Add NBGV_GitEngine=Disabled option

src/NerdBank.GitVersioning/Commands/CloudCommand.cs

@@ -94,7 +94,7 @@ public void SetBuildVariables(string projectDirectory, IEnumerable<string> metad
             activeCloudBuild = CloudBuild.SupportedCloudBuilds[matchingIndex];
         }
 
-        using var context = GitContext.Create(projectDirectory, writable: alwaysUseLibGit2);
+        using var context = GitContext.Create(projectDirectory, engine: alwaysUseLibGit2 ? GitContext.Engine.ReadWrite : GitContext.Engine.ReadOnly);
         var oracle = new VersionOracle(context, cloudBuild: activeCloudBuild);
         if (metadata is not null)
         {

src/NerdBank.GitVersioning/DisabledGit/DisabledGitContext.cs

@@ -0,0 +1,42 @@
+// Copyright (c) .NET Foundation and Contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+#nullable enable
+
+using System.Diagnostics;
+
+namespace Nerdbank.GitVersioning;
+
+[DebuggerDisplay("{" + nameof(DebuggerDisplay) + ",nq}")]
+internal class DisabledGitContext : GitContext
+{
+    public DisabledGitContext(string workingTreePath)
+        : base(workingTreePath, null)
+    {
+        this.VersionFile = new DisabledGitVersionFile(this);
+    }
+
+    public override VersionFile VersionFile { get; }
+
+    public override string? GitCommitId => null;
+
+    public override bool IsHead => false;
+
+    public override DateTimeOffset? GitCommitDate => null;
+
+    public override string? HeadCanonicalName => null;
+
+    private string DebuggerDisplay => $"\"{this.WorkingTreePath}\" (disabled-git)";
+
+    public override void ApplyTag(string name) => throw new NotSupportedException();
+
+    public override void Stage(string path) => throw new NotSupportedException();
+
+    public override string GetShortUniqueCommitId(int minLength) => "nerdbankdisabled";
+
+    public override bool TrySelectCommit(string committish) => true;
+
+    internal override int CalculateVersionHeight(VersionOptions? committedVersion, VersionOptions? workingVersion) => 0;
+
+    internal override Version GetIdAsVersion(VersionOptions? committedVersion, VersionOptions? workingVersion, int versionHeight) => Version0;
+}

src/NerdBank.GitVersioning/DisabledGit/DisabledGitVersionFile.cs

@@ -0,0 +1,22 @@
+// Copyright (c) .NET Foundation and Contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+#nullable enable
+
+namespace Nerdbank.GitVersioning;
+
+internal class DisabledGitVersionFile : VersionFile
+{
+    public DisabledGitVersionFile(GitContext context)
+        : base(context)
+    {
+    }
+
+    protected new DisabledGitContext Context => (DisabledGitContext)base.Context;
+
+    protected override VersionOptions? GetVersionCore(out string? actualDirectory)
+    {
+        actualDirectory = null;
+        return null;
+    }
+}

src/NerdBank.GitVersioning/GitContext.cs

@@ -41,6 +41,24 @@ protected GitContext(string workingTreePath, string? dotGitPath)
         this.DotGitPath = dotGitPath;
     }
 
+    public enum Engine
+    {
+        /// <summary>
+        /// Uses the faster and platform independent managed git implementation.
+        /// </summary>
+        ReadOnly,
+
+        /// <summary>
+        /// Uses the LibGit2 engine.
+        /// </summary>
+        ReadWrite,
+
+        /// <summary>
+        /// Uses a stubbed out engine that doesn't compute versions at all.
+        /// </summary>
+        Disabled,
+    }
+
     /// <summary>
     /// Gets the absolute path to the base directory of the git working tree.
     /// </summary>
@@ -110,17 +128,21 @@ public string RepoRelativeProjectDirectory
     /// </summary>
     /// <param name="path">The path to a directory for which version information is required.</param>
     /// <param name="committish">The SHA-1 or ref for a git commit.</param>
-    /// <param name="writable"><see langword="true"/> if mutating the git repository may be required; <see langword="false" /> otherwise.</param>
+    /// <param name="engine">The git engine to use.</param>
     /// <returns>The newly created <see cref="GitContext"/>.</returns>
-    public static GitContext Create(string path, string? committish = null, bool writable = false)
+    public static GitContext Create(string path, string? committish = null, Engine engine = Engine.ReadOnly)
     {
         Requires.NotNull(path, nameof(path));
 
