Support wasm variants in NuGet packages

[lambdageek]

We need some way for NuGet package authors to provide single threaded and multi-threaded versions of native assets.

Additionally, library authors should be able to identify whether they are in a single- or multi-threaded app.
(For example consider making System.Threading.Thread.IsThreadStartSupported a public property (and also linker-friendly))

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ghost]

Tagging subscribers to 'arch-wasm': @lewing
See info in area-owners.md if you want to be subscribed.

Issue Details

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We need some way for NuGet package authors to provide single threaded and multi-threaded versions of native assets.

Additionally, library authors should be able to identify whether they are in a single- or multi-threaded app.
(For example consider making System.Threading.Thread.IsThreadStartSupported a public property (and also linker-friendly))

Author:	lambdageek
Assignees:	-
Labels:	arch-wasm, untriaged, area-Threading-mono
Milestone:	-

[lambdageek]

@lewing @richlander This probably needs to be part of a broader discussion than just the runtime

[lewing]

yes

cc @jeromelaban

[jeromelaban]

Thanks Larry! Indeed, this is needed. For native assets, here's what we've been using with Uno to address that problem (along with the emscripten version number, for backward compatibility). SkiaSharp is already built this way, in the build folder of the package (mono/SkiaSharp#2286 is coming to support multi-threading).

For a moment, Wasm exceptions support was considered a variant, but since it is supported by all major browsers, that leaves simd and threading for now.

As for capabilities detection, it's for now available as part of an environment variable for threading, yet it's a quite weak API at this point.

This issue should also be linked to another issue about startup time features detection. This is to include multiple builds of the same payload with different variants, based on browser capabilities, considering there's no current way (as far as I can see) to make fined-grained feature switching at IL level, or runtime level.

[lambdageek]

Related #77541