Support getting git information from a sparse-checkout repo

Add tests for getting git info

.gitattributes

@@ -18,3 +18,4 @@
 
 *.sh eol=lf
 *.bin binary
+*.zip binary

modules/BuildTools.Tasks/Utilities/GitRepoInfo.cs

@@ -138,12 +138,41 @@ private static string ResolveHashFromBranch(string gitDir, string branch)
             }
 
             var branchFile = Path.Combine(gitDir, "refs", "heads", branch);
-            if (!File.Exists(branchFile))
+            if (File.Exists(branchFile))
             {
-                throw new FileNotFoundException("Unable to determine current git commit hash");
+                return File.ReadAllText(branchFile).Trim();
             }
 
-            return File.ReadAllText(branchFile).Trim();
+            var packedRefs = Path.Combine(gitDir, "packed-refs");
+            if (File.Exists(packedRefs))
+            {
+                var lines = File.ReadAllLines(packedRefs);
+                foreach (var line in lines)
+                {
+                    if (line.Length == 0)
+                    {
+                        continue;
+                    }
+
+                    if (line[0] == '#' || line[0] == '^')
+                    {
+                        continue;
+                    }
+
+                    var split = line.Split(new[] { ' ' }, 2);
+                    if (split.Length < 2)
+                    {
+                        continue;
+                    }
+
+                    if (string.Equals("refs/heads/" + branch, split[1], StringComparison.Ordinal))
+                    {
+                        return split[0];
+                    }
+                }
+            }
+
+            throw new FileNotFoundException("Unable to determine current git commit hash");
         }
 
         private static DirectoryInfo GetRepositoryRoot(string start)

test/BuildTools.Tasks.Tests/GetGitCommitInfoTests.cs

@@ -0,0 +1,119 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.IO;
+using System.IO.Compression;
+using BuildTools.Tasks.Tests;
+using Xunit;
+
+namespace Microsoft.AspNetCore.BuildTools.Tests
+{
+    public class GetGitCommitInfoTests : IDisposable
+    {
+        private readonly string _tempDir;
+
+        public GetGitCommitInfoTests()
+        {
+            _tempDir = Path.Combine(AppContext.BaseDirectory, Path.GetRandomFileName());
+            Directory.CreateDirectory(_tempDir);
+        }
+
+        public void Dispose()
+        {
+            Directory.Delete(_tempDir, recursive: true);
+        }
+
+        private string CreateTestRepro(string name)
+        {
+            ZipFile.ExtractToDirectory(Path.Combine(AppContext.BaseDirectory, "Resources", name), _tempDir, overwriteFiles: true);
+            return Path.Combine(_tempDir, Path.GetFileNameWithoutExtension(name));
+        }
+
+        [Fact]
+        public void ItFindsCommitHashFromSparseCheckout()
+        {
+            var engine = new MockEngine();
+            var task = new GetGitCommitInfo
+            {
+                BuildEngine = engine,
+                WorkingDirectory = CreateTestRepro("SparseCheckout.zip"),
+            };
+            Assert.True(task.Execute(), "Task should pass");
+
+            Assert.Equal("7896eb0373dac70940819ef6a6494fdeb4880391", task.CommitHash);
+            Assert.Equal("dev", task.Branch);
+        }
+
+        [Fact]
+        public void ItFindsCommitHash()
+        {
+            var engine = new MockEngine();
+            var task = new GetGitCommitInfo
+            {
+                BuildEngine = engine,
+                WorkingDirectory = CreateTestRepro("SimpleGitRepo.zip"),
+            };
+            Assert.True(task.Execute(), "Task should pass");
+
+            Assert.Equal("9c03629e11679f5f3d9dbbd27286239588e0d296", task.CommitHash);
+            Assert.Equal("master", task.Branch);
+        }
+
+        [Fact]
+        public void ItFindsCommitHashInWorktree()
+        {
+            var root = CreateTestRepro("WorktreeRepo.zip");
+
+            var engine = new MockEngine();
+            var task = new GetGitCommitInfo
+            {
+                BuildEngine = engine,
+                WorkingDirectory = Path.Combine(root, "SourceRoot"),
+            };
+
+            Assert.True(task.Execute(), "Task should pass");
+            Assert.Equal("27a9c92f96a117ff926c12beb9d4ea8d0f127e42", task.CommitHash);
+            Assert.Equal("master", task.Branch);
+
+            engine = new MockEngine();
+            task = new GetGitCommitInfo
+            {
+                BuildEngine = engine,
+                WorkingDirectory = Path.Combine(root, "Worktree1"),
+            };
+
+            Assert.True(task.Execute(), "Task should pass");
+            Assert.Equal("51bd19b3825fbc3e96fbdabc9f2cfa9972999bfa", task.CommitHash);
+            Assert.Equal("worktree1", task.Branch);
+        }
+
+        [Fact]
+        public void ItFindsCommitHashInSubmodule()
+        {
+            var root = CreateTestRepro("SubmoduleRepo.zip");
+
+            var engine = new MockEngine();
+            var task = new GetGitCommitInfo
+            {
+                BuildEngine = engine,
+                WorkingDirectory = root,
+            };
+
+            Assert.True(task.Execute(), "Task should pass");
+            Assert.Equal("91314ecce9e7d3cbd1d3f4d1f35664f52a301479", task.CommitHash);
+            Assert.Equal("master", task.Branch);
+
+            engine = new MockEngine();
+            task = new GetGitCommitInfo
+            {
+                BuildEngine = engine,
+                WorkingDirectory = Path.Combine(root, "modules", "submodule1"),
+            };
+
+            Assert.True(task.Execute(), "Task should pass");
+            Assert.Equal("599e691c41f502ed9e062b1822ce13b673fc916e", task.CommitHash);
+            Assert.Equal("dev", task.Branch);
+        }
+    }
+}