Fix loading native libgit2 binary

Author: AArnott

src/Nerdbank.GitVersioning.NuGet/build/Nerdbank.GitVersioning.targets

@@ -31,6 +31,8 @@
     <GenerateAssemblyInformationalVersionAttribute>false</GenerateAssemblyInformationalVersionAttribute>
     <GenerateAssemblyVersionAttribute>false</GenerateAssemblyVersionAttribute>
     <GenerateAssemblyFileVersionAttribute>false</GenerateAssemblyFileVersionAttribute>
+
+    <NerdbankGitVersioningTasksPath Condition=" '$(NerdbankGitVersioningTasksPath)' == '' ">$(MSBuildThisFileDirectory)</NerdbankGitVersioningTasksPath>
   </PropertyGroup>
 
   <UsingTask AssemblyFile="$(NerdbankGitVersioningTasksPath)Nerdbank.GitVersioning.Tasks.dll" TaskName="Nerdbank.GitVersioning.Tasks.AssemblyVersionInfo"/>
@@ -55,7 +57,8 @@
       BuildingRef="$(_NBGV_BuildingRef)"
       BuildMetadata="@(BuildMetadata)"
       DefaultPublicRelease="$(PublicRelease)"
-      GitRepoRoot="$(GitRepoRoot)">
+      GitRepoRoot="$(GitRepoRoot)"
+      TaskAssemblyPath="$(NerdbankGitVersioningTasksPath)">
 
       <Output TaskParameter="Version" PropertyName="BuildVersion" />
       <Output TaskParameter="AssemblyInformationalVersion" PropertyName="AssemblyInformationalVersion" />

src/Nerdbank.GitVersioning.Tasks/GetBuildVersion.cs

@@ -6,6 +6,7 @@
     using System.Diagnostics;
     using System.IO;
     using System.Linq;
+    using System.Runtime.InteropServices;
     using System.Text;
     using System.Text.RegularExpressions;
     using Microsoft.Build.Framework;
@@ -14,6 +15,13 @@
 
     public class GetBuildVersion : Task
     {
+#if !NET45
+        /// <summary>
+        /// An AppDomain-wide variable used on 
+        /// </summary>
+        private static bool libgit2PathInitialized;
+#endif
+
         /// <summary>
         /// Initializes a new instance of the <see cref="GetBuildVersion"/> class.
         /// </summary>
@@ -43,6 +51,15 @@ public GetBuildVersion()
         /// </summary>
         public string GitRepoRoot { get; set; }
 
+        /// <summary>
+        /// Gets or sets the path to the folder that contains this task's assembly.
+        /// </summary>
+        /// <remarks>
+        /// This is particularly useful in .NET Core where discovering one's own assembly path
+        /// is not allowed before .NETStandard 2.0.
+        /// </remarks>
+        public string TaskAssemblyPath { get; set; }
+
         /// <summary>
         /// Gets or sets a value indicating whether the project is building
         /// in PublicRelease mode.
@@ -146,6 +163,11 @@ public override bool Execute()
         {
             try
             {
+                if (!this.HelpFindLibGit2NativeBinaries())
+                {
+                    return false;
+                }
+
                 var cloudBuild = CloudBuild.Active;
                 var oracle = VersionOracle.Create(Directory.GetCurrentDirectory(), this.GitRepoRoot, cloudBuild);
                 if (!string.IsNullOrEmpty(this.DefaultPublicRelease))
@@ -190,5 +212,25 @@ public override bool Execute()
 
             return true;
         }
+
+        private bool HelpFindLibGit2NativeBinaries()
+        {
+#if !NET45
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows) && !libgit2PathInitialized)
+            {
+                if (string.IsNullOrWhiteSpace(this.TaskAssemblyPath))
+                {
+                    this.Log.LogError("The TaskAssemblyPath parameter is required on .NET Core running on Windows.");
+                    return false;
+                }
+
+                string nativeDllPath = Path.Combine(this.TaskAssemblyPath, "lib", "win32", IntPtr.Size == 4 ? "x86" : "x64");
+                Environment.SetEnvironmentVariable("PATH", Environment.GetEnvironmentVariable("PATH") + Path.PathSeparator + nativeDllPath);
+                libgit2PathInitialized = true;
+            }
+#endif
+
+            return true;
+        }
     }
 }