Re-enable telemetry for global.json state - set error writer to swallow expected error messages (dotnet#51089)

When getting the global.json state for telemetry, we avoid the work to actually resolve an SDK. However, this means that the actual SDK resolution (`hostfxr_resolve_sdk2`) fails, resulting in messages printed to stderr by default. This change uses `hostfxr_set_error_writer` to swallow the expected errors for the duration of that call.

Author: elinor-fung

src/Cli/dotnet/Program.cs

@@ -240,10 +240,7 @@ internal static int ProcessArgs(string[] args, TimeSpan startupTime)
         if (TelemetryClient.Enabled)
         {
             // Get the global.json state to report in telemetry along with this command invocation.
-            // We don't care about the actual SDK resolution, just the global.json information,
-            // so just pass empty string as executable directory for resolution.
-            // NativeWrapper.SdkResolutionResult result = NativeWrapper.NETCoreSdkResolverNativeWrapper.ResolveSdk(string.Empty, Environment.CurrentDirectory);
-            // globalJsonState = result.GlobalJsonState;
+            globalJsonState = NativeWrapper.NETCoreSdkResolverNativeWrapper.GetGlobalJsonState(Environment.CurrentDirectory);
         }
 
         TelemetryEventEntry.SendFiltered(Tuple.Create(parseResult, performanceData, globalJsonState));

src/Resolvers/Microsoft.DotNet.NativeWrapper/Interop.cs

@@ -28,7 +28,7 @@ static Interop()
         }
 
         // MSBuild SDK resolvers are required to be AnyCPU, but we have a native dependency and .NETFramework does not
-        // have a built-in facility for dynamically loading user native dlls for the appropriate platform. We therefore 
+        // have a built-in facility for dynamically loading user native dlls for the appropriate platform. We therefore
         // preload the version with the correct architecture (from a corresponding sub-folder relative to us) on static
         // construction so that subsequent P/Invokes can find it.
         private static void PreloadWindowsLibrary(string dllFileName)
@@ -124,6 +124,12 @@ internal static extern int hostfxr_get_dotnet_environment_info(
             hostfxr_get_dotnet_environment_info_result_fn result,
             IntPtr result_context);
 
+        [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
+        internal delegate void hostfxr_error_writer_fn(IntPtr message);
+
+        [DllImport(Constants.HostFxr, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
+        internal static extern IntPtr hostfxr_set_error_writer(IntPtr error_writer);
+
         public static class Windows
         {
             private const CharSet UTF16 = CharSet.Unicode;

src/Resolvers/Microsoft.DotNet.NativeWrapper/NETCoreSdkResolverNativeWrapper.cs

@@ -23,6 +23,26 @@ public static SdkResolutionResult ResolveSdk(
             return result;
         }
 
+        public static string? GetGlobalJsonState(string globalJsonStartDirectory)
+        {
+            // We don't care about the actual SDK resolution, just the global.json information,
+            // so just pass empty string as executable directory for resolution. This means that
+            // we expect the call to fail to resolve an SDK. Set up the error writer to avoid
+            // output going to stderr. We reset it after the call.
+            var swallowErrors = new Interop.hostfxr_error_writer_fn(message => { });
+            IntPtr errorWriter = Marshal.GetFunctionPointerForDelegate(swallowErrors);
+            IntPtr previousErrorWriter = Interop.hostfxr_set_error_writer(errorWriter);
+            try
+            {
+                SdkResolutionResult result = ResolveSdk(string.Empty, globalJsonStartDirectory);
+                return result.GlobalJsonState;
+            }
+            finally
+            {
+                Interop.hostfxr_set_error_writer(previousErrorWriter);
+            }
+        }
+
         private sealed class SdkList
         {
             public string[]? Entries;