         if (TryFindGitPaths(path, out string? gitDirectory, out string? workingTreeDirectory, out string? workingTreeRelativePath))
         {
-            GitContext result = writable
-                ? new LibGit2.LibGit2Context(workingTreeDirectory, gitDirectory, committish)
-                : new Managed.ManagedGitContext(workingTreeDirectory, gitDirectory, committish);
+            GitContext result = engine switch
+            {
+                Engine.Disabled => new DisabledGitContext(workingTreeDirectory),
+                Engine.ReadWrite => new LibGit2.LibGit2Context(workingTreeDirectory, gitDirectory, committish),
+                Engine.ReadOnly => new Managed.ManagedGitContext(workingTreeDirectory, gitDirectory, committish),
+                _ => throw new ArgumentException("Unrecognized value.", nameof(engine)),
+            };
             result.RepoRelativeProjectDirectory = workingTreeRelativePath;
             return result;
         }

src/NerdBank.GitVersioning/ReleaseManager.cs

@@ -311,7 +311,7 @@ private LibGit2Context GetRepository(string projectDirectory)
     {
         // open git repo and use default configuration (in order to commit we need a configured user name and email
         // which is most likely configured on a user/system level rather than the repo level.
-        var context = GitContext.Create(projectDirectory, writable: true);
+        var context = GitContext.Create(projectDirectory, engine: GitContext.Engine.ReadWrite);
         if (!context.IsRepository)
         {
             this.stderr.WriteLine($"No git repository found above directory '{projectDirectory}'.");

src/Nerdbank.GitVersioning.Tasks/GetBuildVersion.cs

@@ -219,21 +219,24 @@ protected override bool ExecuteInner()
                     Requires.Argument(!containsDotDotSlash, nameof(this.ProjectPathRelativeToGitRepoRoot), "Path must not use ..\\");
                 }
 
-                bool useLibGit2 = false;
+                GitContext.Engine engine = GitContext.Engine.ReadOnly;
                 if (!string.IsNullOrWhiteSpace(this.GitEngine))
                 {
-                    useLibGit2 =
-                        this.GitEngine == "Managed" ? false :
-                        this.GitEngine == "LibGit2" ? true :
-                        throw new ArgumentException("GitEngine property must be set to either \"Managed\" or \"LibGit2\" or left empty.");
+                    engine = this.GitEngine switch
+                    {
+                        "Managed" => GitContext.Engine.ReadOnly,
+                        "LibGit2" => GitContext.Engine.ReadWrite,
+                        "Disabled" => GitContext.Engine.Disabled,
+                        _ => throw new ArgumentException("GitEngine property must be set to either \"Disabled\", \"Managed\" or \"LibGit2\" or left empty."),
+                    };
                 }
 
                 ICloudBuild cloudBuild = CloudBuild.Active;
                 int? overrideBuildNumberOffset = (this.OverrideBuildNumberOffset == int.MaxValue) ? (int?)null : this.OverrideBuildNumberOffset;
                 string projectDirectory = this.ProjectPathRelativeToGitRepoRoot is object && this.GitRepoRoot is object
                     ? Path.Combine(this.GitRepoRoot, this.ProjectPathRelativeToGitRepoRoot)
                     : this.ProjectDirectory;
-                using var context = GitContext.Create(projectDirectory, writable: useLibGit2);
+                using var context = GitContext.Create(projectDirectory, engine: engine);
                 var oracle = new VersionOracle(context, cloudBuild, overrideBuildNumberOffset);
                 if (!string.IsNullOrEmpty(this.DefaultPublicRelease))
                 {

