Fix discovery of working directory for worktrees (#734)

SourceLink used gitdir to determine the directory (and incorrectly interpreting the content) but turns out git doesn't use it at all for this purpose.

src/Microsoft.Build.Tasks.Git.UnitTests/GitRepositoryTests.cs

@@ -95,19 +95,33 @@ public void TryFindRepository_Worktree_Realistic()
             Assert.Equal(mainGitDir.Path, location.CommonDirectory);
             Assert.Null(location.WorkingDirectory);
 
+            var repository = GitRepository.OpenRepository(location, GitEnvironment.Empty);
+            Assert.Equal(location.GitDirectory, repository.GitDirectory);
+            Assert.Equal(location.CommonDirectory, repository.CommonDirectory);
+            Assert.Null(repository.WorkingDirectory);
+
             // start under worktree directory:
             Assert.True(GitRepository.TryFindRepository(worktreeSubDir.Path, out location));
-
             Assert.Equal(worktreeGitDir.Path, location.GitDirectory);
             Assert.Equal(mainGitDir.Path, location.CommonDirectory);
             Assert.Equal(worktreeDir.Path, location.WorkingDirectory);
 
+            repository = GitRepository.OpenRepository(location, GitEnvironment.Empty);
+            Assert.Equal(location.GitDirectory, repository.GitDirectory);
+            Assert.Equal(location.WorkingDirectory, repository.WorkingDirectory);
+            Assert.Equal(location.CommonDirectory, repository.CommonDirectory);
+
             // start under worktree git directory (git config works from this dir, but git status requires work dir):
             Assert.True(GitRepository.TryFindRepository(worktreeGitSubDir.Path, out location));
 
             Assert.Equal(worktreeGitDir.Path, location.GitDirectory);
             Assert.Equal(mainGitDir.Path, location.CommonDirectory);
             Assert.Null(location.WorkingDirectory);
+
+            repository = GitRepository.OpenRepository(location, GitEnvironment.Empty);
+            Assert.Equal(location.GitDirectory, repository.GitDirectory);
+            Assert.Equal(location.CommonDirectory, repository.CommonDirectory);
+            Assert.Null(repository.WorkingDirectory);
         }
 
         [Fact]
@@ -170,6 +184,31 @@ public void OpenRepository()
             Assert.Equal("0000000000000000000000000000000000000000", repository.GetHeadCommitSha());
         }
 
+        [Fact]
+        public void OpenRepository_WorkingDirectorySpecifiedInConfig()
+        {
+            using var temp = new TempRoot();
+
+            var homeDir = temp.CreateDirectory();
+
+            var workingDir = temp.CreateDirectory();
+            var workingDir2 = temp.CreateDirectory();
+            var gitDir = workingDir.CreateDirectory(".git");
+
+            gitDir.CreateFile("HEAD");
+            gitDir.CreateFile("config").WriteAllText("[core]worktree = " + workingDir2.Path.Replace(@"\", @"\\"));
+
+            Assert.True(GitRepository.TryFindRepository(gitDir.Path, out var location));
+            Assert.Equal(gitDir.Path, location.CommonDirectory);
+            Assert.Equal(gitDir.Path, location.GitDirectory);
+            Assert.Null(location.WorkingDirectory);
+
+            var repository = GitRepository.OpenRepository(location, GitEnvironment.Empty);
+            Assert.Equal(gitDir.Path, repository.CommonDirectory);
+            Assert.Equal(gitDir.Path, repository.GitDirectory);
+            Assert.Equal(workingDir2.Path, repository.WorkingDirectory);
+        }
+
         [Fact]
         public void OpenRepository_VersionNotSupported()
         {
@@ -191,6 +230,72 @@ public void OpenRepository_VersionNotSupported()
             Assert.Throws<NotSupportedException>(() => GitRepository.OpenRepository(src.Path, new GitEnvironment(homeDir.Path)));
         }
 
