Add NBGV_GitEngine=Disabled option

src/NerdBank.GitVersioning.Tests/BuildIntegrationTests.cs

@@ -55,6 +55,22 @@ protected override void ApplyGlobalProperties(IDictionary<string, string> global
         => globalProperties["NBGV_GitEngine"] = "LibGit2";
 }
 
+[Trait("Engine", "Disabled")]
+[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
+{
+    public BuildIntegrationDisabledTests(ITestOutputHelper logger)
+        : base(logger)
+    {
+    }
+
+    protected override GitContext CreateGitContext(string path, string committish = null)
+        => GitContext.Create(path, committish, disabled: true);
+
+    protected override void ApplyGlobalProperties(IDictionary<string, string> globalProperties)
+        => globalProperties["NBGV_GitEngine"] = "Disabled";
+}
+
 public abstract class BuildIntegrationTests : RepoTestBase, IClassFixture<MSBuildFixture>
 {
     private const string GitVersioningTargetsFileName = "NerdBank.GitVersioning.targets";

src/NerdBank.GitVersioning/DisabledGit/DisabledGitContext.cs

@@ -0,0 +1,36 @@
+#nullable enable
+
+using System;
+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}\" (no-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 +1,20 @@
+namespace Nerdbank.GitVersioning
+{
+    using Validation;
+
+    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

@@ -119,12 +119,13 @@ public void Dispose()
         /// <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="disabled"><see langword="true"/> if no git operations should be disabled (implies <paramref name="writable"/> false); <see langword="false" /> otherwise.</param>
         /// <returns></returns>
-        public static GitContext Create(string path, string? committish = null, bool writable = false)
+        public static GitContext Create(string path, string? committish = null, bool writable = false, bool disabled = false)
         {
             if (TryFindGitPaths(path, out string? gitDirectory, out string? workingTreeDirectory, out string? workingTreeRelativePath))
             {
-                GitContext result = writable
+                GitContext result = disabled ? new DisabledGitContext(workingTreeDirectory) : writable
                     ? (GitContext)new LibGit2.LibGit2Context(workingTreeDirectory, gitDirectory, committish)
                     : new Managed.ManagedGitContext(workingTreeDirectory, gitDirectory, committish);
                 result.RepoRelativeProjectDirectory = workingTreeRelativePath;

src/Nerdbank.GitVersioning.Tasks/GetBuildVersion.cs

@@ -217,20 +217,30 @@ protected override bool ExecuteInner()
                 }
 
                 bool useLibGit2 = false;
+                bool disabled = false;
                 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.");
+                    switch (this.GitEngine)
+                    {
+                        case "Managed":
+                            break;
+                        case "LibGit2":
+                            useLibGit2 = true;
+                            break;
+                        case "Disabled":
+                            disabled = true;
+                            break;
+                        default:
+                            throw new ArgumentException("GitEngine property must be set to either \"Disabled\", \"Managed\" or \"LibGit2\" or left empty.");
+                    }
                 }
 
                 var cloudBuild = CloudBuild.Active;
                 var 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, writable: useLibGit2, disabled: disabled);
                 var oracle = new VersionOracle(context, cloudBuild, overrideBuildNumberOffset);
                 if (!string.IsNullOrEmpty(this.DefaultPublicRelease))
                 {

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

@@ -14,6 +14,9 @@
           Returns="@(CloudBuildVersionVars)" />
 
   <Target Name="GetBuildVersionCore">
+    <PropertyGroup Condition=" '$(NBGV_Disabled)' == 'true' ">
+      <NBGV_GitEngine>Disabled</NBGV_GitEngine>
+    </PropertyGroup>
     <Nerdbank.GitVersioning.Tasks.GetBuildVersion
       BuildingRef="$(_NBGV_BuildingRef)"
       BuildMetadata="$(BuildMetadata.Replace(',',';'))"

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

@@ -75,6 +75,7 @@
         ProjectPathRelativeToGitRepoRoot=$(ProjectPathRelativeToGitRepoRoot);
         ProjectDirectory=$(GitVersionBaseDirectory);
         OverrideBuildNumberOffset=$(OverrideBuildNumberOffset);
+        NGBV_Disabled=$(NBGV_Disabled);
       </NBGV_InnerGlobalProperties>
       <NBGV_CoreTargets>$(MSBuildThisFileDirectory)Nerdbank.GitVersioning.Inner.targets</NBGV_CoreTargets>
     </PropertyGroup>