Fix git system hash for worktrees

src/GitVersionCore.Tests/ExecuteCoreTests.cs

@@ -8,6 +8,8 @@
 using System.IO;
 using System.Text;
 
+using LibGit2Sharp;
+
 [TestFixture]
 [Parallelizable(ParallelScope.None)]
 public class ExecuteCoreTests : TestBase
@@ -40,6 +42,34 @@ public void CacheKeySameAfterReNormalizing()
         });
     }
 
+    [Test]
+    [Category("NoMono")]
+    [Description("LibGit2Sharp fails here when running under Mono")]
+    public void CacheKeyForWorktree() {
+        var versionAndBranchFinder = new ExecuteCore(fileSystem);
+
+        RepositoryScope(versionAndBranchFinder, (fixture, vv) =>
+        {
+            var worktreePath = Path.Combine(Path.GetTempPath(), "TestRepositories", Guid.NewGuid().ToString());
+            try
+            {
+                // create a branch and a new worktree for it
+                var repo = new Repository(fixture.RepositoryPath);
+                repo.Worktrees.Add("worktree", worktreePath, false);
+
+                var targetUrl = "https://github.com/GitTools/GitVersion.git";
+                var gitPreparer = new GitPreparer(targetUrl, null, new Authentication(), false, worktreePath);
+                var configFileLocator = new DefaultConfigFileLocator();
+                var cacheKey = GitVersionCacheKeyFactory.Create(fileSystem, gitPreparer, null, configFileLocator);
+                cacheKey.Value.ShouldNotBeEmpty();
+            }
+            finally
+            {
+                DirectoryHelper.DeleteDirectory(worktreePath);
+            }
+        });
+    }
+
     [Test]
     public void CacheFileExistsOnDisk()
     {

src/GitVersionCore/GitVersionCacheKeyFactory.cs

@@ -25,12 +25,30 @@ static string GetGitSystemHash(GitPreparer gitPreparer)
         {
             var dotGitDirectory = gitPreparer.GetDotGitDirectory();
 
+            // get repo .git directory -- not the worktree gitdir
+            dotGitDirectory = GetWorktreeParentDotGetDirectory(dotGitDirectory);
+
             // traverse the directory and get a list of files, use that for GetHash
             var contents = CalculateDirectoryContents(Path.Combine(dotGitDirectory, "refs"));
 
             return GetHash(contents.ToArray());
         }
 
+        static string GetWorktreeParentDotGetDirectory(string path)
+        {
+            if (string.IsNullOrEmpty(path)) return path;
+
+            var parentPath = Path.GetDirectoryName(path);
+            if (string.IsNullOrEmpty(parentPath)) return path;
+
+            var worktreesPath = Path.Combine(".git", "worktrees");
+
+            if (!parentPath.EndsWith(worktreesPath, StringComparison.Ordinal))
+                return path;
+
+            return parentPath.Substring(0, parentPath.Length - 10);
+        }
+
         // based on https://msdn.microsoft.com/en-us/library/bb513869.aspx
         static List<string> CalculateDirectoryContents(string root)
         {
@@ -42,7 +60,7 @@ static List<string> CalculateDirectoryContents(string root)
 
             if (!Directory.Exists(root))
             {
-                throw new ArgumentException();
+                throw new ArgumentException($"Root directory does not exist: {root}");
             }
 
             dirs.Push(root);