test(worker): isolate JobExtensionL0 from host job-hook env vars

.github/workflows/build.yml

@@ -75,7 +75,16 @@ jobs:
       run: |
         ./dev.sh test
       working-directory: src
-      if: matrix.runtime == 'arm64' || matrix.runtime == 'amd64'
+      # Compares against matrix.runs_on (bare arch names: amd64/arm64/ppc64le/s390x), not
+      # matrix.runtime (always 'linux-*'-prefixed) -- the two were previously confused here,
+      # which meant this condition never evaluated true for any matrix leg and L0 silently
+      # never ran in this workflow. linux-x64/linux-arm64 build/run natively on matching
+      # self-hosted hardware with no patches applied, so they're safe to test directly here.
+      # linux-arm (32-bit armv7l, cross-targeted even though it builds on arm64 hardware),
+      # ppc64le, and s390x are intentionally left out: they require the arm/ppc64le/s390x
+      # patches to be applied first and their 32-bit/cross-arch test-tooling behavior under
+      # dotnet test is not yet validated.
+      if: matrix.runtime == 'linux-x64' || matrix.runtime == 'linux-arm64'
 
     # Create runner package tar.gz/zip
     - name: Package Release

src/Test/L0/Worker/JobExtensionL0.cs

@@ -15,8 +15,17 @@
 
 namespace GitHub.Runner.Common.Tests.Worker
 {
-    public sealed class JobExtensionL0
+    public sealed class JobExtensionL0 : IDisposable
     {
+        // JobExtension.InitializeJob reads these environment variables directly to decide whether to inject
+        // extra "job started"/"job completed" hook steps. Some self-hosted runner hosts configure these hooks
+        // for their own lifecycle management, and since they're real process-level environment variables they
+        // would otherwise leak into this test process and silently change the expected step count.
+        private const string JobStartedHookVariable = "ACTIONS_RUNNER_HOOK_JOB_STARTED";
+        private const string JobCompletedHookVariable = "ACTIONS_RUNNER_HOOK_JOB_COMPLETED";
+        private readonly string _originalJobStartedHook;
+        private readonly string _originalJobCompletedHook;
+
         private IExecutionContext _jobEc;
         private Pipelines.AgentJobRequestMessage _message;
 
@@ -33,6 +42,24 @@ public sealed class JobExtensionL0
         private Pipelines.Snapshot _requestedSnapshot;
 
         private CancellationTokenSource _tokenSource;
+
+        public JobExtensionL0()
+        {
+            // xUnit creates a new instance per test, so this runs before every test: capture whatever the host
+            // environment has configured and force a clean (unset) state so tests aren't affected by job hooks
+            // that a self-hosted runner's own environment may have configured for itself.
+            _originalJobStartedHook = Environment.GetEnvironmentVariable(JobStartedHookVariable);
+            _originalJobCompletedHook = Environment.GetEnvironmentVariable(JobCompletedHookVariable);
+            Environment.SetEnvironmentVariable(JobStartedHookVariable, null);
+            Environment.SetEnvironmentVariable(JobCompletedHookVariable, null);
+        }
+
+        public void Dispose()
+        {
+            Environment.SetEnvironmentVariable(JobStartedHookVariable, _originalJobStartedHook);
+            Environment.SetEnvironmentVariable(JobCompletedHookVariable, _originalJobCompletedHook);
+        }
+
         private TestHostContext CreateTestContext([CallerMemberName] String testName = "")
         {
             var hc = new TestHostContext(this, testName);