Avoid multiple exceptions at startup from MsQuic support tests

[stephentoub]

Fixes #49918
cc: @brianrob, @scalablecory, @geoffkizer

Cory, Geoff, I'm not sure how to install msquic to test this. Could you either a) confirm CI will appropriately validate it, b) validate it, or c) tell me how to get/install the relevant dll I'd need to validate it?

[ghost]

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Fixes #49918
cc: @brianrob, @scalablecory, @geoffkizer

Cory, Geoff, I'm not sure how to install msquic to test this. Could you either a) confirm CI will appropriately validate it, b) validate it, or c) tell me how to get/install the relevant dll I'd need to validate it?

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

[karelz]

Thanks @stephentoub!
Also adding @ManickaP @CarnaViire who are now actively working on msquic.
Won't it collide with @ManickaP's PR?

[ManickaP]

I'd love to understand the problem before we dive into fixing it so that we can avoid making the same mistake in the future. I've posted some question in the original issue about the exception and its circumstances. I also have some questions about this PR:

• Does this somehow relates to this PR: Don't call into unsupported APIs on build targets which don't support quic #49261?
• Should I consider this PR as some guideline I should take into account when fixing the above mentioned PR?

Otherwise, this will clash with #49823. Depending on the priority of the original issue:

• If not urgent, I'd prefer to get in my PR sooner, since this is smaller and thus easier to solve the conflicts.
• If this needs to get in ASAP, then I'll do a review to minimize impact on [QUIC] API Update #49823.

Finally, to answer your question @stephentoub:

Could you either a) confirm CI will appropriately validate it, b) validate it, or c) tell me how to get/install the relevant dll I'd need to validate it?

a) Nope, CI won't validate: #49138
b) Me or @CarnaViire can do it
c) If you want to do it yourself, instructions are here: https://github.com/dotnet/runtimelab/tree/feature/System.Net.Experimental.MsQuic

[stephentoub]

Does this somehow relates to this PR: #49261?

Only tangentially. That PR is about changing System.Net.Http to avoid accessing APIs from System.Net.Quic on platforms where Quic is definitely not supported. This issue is about the impact of accessing those APIs (and in particular IsSupported) on platforms where it might be supported.

If not urgent, I'd prefer to get in my PR sooner, since this is smaller and thus easier to solve the conflicts.

It's not urgent. Feel free to cherry-pick this into your other PR if that makes things easier for you.

I'd love to understand the problem before we dive into fixing it so that we can avoid making the same mistake in the future.

Exceptions are expensive, in particular throwing and catching them, and way more so if a debugger is attached. We want exceptions to be "exceptional", not the common case.

Here, we're P/Invoking into MsQuicOpen from the msquic.dll library. If that library doesn't exist, that P/Invoke will generate a DllNotFoundException. The code is then catching that exception, and throwing a new NotSupportedException, which it then also catches and turns into setting IsSupported to false.

Since for the foreseeable future it's very likely that the common case on many systems will be that they don't have msquic.dll, and we're throwing two exceptions as part of computing IsSupported when it's not supported, we're putting multiple exceptions onto the common path, and a path that's often used as part of an app starting up (since it's used from HttpClient as part of determining whether HTTP/3 is available in SocketsHttpHandler's connection pool).

This PR avoids making such a P/Invoke by instead using NativeLibrary.TryLoad/TryGetExport to get MsQuicOpen, changing the code to not use exceptions for the control flow around determining whether IsSupported should be set to true.

[ManickaP]

Let's merge this, I'll deal with the rebase. The change makes sense. I was not aware that we had enabled H3 by default.
Thanks.

[stephentoub]

Ok. Thanks.