Need better logging for the native SDK resolver

[KirillOsenkov]

I don't see a way to debug/investigate the decision process of the SDK resolver native .dll.

I've gotten as far as

sdk/src/Resolvers/Microsoft.DotNet.SdkResolver/NETCoreSdkResolverNativeWrapper.cs

Lines 21 to 23 in 917b98c

	int errorCode = Interop.RunningOnWindows
	? Interop.Windows.hostfxr_resolve_sdk2(dotnetExeDirectory, globalJsonStartDirectory, flags, result.Initialize)
	: Interop.Unix.hostfxr_resolve_sdk2(dotnetExeDirectory, globalJsonStartDirectory, flags, result.Initialize);

but from then on it's a black box and I don't see anything that would explain the resulting SDK that comes out of it. I want to know what factors influenced the decision:

• did it use an environment variable?
• did it use PATH?
• did it use global.json?
• which global.json?
• was IsRunningInVisualStudio specified?

The resolver should explain its choice and clearly communicate which factors made a difference and which didn't. Otherwise I have to resort to cargo culting it and so far unsuccessfully.

[KirillOsenkov]

As an example, dotnet/msbuild#5999 is an issue that took me a whole day to figure out, and as a result of that investigation I have emerged with the knowledge that I should have set these three environment variables to influence the SDK resolution process:

PATH=C:\msbuild\.dotnet;%PATH%
DOTNET_INSTALL_DIR=C:\msbuild\.dotnet
DOTNET_MULTILEVEL_LOOKUP=0

Had there been a log entry like "choosing 3.1.100 because the DOTNET_INSTALL_DIR was set explicitly and pointed to a 3.1.100 SDK at C:\msbuild.dotnet" I could have binged an entire season of something wholesome today instead of debugging deep into the SDK resolution.

Currently the only logging I see is

[KirillOsenkov]

Correction, I also had to set PATH=C:\msbuild\.dotnet;%PATH%

[rainersigwald]

I don't know if the logging interface currently exposed by MSBuild makes this possible--it's fairly minimal. We might need to augment that first before improving the dotnet resolver.

[dsplaisted]

@KirillOsenkov Are you saying you need more visibility into what the native hostfxr_resolve_sdk2 method is doing, or what the managed code calling it does with the result? (Or are you not sure?)

[KirillOsenkov]

The former, I want to know what decisions the native hostfxr_resolve_sdk2 did and why. Because it's impossible to debug, all I get is a decision, but why it chose that SDK is completely unclear. It should say "used this environment variable" or something.

[KirillOsenkov]

And then I guess it's a separate story to thread that output through the managed code so it ends up as an Evaluation message in the binlog