src/Nerdbank.GitVersioning.Tasks/build/MSBuildTargetCaching.targets

@@ -7,6 +7,7 @@
     <NBGV_InnerGlobalProperties Condition=" '$(GitVersionBaseDirectory)' != '' ">$(NBGV_InnerGlobalProperties)GitVersionBaseDirectory=$(GitVersionBaseDirectory);</NBGV_InnerGlobalProperties>
     <NBGV_InnerGlobalProperties Condition=" '$(OverrideBuildNumberOffset)' != '' ">$(NBGV_InnerGlobalProperties)OverrideBuildNumberOffset=$(OverrideBuildNumberOffset);</NBGV_InnerGlobalProperties>
     <NBGV_InnerGlobalProperties Condition=" '$(NBGV_PrivateP2PAuxTargets)' != '' ">$(NBGV_InnerGlobalProperties)NBGV_PrivateP2PAuxTargets=$(NBGV_PrivateP2PAuxTargets);</NBGV_InnerGlobalProperties>
+    <NBGV_InnerGlobalProperties Condition=" '$(NBGV_GitEngine)' != '' ">$(NBGV_InnerGlobalProperties)NBGV_GitEngine=$(NBGV_GitEngine);</NBGV_InnerGlobalProperties>
   </PropertyGroup>
 
   <!-- Compile a list of global properties that may vary when a project builds but that would never influence the result of the GetBuildVersion task. -->

src/nbgv/Program.cs

@@ -315,7 +315,7 @@ private static async Task<int> OnInstallCommand(string path, string version, str
                 return (int)ExitCodes.NoGitRepo;
             }
 
-            using var context = GitContext.Create(searchPath, writable: true);
+            using var context = GitContext.Create(searchPath, engine: GitContext.Engine.ReadWrite);
             if (!context.IsRepository)
             {
                 Console.Error.WriteLine("No git repo found at or above: \"{0}\"", searchPath);
@@ -417,7 +417,7 @@ private static Task<int> OnGetVersionCommand(string project, string[] metadata,
 
             string searchPath = GetSpecifiedOrCurrentDirectoryPath(project);
 
-            using var context = GitContext.Create(searchPath, writable: AlwaysUseLibGit2);
+            using var context = GitContext.Create(searchPath, engine: AlwaysUseLibGit2 ? GitContext.Engine.ReadWrite : GitContext.Engine.ReadOnly);
             if (!context.IsRepository)
             {
                 Console.Error.WriteLine("No git repo found at or above: \"{0}\"", searchPath);
@@ -500,7 +500,7 @@ private static Task<int> OnSetVersionCommand(string project, string version)
             };
 
             string searchPath = GetSpecifiedOrCurrentDirectoryPath(project);
-            using var context = GitContext.Create(searchPath, writable: true);
+            using var context = GitContext.Create(searchPath, engine: GitContext.Engine.ReadWrite);
             VersionOptions existingOptions = context.VersionFile.GetVersion(out string actualDirectory);
             string versionJsonPath;
             if (existingOptions is not null)
@@ -540,7 +540,7 @@ private static Task<int> OnTagCommand(string project, string versionOrRef)
 
             string searchPath = GetSpecifiedOrCurrentDirectoryPath(project);
 
-            using var context = (LibGit2Context)GitContext.Create(searchPath, writable: true);
+            using var context = (LibGit2Context)GitContext.Create(searchPath, engine: GitContext.Engine.ReadWrite);
             if (context is null)
             {
                 Console.Error.WriteLine("No git repo found at or above: \"{0}\"", searchPath);
@@ -639,7 +639,7 @@ private static Task<int> OnGetCommitsCommand(string project, bool quiet, string
 
             string searchPath = GetSpecifiedOrCurrentDirectoryPath(project);
 
-            using var context = (LibGit2Context)GitContext.Create(searchPath, writable: true);
+            using var context = (LibGit2Context)GitContext.Create(searchPath, engine: GitContext.Engine.ReadWrite);
             if (!context.IsRepository)
             {
                 Console.Error.WriteLine("No git repo found at or above: \"{0}\"", searchPath);

test/Nerdbank.GitVersioning.Benchmarks/GetVersionBenchmarks.cs

@@ -28,15 +28,15 @@ public class GetVersionBenchmarks
         [Benchmark(Baseline = true)]
         public void GetVersionLibGit2()
         {
-            using var context = GitContext.Create(GetPath(this.ProjectDirectory), writable: true);
+            using var context = GitContext.Create(GetPath(this.ProjectDirectory), engine: GitContext.Engine.ReadWrite);
             var oracle = new VersionOracle(context, cloudBuild: null);
             this.Version = oracle.Version;
         }
 
