Fix SpaServices extensions hangs (#17277)

[artar94]

Fixed adding a string with a large number of trailing zeros to StringBuilder, which sometimes caused the thread to hang.

Summary of the changes

• Only a significant portion of the buffer will be added.

Addresses #17277

[Tratcher]

This code would be clearer if re-written using Spans and Slicing.

[Rinsen]

@Tratcher Will this be merged to next 3.1.x release also?

[Tratcher]

Not automatically. Fixes in 3.1 have to meet a high bar. While there was a clear bug here, we'll need people to verify this really fixes the hang they were experiencing. E.g. reproducing the issue in 5.0.0-preview4, but not in a nightly build or preview after this fix (preview 6?).

[Rinsen]

That I 100% understand.

I tried to brute force the change in on a branch on my own under the 3.1 tag but that ended up in a mess of broken builds so it was kind of a dead end to verify that this was compleatly fixing the issue for us...

What is the easies way to know what preview this will be included in? Will it be in some list or is it just to check when the preview was taken via some tag or branch?

[Tratcher]

From our current schedule it should go into preview6, but that's a ways out. Do you have a reliable repro using the current preview3? If so we can find you a nightly build with the changes to test.

[Rinsen]

@Tratcher I can't reproduce this on Microsoft.AspNetCore.SpaServices.Extensions" Version="5.0.0-preview.3.20215.14"

So on .NET 5 it seem to be working just fine with preview 3 already.

If I go back to 3.1.3 I directly have the issue. So something else must have changed related to this so we never end up in the breaking case that this is fixing, I will try to investigate 3.1.3 a bit more and see what is going on.

Edit:
The source code I have been using
https://github.com/Rinsen/Angular9IssueWithSpaServicesExtensions

It's only a file new project on the Angular Template and an updated Angular application to Angular 9 and nothing else.