Background steps execution engine

src/Runner.Worker/BackgroundStepCoordinator.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using GitHub.DistributedTask.WebApi;
+using GitHub.Runner.Common;
+using GitHub.Runner.Common.Util;
+using GitHub.Runner.Sdk;
+using Pipelines = GitHub.DistributedTask.Pipelines;
+
+namespace GitHub.Runner.Worker
+{
+    [ServiceLocator(Default = typeof(BackgroundStepCoordinator))]
+    public interface IBackgroundStepCoordinator : IRunnerService
+    {
+        void InitializeCoordinator(int maxConcurrent);
+        void StartBackgroundStep(IStep step, CancellationToken jobCancellationToken);
+        Task WaitForUnwaitedStepsAsync(IExecutionContext jobContext);
+        Task RunControlFlowAsync(IExecutionContext stepContext, object data);
+    }
+
+    /// <summary>
+    /// Coordinates background step execution, waiting, cancellation, and deferred state.
+    /// Extracted from StepsRunner so the main step loop stays clean.
+    /// </summary>
+    public sealed class BackgroundStepCoordinator : RunnerService, IBackgroundStepCoordinator
+    {
+        private const int DefaultMaxBackgroundSteps = 10;
+        private readonly Dictionary<string, (IStep Step, Task Task, CancellationTokenSource Cts)> _backgroundSteps = new();
+        private readonly HashSet<string> _waitedStepIds = new();
+        private SemaphoreSlim _backgroundSlotSemaphore = new SemaphoreSlim(DefaultMaxBackgroundSteps);
+
+        /// <summary>
+        /// Reset per-job state. Call at the start of each job.
+        /// </summary>
+        public void InitializeCoordinator(int maxConcurrent)
+        {
+            _backgroundSteps.Clear();
+            _waitedStepIds.Clear();
+            var max = maxConcurrent > 0 ? maxConcurrent : DefaultMaxBackgroundSteps;
+            _backgroundSlotSemaphore = new SemaphoreSlim(max);
+        }
+
+        // -----------------------------------------------------------------
+        // Starting background steps
+        // -----------------------------------------------------------------
+
+        /// <summary>
+        /// Prepare and launch a background step. Does not block the caller.
+        /// </summary>
+        public void StartBackgroundStep(IStep step, CancellationToken jobCancellationToken)
+        {
+            var stepId = step.ExecutionContext?.ContextName ?? step.DisplayName;
+
+            // Isolate GitHubContext so concurrent steps don't overwrite each other's GITHUB_OUTPUT paths
+            if (step.ExecutionContext.ExpressionValues.TryGetValue("github", out var ghCtx) && ghCtx is GitHubContext sharedGitHub)
+            {
+                step.ExecutionContext.ExpressionValues["github"] = sharedGitHub.ShallowCopy();
+            }
+
+            var bgCts = CancellationTokenSource.CreateLinkedTokenSource(jobCancellationToken);
+
+            // Evaluate timeout on the main thread (needs expression context)
+            var timeoutMinutes = 0;
+            try
+            {
+                var templateEvaluator = step.ExecutionContext.ToPipelineTemplateEvaluator();
+                timeoutMinutes = templateEvaluator.EvaluateStepTimeout(step.Timeout, step.ExecutionContext.ExpressionValues, step.ExecutionContext.ExpressionFunctions);
+            }
+            catch (Exception ex)
+            {
+                Trace.Info($"Error determining timeout for background step '{stepId}': {ex.Message}");
+            }
+
+            var task = ExecuteBackgroundStepCoreAsync(step, bgCts, stepId, timeoutMinutes);
+            _backgroundSteps[stepId] = (step, task, bgCts);
+            Trace.Info($"Background step '{stepId}' queued (slot will be acquired asynchronously).");
+        }
+
+        private async Task ExecuteBackgroundStepCoreAsync(
+            IStep step, CancellationTokenSource bgCts,
+            string stepId, int timeoutMinutes)
+        {
+            Trace.Info($"Background step '{stepId}' waiting for slot.");
+            await _backgroundSlotSemaphore.WaitAsync(bgCts.Token);
+            Trace.Info($"Background step '{stepId}' acquired slot.");
+
+            step.ExecutionContext.Start();
+
