Use async name resolution on Linux

[scalablecory]

Edit by @antonfirsov:

this issue has been reopened to track the missing feature, and the related disabled test (see #70089). We should not use getaddrinfo_a as proposed by the original opening comment, since it turned out to be sub-optimal (see #48666).

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We should use getaddrinfo_a on Linux for async name resolution. It supports cancellation as well, bringing us in line with Windows support. See sample code for a usage example.

Note: Today we do async over sync on Linux.

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[karelz]

up-for-grabs: We just need to add function to our PAL layer. Then call it from the C# code.
Should be fairly straightforward -- the Linux code will be very similar to Windows code.

[gfoidl]

I'd like to give this a try.

[gfoidl]

I have a question regarding the workflow for testing the native parts.

Steps I've done (on linux):

1. building
2. $ cd src/libraries/System.Net.NameResolution/tests/PalTests
3. $ pushd ../../src & dotnet build & popd & dotnet build /t:test
4. verify tests pass
5. make changes in native code

in a second shell:

1. $ cd /src/libraries/Native
2. $ ./build-native.sh debug x64
3. make sure build is OK

When I test then the changes in native code aren't picked up. Even if i delete libSystem.Native.so then the test pass -- I'd expect them to fail without the native counterpart.

How can I ensure the current libSystem.Native.so is used?

Additional info:
.NET Core 5 SDK was installed before a build of the repo was done.
No custom environment variables got set or any of the default variables got overwritten.

[gfoidl]

LD_DEBUG=libs dotnet build /t:test 2>&1 > /dev/null | grep System.Native

outputs

       678:	calling init: /home/runtime/.dotnet/shared/Microsoft.NETCore.App/5.0.0-preview.3.20169.1/libSystem.Native.so
       701:	calling init: /home/runtime/.dotnet/shared/Microsoft.NETCore.App/5.0.0-preview.3.20169.1/libSystem.Native.so
       701:	calling fini: /home/runtime/.dotnet/shared/Microsoft.NETCore.App/5.0.0-preview.3.20169.1/libSystem.Native.so [0]
       678:	calling fini: /home/runtime/.dotnet/shared/Microsoft.NETCore.App/5.0.0-preview.3.20169.1/libSystem.Native.so [0]

so the native-libs get loaded from the shared installations (from the build) and not from the artifacts-location.

With some hacks* this could be solved, but is there a "nice way" to use the currently built native libraries in the managed tests?

* even LD_PRELOAD doesn't do the job

[stephentoub]

I agree it'd be nice if there were a better solution (and there may be... @ViktorHofer?), but I always just copy the built binaries into the testhost shared folder after building them.

[rzikm]

Opening as the async name resolution implementation introduced in #34633 was reverted in #48666

[ViktorHofer]

so the native-libs get loaded from the shared installations (from the build) and not from the artifacts-location.

That looks like a bug as the testhost should be the single point of truth. @gfoidl can you please open a separate issue for that with repro steps so that we can take a look?