Make SDK resolution error messages more helpful. (#341)

* Make SDK resolution error messages more helpful.

* Shorten exception message.

Author: teo-tsirpanis

src/MSBuildLocator/DotNetSdkLocationHelper.cs

@@ -11,6 +11,7 @@
 using System.Reflection;
 using System.Runtime.InteropServices;
 using System.Runtime.Loader;
+using System.Text;
 using System.Text.RegularExpressions;
 
 #nullable enable
@@ -117,11 +118,13 @@ public static IEnumerable<VisualStudioInstance> GetInstances(string workingDirec
             static IEnumerable<string> GetAllAvailableSDKs(bool allowAllDotnetLocations)
             {
                 bool foundSdks = false;
+                int rc = 0;
+                StringBuilder? errorMessage = null;
                 foreach (string dotnetPath in s_dotnetPathCandidates.Value)
                 {
-                    int rc = NativeMethods.hostfxr_get_available_sdks(exe_dir: dotnetPath, out string[]? resolvedPaths);
+                    rc = NativeMethods.hostfxr_get_available_sdks(exe_dir: dotnetPath, out string[]? resolvedPaths, out errorMessage);
 
-                    if (rc == 0 && resolvedPaths != null)
+                    if (resolvedPaths != null)
                     {
                         foundSdks = true;
 
@@ -140,17 +143,19 @@ static IEnumerable<string> GetAllAvailableSDKs(bool allowAllDotnetLocations)
                 // Errors are automatically printed to stderr. We should not continue to try to output anything if we failed.
                 if (!foundSdks)
                 {
-                    throw new InvalidOperationException(SdkResolutionExceptionMessage(nameof(NativeMethods.hostfxr_get_available_sdks)));
+                    throw new InvalidOperationException(SdkResolutionExceptionMessage(nameof(NativeMethods.hostfxr_get_available_sdks), rc, errorMessage));
                 }
             }
 
             // Determines the directory location of the SDK accounting for global.json and multi-level lookup policy.
             static string? GetSdkFromGlobalSettings(string workingDirectory)
             {
                 string? resolvedSdk = null;
+                int rc = 0;
+                StringBuilder? errorMessage = null;
                 foreach (string dotnetPath in s_dotnetPathCandidates.Value)
                 {
-                    int rc = NativeMethods.hostfxr_resolve_sdk2(exe_dir: dotnetPath, working_dir: workingDirectory, flags: 0, out resolvedSdk, out _);
+                    rc = NativeMethods.hostfxr_resolve_sdk2(exe_dir: dotnetPath, working_dir: workingDirectory, flags: 0, out resolvedSdk, out _, out errorMessage);
 
                     if (rc == 0)
                     {
@@ -160,7 +165,7 @@ static IEnumerable<string> GetAllAvailableSDKs(bool allowAllDotnetLocations)
                 }
 
                 return string.IsNullOrEmpty(resolvedSdk)
-                    ? throw new InvalidOperationException(SdkResolutionExceptionMessage(nameof(NativeMethods.hostfxr_resolve_sdk2)))
+                    ? throw new InvalidOperationException(SdkResolutionExceptionMessage(nameof(NativeMethods.hostfxr_resolve_sdk2), rc, errorMessage))
                     : resolvedSdk;
             }
         }
@@ -229,7 +234,8 @@ private static IntPtr HostFxrResolver(Assembly assembly, string libraryName)
             throw new InvalidOperationException(error);
         }
 
-        private static string SdkResolutionExceptionMessage(string methodName) => $"Failed to find all versions of .NET Core MSBuild. Call to {methodName}. There may be more details in stderr.";
+        private static string SdkResolutionExceptionMessage(string methodName, int rc, StringBuilder? errorMessage) =>
+            $"Error while calling hostfxr function {methodName}. Error code: {rc} Detailed error: {errorMessage}";
 
         private static List<string> ResolveDotnetPathCandidates()
         {

src/MSBuildLocator/NativeMethods.cs

@@ -7,6 +7,7 @@
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Runtime.InteropServices.Marshalling;
+using System.Text;
 
 namespace Microsoft.Build.Locator
 {
@@ -25,17 +26,19 @@ private enum hostfxr_resolve_sdk2_result_key_t
             global_json_path = 1,
         };
 