+        [Fact]
+        public void OpenRepository_Worktree_GitdirFileMissing()
+        {
+            using var temp = new TempRoot();
+
+            var mainWorkingDir = temp.CreateDirectory();
+            var mainGitDir = mainWorkingDir.CreateDirectory(".git");
+            mainGitDir.CreateFile("HEAD");
+
+            var worktreesDir = mainGitDir.CreateDirectory("worktrees");
+            var worktreeGitDir = worktreesDir.CreateDirectory("myworktree");
+            var worktreeDir = temp.CreateDirectory();
+            var worktreeGitFile = worktreeDir.CreateFile(".git").WriteAllText("gitdir: " + worktreeGitDir + " \r\n\t\v");
+
+            worktreeGitDir.CreateFile("HEAD");
+            worktreeGitDir.CreateFile("commondir").WriteAllText("../..\n");
+            // gitdir file that links back to the worktree working directory is missing from worktreeGitDir
+
+            Assert.True(GitRepository.TryFindRepository(worktreeDir.Path, out var location));
+            Assert.Equal(worktreeGitDir.Path, location.GitDirectory);
+            Assert.Equal(mainGitDir.Path, location.CommonDirectory);
+            Assert.Equal(worktreeDir.Path, location.WorkingDirectory);
+
+            var repository = GitRepository.OpenRepository(location, GitEnvironment.Empty);
+            Assert.Equal(repository.GitDirectory, location.GitDirectory);
+            Assert.Equal(repository.CommonDirectory, location.CommonDirectory);
+            Assert.Equal(repository.WorkingDirectory, location.WorkingDirectory);
+        }
+
+        /// <summary>
+        /// The directory in gitdir file is ignored for the purposes of determining repository working directory.
+        /// </summary>
+        [Fact]
+        public void OpenRepository_Worktree_GitdirFileDifferentPath()
+        {
+            using var temp = new TempRoot();
+
+            var mainWorkingDir = temp.CreateDirectory();
+            var mainGitDir = mainWorkingDir.CreateDirectory(".git");
+            mainGitDir.CreateFile("HEAD");
+
+            var worktreesDir = mainGitDir.CreateDirectory("worktrees");
+            var worktreeGitDir = worktreesDir.CreateDirectory("myworktree");
+            var worktreeDir = temp.CreateDirectory();
+            var worktreeGitFile = worktreeDir.CreateFile(".git").WriteAllText("gitdir: " + worktreeGitDir + " \r\n\t\v");
+
+            var worktreeDir2 = temp.CreateDirectory();
+            var worktreeGitFile2 = worktreeDir2.CreateFile(".git").WriteAllText("gitdir: " + worktreeGitDir + " \r\n\t\v");
+
+            worktreeGitDir.CreateFile("HEAD");
+            worktreeGitDir.CreateFile("commondir").WriteAllText("../..\n");
+            worktreeGitDir.CreateFile("gitdir").WriteAllText(worktreeGitFile2.Path + " \r\n\t\v");
+
+            Assert.True(GitRepository.TryFindRepository(worktreeDir.Path, out var location));
+            Assert.Equal(worktreeGitDir.Path, location.GitDirectory);
+            Assert.Equal(mainGitDir.Path, location.CommonDirectory);
+            Assert.Equal(worktreeDir.Path, location.WorkingDirectory);
+
+            var repository = GitRepository.OpenRepository(location, GitEnvironment.Empty);
+            Assert.Equal(repository.GitDirectory, location.GitDirectory);
+            Assert.Equal(repository.CommonDirectory, location.CommonDirectory);
+
+            // actual working dir is not affected:
+            Assert.Equal(worktreeDir.Path, location.WorkingDirectory);
+        }
+
         [Fact]
         public void Submodules()
         {

src/Microsoft.Build.Tasks.Git/GitDataReader/GitRepository.cs

@@ -5,7 +5,6 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
-using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
@@ -19,7 +18,6 @@ internal sealed class GitRepository
         private const string CommonDirFileName = "commondir";
         private const string GitDirName = ".git";
         private const string GitDirPrefix = "gitdir: ";
-        private const string GitDirFileName = "gitdir";
 
         // See https://git-scm.com/docs/gitrepository-layout#Documentation/gitrepository-layout.txt-HEAD
         internal const string GitHeadFileName = "HEAD";
@@ -131,46 +129,10 @@ public static GitRepository OpenRepository(GitRepositoryLocation location, GitEn
 
         private static string? GetWorkingDirectory(GitConfig config, GitRepositoryLocation location)
         {
-            // Working trees cannot have the same common directory and git directory.
-            // 'gitdir' file in a git directory indicates a working tree.
-
-            var gitdirFilePath = Path.Combine(location.GitDirectory, GitDirFileName);
-
-            var isLinkedWorkingTree = PathUtils.ToPosixDirectoryPath(location.CommonDirectory) != PathUtils.ToPosixDirectoryPath(location.GitDirectory) && 
-                File.Exists(gitdirFilePath);
-
-            if (isLinkedWorkingTree)
-            {
-                // https://git-scm.com/docs/gitrepository-layout#Documentation/gitrepository-layout.txt-worktreesltidgtgitdir
-
-                string workingDirectory;
-                try
-                {
-                    workingDirectory = File.ReadAllText(gitdirFilePath);
-                }
-                catch (Exception e) when (!(e is IOException))
-                {
-                    throw new IOException(e.Message, e);
-                }
-
-                workingDirectory = workingDirectory.TrimEnd(CharUtils.AsciiWhitespace);
-
-                // Path in gitdir file must be absolute.
-                if (!PathUtils.IsAbsolute(workingDirectory))
-                {
-                    throw new InvalidDataException(string.Format(Resources.PathSpecifiedInFileIsNotAbsolute, gitdirFilePath, workingDirectory));
-                }
-
-                try
-                {
-                    return Path.GetFullPath(workingDirectory);
-                }
-                catch
-                {
-                    throw new InvalidDataException(string.Format(Resources.PathSpecifiedInFileIsInvalid, gitdirFilePath, workingDirectory));
-                }
-            }
+            // TODO (https://github.com/dotnet/sourcelink/issues/301):
+            // GIT_WORK_TREE environment variable can also override working directory.
 
+            // Working directory can be overridden by a config option.
             // See https://git-scm.com/docs/git-config#Documentation/git-config.txt-coreworktree
             string? value = config.GetVariableValue("core", "worktree");
             if (value != null)

src/Microsoft.Build.Tasks.Git/GitOperations.cs

@@ -357,7 +357,7 @@ internal sealed class DirectoryNode
             public readonly string Name;
             public readonly List<DirectoryNode> OrderedChildren;
 
-            // set on nodes that represent submodule working directory:
+            // set on nodes that represent working directory of the repository or a submodule:
             public Lazy<GitIgnore.Matcher?>? Matcher;
 
             public DirectoryNode(string name, List<DirectoryNode> orderedChildren)