         [Benchmark]
         public void GetVersionManaged()
         {
-            using var context = GitContext.Create(GetPath(this.ProjectDirectory), writable: false);
+            using var context = GitContext.Create(GetPath(this.ProjectDirectory), engine: GitContext.Engine.ReadOnly);
             var oracle = new VersionOracle(context, cloudBuild: null);
             this.Version = oracle.Version;
         }

test/Nerdbank.GitVersioning.Tests/BuildIntegrationDisabledTests.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation and Contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Nerdbank.GitVersioning;
+using Xunit;
+using Xunit.Abstractions;
+
+[Trait("Engine", EngineString)]
+[Collection("Build")] // msbuild sets current directory in the process, so we can't have it be concurrent with other build tests.
+public class BuildIntegrationDisabledTests : BuildIntegrationTests
+{
+    private const string EngineString = "Disabled";
+
+    public BuildIntegrationDisabledTests(ITestOutputHelper logger)
+        : base(logger)
+    {
+    }
+
+    protected override GitContext CreateGitContext(string path, string committish = null)
+        => GitContext.Create(path, committish, GitContext.Engine.Disabled);
+
+    protected override void ApplyGlobalProperties(IDictionary<string, string> globalProperties)
+        => globalProperties["NBGV_GitEngine"] = EngineString;
+}

test/Nerdbank.GitVersioning.Tests/BuildIntegrationInProjectManagedTests.cs

@@ -0,0 +1,22 @@
+// Copyright (c) .NET Foundation and Contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Xunit;
+using Xunit.Abstractions;
+
+[Trait("Engine", EngineString)]
+[Collection("Build")] // msbuild sets current directory in the process, so we can't have it be concurrent with other build tests.
+public class BuildIntegrationInProjectManagedTests : BuildIntegrationManagedTests
+{
+    public BuildIntegrationInProjectManagedTests(ITestOutputHelper logger)
+        : base(logger)
+    {
+    }
+
+    /// <inheritdoc/>
+    protected override void ApplyGlobalProperties(IDictionary<string, string> globalProperties)
+    {
+        base.ApplyGlobalProperties(globalProperties);
+        globalProperties["NBGV_CacheMode"] = "None";
+    }
+}

test/Nerdbank.GitVersioning.Tests/BuildIntegrationLibGit2Tests.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation and Contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Nerdbank.GitVersioning;
+using Xunit;
+using Xunit.Abstractions;
+
+[Trait("Engine", EngineString)]
+[Collection("Build")] // msbuild sets current directory in the process, so we can't have it be concurrent with other build tests.
+public class BuildIntegrationLibGit2Tests : SomeGitBuildIntegrationTests
+{
+    private const string EngineString = "LibGit2";
+
+    public BuildIntegrationLibGit2Tests(ITestOutputHelper logger)
+        : base(logger)
+    {
+    }
+
+    protected override GitContext CreateGitContext(string path, string committish = null)
+        => GitContext.Create(path, committish, GitContext.Engine.ReadWrite);
+
+    protected override void ApplyGlobalProperties(IDictionary<string, string> globalProperties)
+        => globalProperties["NBGV_GitEngine"] = EngineString;
+}

test/Nerdbank.GitVersioning.Tests/BuildIntegrationManagedTests.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation and Contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Nerdbank.GitVersioning;
+using Xunit;
+using Xunit.Abstractions;
+
+[Trait("Engine", EngineString)]
+[Collection("Build")] // msbuild sets current directory in the process, so we can't have it be concurrent with other build tests.
+public class BuildIntegrationManagedTests : SomeGitBuildIntegrationTests
+{
+    protected const string EngineString = "Managed";
+
+    public BuildIntegrationManagedTests(ITestOutputHelper logger)
+        : base(logger)
+    {
+    }
+
+    protected override GitContext CreateGitContext(string path, string committish = null)
+        => GitContext.Create(path, committish, GitContext.Engine.ReadOnly);
+
+    protected override void ApplyGlobalProperties(IDictionary<string, string> globalProperties)
+        => globalProperties["NBGV_GitEngine"] = EngineString;
+}