+            if (timeoutMinutes > 0)
+            {
+                step.ExecutionContext.SetTimeout(TimeSpan.FromMinutes(timeoutMinutes));
+            }
+
+            using var cancelReg = bgCts.Token.Register(() =>
+            {
+                Trace.Info($"Background step '{stepId}': cancellation signalled, sending CancelToken to process.");
+                step.ExecutionContext.CancelToken();
+            });
+
+            TaskResult? result = null;
+            try
+            {
+                await step.RunAsync();
+                result = step.ExecutionContext.Result ?? TaskResult.Succeeded;
+            }
+            catch (OperationCanceledException) when (bgCts.Token.IsCancellationRequested)
+            {
+                result = TaskResult.Canceled;
+            }
+            catch (OperationCanceledException) when (step.ExecutionContext.CancellationToken.IsCancellationRequested)
+            {
+                Trace.Info($"Background step '{stepId}' timed out after {timeoutMinutes} minutes.");
+                step.ExecutionContext.Error($"The background step '{step.DisplayName}' has timed out after {timeoutMinutes} minutes.");
+                result = TaskResult.Failed;
+            }
+            catch (Exception ex)
+            {
+                Trace.Info($"Background step '{stepId}' failed: {ex.Message}");
+                step.ExecutionContext.Error(ex);
+                result = TaskResult.Failed;
+            }
+            finally
+            {
+                _backgroundSlotSemaphore.Release();
+
+                if (step.ExecutionContext.CommandResult != null)
+                {
+                    result = TaskResultUtil.MergeTaskResults(result, step.ExecutionContext.CommandResult.Value);
+                }
+
+                step.ExecutionContext.Result = result;
+                step.ExecutionContext.ApplyContinueOnError(step.ContinueOnError);
+
+                step.ExecutionContext.Complete(step.ExecutionContext.Result);
+                Trace.Info($"Background step '{stepId}' completed with result: {step.ExecutionContext.Result}");
+            }
+        }
+
+        // -----------------------------------------------------------------
+        // Wait
+        // -----------------------------------------------------------------
+
+        private async Task<TaskResult> WaitForStepsAsync(string[] stepIds, CancellationToken cancellationToken)
+        {
+            var ids = stepIds ?? Array.Empty<string>();
+            await WaitForStepTasksAsync(ids, cancellationToken);
+            return CompleteWaitedSteps(ids);
+        }
+
+        private async Task<TaskResult> WaitForAllAsync(CancellationToken cancellationToken)
+        {
+            var remaining = _backgroundSteps.Keys.Where(id => !_waitedStepIds.Contains(id)).ToList();
+            await WaitForStepTasksAsync(remaining, cancellationToken);
+            return CompleteWaitedSteps(remaining);
+        }
+
+        // -----------------------------------------------------------------
+        // Cancel
+        // -----------------------------------------------------------------
+
+        private async Task CancelStepsAsync(string[] cancelStepIds)
+        {
+            if (cancelStepIds == null || cancelStepIds.Length == 0) return;
+
+            var idsToCancel = cancelStepIds
+                .Where(id => _backgroundSteps.ContainsKey(id) && !_backgroundSteps[id].Task.IsCompleted)
+                .ToArray();
+
+            if (idsToCancel.Length > 0)
+            {
+                Trace.Info($"Cancelling {idsToCancel.Length} background step(s): {string.Join(", ", idsToCancel)}");
+                await CancelWithGracePeriodAsync(idsToCancel);
+            }
+
+            foreach (var id in cancelStepIds)
+            {
+                FlushDeferredState(id);
+            }
+        }
+
+        // -----------------------------------------------------------------
+        // Safety net
+        // -----------------------------------------------------------------
+
+        public async Task WaitForUnwaitedStepsAsync(IExecutionContext jobContext)
+        {
