@danmosemsft to make sure I understand, corefx never merges from release/3.0 into master. Should I just cherry-pick this commit onto master?

[danmoseley]

@jkotalik yep