-        internal static int hostfxr_resolve_sdk2(string exe_dir, string working_dir, hostfxr_resolve_sdk2_flags_t flags, out string resolved_sdk_dir, out string global_json_path)
+        internal static int hostfxr_resolve_sdk2(string exe_dir, string working_dir, hostfxr_resolve_sdk2_flags_t flags, out string resolved_sdk_dir, out string global_json_path, out StringBuilder errorMessage)
         {
             Debug.Assert(t_resolve_sdk2_resolved_sdk_dir is null);
             Debug.Assert(t_resolve_sdk2_global_json_path is null);
             try
             {
                 unsafe
                 {
+                    using var errorHandler = new ErrorHandler();
                     int result = hostfxr_resolve_sdk2(exe_dir, working_dir, flags, &hostfxr_resolve_sdk2_callback);
                     resolved_sdk_dir = t_resolve_sdk2_resolved_sdk_dir;
                     global_json_path = t_resolve_sdk2_global_json_path;
+                    errorMessage = t_hostfxr_error_builder;
                     return result;
                 }
             }
@@ -72,15 +75,17 @@ private static unsafe void hostfxr_resolve_sdk2_callback(hostfxr_resolve_sdk2_re
             }
         }
 
-        internal static int hostfxr_get_available_sdks(string exe_dir, out string[] sdks)
+        internal static int hostfxr_get_available_sdks(string exe_dir, out string[] sdks, out StringBuilder errorMessage)
         {
             Debug.Assert(t_get_available_sdks_result is null);
             try
             {
                 unsafe
                 {
+                    using var errorHandler = new ErrorHandler();
                     int result = hostfxr_get_available_sdks(exe_dir, &hostfxr_get_available_sdks_callback);
                     sdks = t_get_available_sdks_result;
+                    errorMessage = t_hostfxr_error_builder;
                     return result;
                 }
             }
@@ -108,6 +113,29 @@ private static unsafe void hostfxr_get_available_sdks_callback(int count, void**
             t_get_available_sdks_result = result;
         }
 
+        [LibraryImport(HostFxrName)]
+        [UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
+        private static unsafe partial delegate* unmanaged[Cdecl]<void*, void> hostfxr_set_error_writer(delegate* unmanaged[Cdecl]<void*, void> error_writer);
+
+        [ThreadStatic]
+        private static StringBuilder t_hostfxr_error_builder;
+
+        [UnmanagedCallersOnly(CallConvs = [typeof(CallConvCdecl)])]
+        private static unsafe void hostfxr_error_writer_callback(void* message)
+        {
+            t_hostfxr_error_builder ??= new StringBuilder();
+            if (OperatingSystem.IsWindows())
+            {
+                // Avoid allocating temporary string on Windows.
+                t_hostfxr_error_builder.Append(MemoryMarshal.CreateReadOnlySpanFromNullTerminated((char*)message));
+                t_hostfxr_error_builder.AppendLine();
+            }
+            else
+            {
+                t_hostfxr_error_builder.AppendLine(Utf8StringMarshaller.ConvertToManaged((byte*)message));
+            }
+        }
+
         [CustomMarshaller(typeof(string), MarshalMode.Default, typeof(AutoStringMarshaller))]
         internal static unsafe class AutoStringMarshaller
         {
@@ -117,6 +145,23 @@ internal static unsafe class AutoStringMarshaller
 
             public static string ConvertToManaged(void* ptr) => Marshal.PtrToStringAuto((nint)ptr);
         }
+
+        private unsafe readonly ref struct ErrorHandler
+        {
+            private readonly delegate* unmanaged[Cdecl]<void*, void> _previousCallback;
+
+            public ErrorHandler()
+            {
+                Debug.Assert(t_hostfxr_error_builder is null);
+                _previousCallback = hostfxr_set_error_writer(&hostfxr_error_writer_callback);
+            }
+
+            public void Dispose()
+            {
+                hostfxr_set_error_writer(_previousCallback);
+                t_hostfxr_error_builder = null;
+            }
+        }
     }
 }
 #endif