Allow explicit Windows SDK version

[reynoldsbd]

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Is your feature request related to a problem? Please describe.

For the sake of stability and reproducibility, my team prefers to build against an explicit version of the Windows SDK, e.g. 10.0.18362.0.

However, when loading its Visual Studio shell environment, vcpkg currently does not specify any particular version of the SDK. vcvarsall.bat will thus choose the latest available version. This limits our ability to control which version of the SDK is used when building vcpkg dependencies.

Proposed solution

Add a mechanism by which users may specify an explicit version of the Windows SDK to use. When calling vcvarsall.bat, vcpkg will pass the specified value using the -winsdk=<ver> argument.

I propose exposing this mechanism to the user by way of a new triplet variable, tentatively named VCPKG_PLATFORM_SDK_VERSION.

Describe alternatives you've considered

It is technically possible to build with a specific Windows SDK version by simply uninstalling any newer versions of the SDK before running vcpkg. This is obviously cumbersome, particularly for users who may jump between projects with different SDK requirements.

My team is currently applying a patch to vcpkg-tool and bootstrapping manually. This patch is a very trivial hack which reads from an env var and injects the desired -winsdk=<ver> argument within visualstudio.cpp. Definitely not something that would be accepted upstream.

Additional context

I'm more than happy to submit a PR for this proposal. Just wanted to open this issue first to hash out the details and get feedback.

Unless there are objections, I plan on largely copying from microsoft/vcpkg-tool#199 which introduced VCPKG_PLATFORM_TOOLSET_VERSION.

[Hoikas]

I think you can do this by setting the WindowsSDKVersion environment variable and passing it through by setting VCPKG_ENV_PASSTHROUGH to WindowsSDKVersion in your triplet.

[reynoldsbd]

I wasn't able to make this work locally.

My understanding is that WindowsSDKVersion is an output from vcvarsall.bat. So even if I set it to the desired value, it will be overwritten to the default (latest available) version once vcvarsall.bat is loaded.

[Neumann-A]

Set the env var in the triplet directly

[Cheney-W]

@BillyONeal @vicroms @ras0219-msft Could you please help take a look this issue?

[BillyONeal]

@Cheney-W I don't expect such a feature is going to make the cut line soon.

[reynoldsbd]

@BillyONeal can you elaborate on what you mean by "make the cut line"? I'm totally willing to implement this feature myself and submit a PR, but are you saying my PR would not be accepted?

[BillyONeal]

I mean "vcpkg engineers working on", not a statement about future PR acceptance. It seems like it would be useful but plumbing into all the build systems will not be a small undertaking.

[reynoldsbd]

Thanks for clarifying!