.NET 9: Enable and better define "portable Linux" behavior

[richlander]

Source build should support both portable and distro-specific Linux build flavors. We'll also need to decide which is the default. There are reasons for both choices.

Today, we have the following dichotomy:

• Microsoft proprietary build produces the portable linux flavor.
• Source build produces distro-specific flavor, like rhel.

We need to support both flavors within source-build going forward. This issue goes into some detail on this topic: dotnet/runtime#66132 (comment)

I'm also not aware of a good spec on portable linux. We should probably have one. It affects topics like dotnet --info, dotnet build --use-current-runtime, and (obviously) dotnet restore. More context: dotnet/sdk#23539 (comment).

[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.

[tmds]

I'm also not aware of a good spec on portable linux.

portable Linux means the binaries work across a range of distros.

When building .NET, it is the build Microsoft makes for linux-{arch} with an old glibc and a set of build flags that make the runtime dynamically look for libraries (like OpenSSL).
(And similar for linux-musl-{arch}).

Anything maintainers build on a specific distro using .NET source-build is not portable linux.
They may use some of the flags Microsoft uses for the portable build
Because they use the distro's glibc (and not the old one Microsoft is pinning to), the resulting binaries won't work across a range of distros.
These builds identify themselves with a non-portable rid (like fedora.36-x64).

build --use-current-runtime

Currently, only Microsoft is publish assets like Microsoft.NETCore.App.Runtime.<rid> which enable features like stand-alone applications.

That is why it makes sense for --use-current-runtime to target those portable assets today.

These features should be made source-buildable. I hope we can invest in this for .NET 8 (#1215).

Then we we have two use-cases:

build --use-portable-runtime // target linux-x64 to have something that works accross a range of distros
build --use-platform-runtime // target source-build non-portable rid (e.g. `fedora.36-x64`) to use assets completely built from source

[MichaelSimons]

Stating the obvious but part of the work is to create documentation for what is supported and the UX.