Add async-local override for BuildServerDetector

Introduce an AsyncLocal-backed override for BuildServerDetector.Detected so tests (and other async-scoped code) can set the detected value without leaking to other threads. The static initialization now stores the computed detection in a private 'detected' field, and the Detected property reads from overrideDetected.Value ?? detected and sets overrideDetected.Value. Added unit tests to verify the override persists in the async context and doesn't leak, and updated readme/source docs with a snippet explaining how to override Detected in tests.

Author: SimonCropp

readme.md

@@ -46,6 +46,7 @@ DiffEngine manages launching and cleanup of diff tools. It is designed to be use
   * [Closing a tool](#closing-a-tool)
   * [File type detection](#file-type-detection)
   * [BuildServerDetector](#buildserverdetector)
+    * [Override in tests](#override-in-tests)
   * [AiCliDetector](#aiclidetector)
   * [Disable for a machine/process](#disable-for-a-machineprocess)
   * [Disable in code](#disable-in-code)
@@ -162,6 +163,50 @@ var isAppVeyor = BuildServerDetector.IsAppVeyor;
 <!-- endSnippet -->
 
 
+### Override in tests
+
+`BuildServerDetector.Detected` can be set at test time. The value is stored in an `AsyncLocal`, so it is scoped to the current async context and does not leak to other threads or tests running in parallel.
+
+<!-- snippet: BuildServerDetectorDetectedOverride -->
+<a id='snippet-BuildServerDetectorDetectedOverride'></a>
+```cs
+[Fact]
+public async Task SetDetectedPersistsInAsyncContext()
+{
+    var original = BuildServerDetector.Detected;
+    try
+    {
+        BuildServerDetector.Detected = true;
+        Assert.True(BuildServerDetector.Detected);
+
+        await Task.Delay(1);
+
+        Assert.True(BuildServerDetector.Detected);
+    }
+    finally
+    {
+        BuildServerDetector.Detected = original;
+    }
+}
+
+[Fact]
+public async Task SetDetectedDoesNotLeakToOtherContexts()
+{
+    var parentValue = BuildServerDetector.Detected;
+
+    await Task.Run(() =>
+    {
+        BuildServerDetector.Detected = true;
+        Assert.True(BuildServerDetector.Detected);
+    });
+
+    Assert.Equal(parentValue, BuildServerDetector.Detected);
+}
+```
+<sup><a href='/src/DiffEngine.Tests/BuildServerDetectorTest.cs#L28-L63' title='Snippet source file'>snippet source</a> | <a href='#snippet-BuildServerDetectorDetectedOverride' title='Start of snippet'>anchor</a></sup>
+<!-- endSnippet -->
+
+
 ## AiCliDetector
 
 `AiCliDetector.Detected` returns true if the current code is running in an AI-powered CLI environment.

readme.source.md

@@ -84,6 +84,13 @@ There are also individual properties to check for each specific build system
 snippet: BuildServerDetectorProps
 
 
+### Override in tests
+
+`BuildServerDetector.Detected` can be set at test time. The value is stored in an `AsyncLocal`, so it is scoped to the current async context and does not leak to other threads or tests running in parallel.
+
+snippet: BuildServerDetectorDetectedOverride
+
+
 ## AiCliDetector
 
 `AiCliDetector.Detected` returns true if the current code is running in an AI-powered CLI environment.

src/DiffEngine.Tests/BuildServerDetectorTest.cs

@@ -24,4 +24,41 @@ public void Props()
 
         // ReSharper restore UnusedVariable
     }
+
+    #region BuildServerDetectorDetectedOverride
+
+    [Fact]
+    public async Task SetDetectedPersistsInAsyncContext()
+    {
+        var original = BuildServerDetector.Detected;
+        try
+        {
+            BuildServerDetector.Detected = true;
+            Assert.True(BuildServerDetector.Detected);
+
+            await Task.Delay(1);
+
+            Assert.True(BuildServerDetector.Detected);
+        }
+        finally
+        {
+            BuildServerDetector.Detected = original;
+        }
+    }
+
+    [Fact]
+    public async Task SetDetectedDoesNotLeakToOtherContexts()
+    {
+        var parentValue = BuildServerDetector.Detected;
+
+        await Task.Run(() =>
+        {
+            BuildServerDetector.Detected = true;
+            Assert.True(BuildServerDetector.Detected);
+        });
+
+        Assert.Equal(parentValue, BuildServerDetector.Detected);
+    }
+
+    #endregion
 }

src/DiffEngine/BuildServerDetector.cs

@@ -52,7 +52,7 @@ static BuildServerDetector()
         // https://docs.microsoft.com/en-us/azure/devops/pipelines/process/variables?view=azure-devops&tabs=yaml%2Cbatch#access-variables-through-the-environment
         IsAzureDevops = ValueEquals(variables, "TF_BUILD", "True");
 
-        Detected = IsTravis ||
+        detected = IsTravis ||
                    IsJenkins ||
                    IsGithubAction ||
                    IsAzureDevops ||
@@ -65,6 +65,9 @@ static BuildServerDetector()
                    IsAppVeyor;
     }
 
+    static bool detected;
+    static AsyncLocal<bool?> overrideDetected = new();
+
     static bool ValueEquals(IDictionary variables, string key, string value)
     {
         var variable = variables[key];
@@ -98,5 +101,9 @@ static bool ValueEquals(IDictionary variables, string key, string value)
 
     public static bool IsJenkins { get; }
 
-    public static bool Detected { get; set; }
+    public static bool Detected
+    {
+        get => overrideDetected.Value ?? detected;
+        set => overrideDetected.Value = value;
+    }
 }