+            var unwaitedIds = _backgroundSteps.Keys.Where(id => !_waitedStepIds.Contains(id)).ToList();
+            if (unwaitedIds.Count == 0)
+            {
+                return;
+            }
+
+            Trace.Info($"Safety net: {unwaitedIds.Count} unwaited background step(s) at post-job boundary: {string.Join(", ", unwaitedIds)}");
+            await WaitForAllAsync(jobContext.CancellationToken);
+
+            // Propagate any background step failures to the job result
+            foreach (var (_, (step, _, _)) in _backgroundSteps)
+            {
+                if (step.ExecutionContext.Result == TaskResult.Failed)
+                {
+                    Trace.Info($"Propagating failure from background step '{step.ExecutionContext.ContextName}' to job result.");
+                    jobContext.Result = TaskResultUtil.MergeTaskResults(jobContext.Result, TaskResult.Failed);
+                    jobContext.JobContext.Status = jobContext.Result?.ToActionResult();
+                    break;
+                }
+            }
+        }
+
+        // -----------------------------------------------------------------
+        // Control-flow step dispatch
+        // -----------------------------------------------------------------
+
+        /// <summary>
+        /// Execute a control-flow step (wait, wait-all, cancel) and propagate results.
+        /// </summary>
+        public async Task RunControlFlowAsync(IExecutionContext stepContext, object data)
+        {
+            var controlFlow = data as ControlFlowStepData;
+            switch (controlFlow.Type)
+            {
+                case Pipelines.BackgroundControlTypes.Wait:
+                    stepContext.Result = await WaitForStepsAsync(controlFlow.StepIds, stepContext.CancellationToken);
+                    break;
+
+                case Pipelines.BackgroundControlTypes.WaitAll:
+                    stepContext.Result = await WaitForAllAsync(stepContext.CancellationToken);
+                    break;
+
+                case Pipelines.BackgroundControlTypes.Cancel:
+                    await CancelStepsAsync(controlFlow.StepIds);
+                    stepContext.Result = TaskResult.Succeeded;
+                    break;
+
+                default:
+                    throw new ArgumentException($"Unknown background step control type '{controlFlow.Type}'.");
+            }
+        }
+
+        // -----------------------------------------------------------------
+        // Private helpers
+        // -----------------------------------------------------------------
+
+        private async Task WaitForStepTasksAsync(IEnumerable<string> stepIds, CancellationToken cancellationToken)
+        {
+            var ids = stepIds.ToList();
+            var tasks = new List<Task>();
+
+            foreach (var stepId in ids)
+            {
+                if (_backgroundSteps.TryGetValue(stepId, out var entry) && !entry.Task.IsCompleted)
+                {
+                    tasks.Add(entry.Task);
+                }
+                else if (!_backgroundSteps.ContainsKey(stepId))
+                {
+                    Trace.Info($"Wait references unknown background step: {stepId}");
+                }
+            }
+
+            if (tasks.Count > 0)
+            {
+                Trace.Info($"Waiting for {tasks.Count} background step(s)...");
+                try
+                {
+                    await Task.WhenAll(tasks).WaitAsync(cancellationToken);
+                }
+                catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+                {
+                    Trace.Info("Wait interrupted by job cancellation — cancelling background steps.");
+                    await CancelWithGracePeriodAsync(ids);
+                }
+            }
+        }
+
+        private async Task CancelWithGracePeriodAsync(IEnumerable<string> stepIds, double graceSeconds = 7.5)
+        {
+            var tasks = new List<Task>();
+            foreach (var stepId in stepIds)
+            {
+                if (_backgroundSteps.TryGetValue(stepId, out var entry) && !entry.Task.IsCompleted)
+                {
+                    entry.Step.ExecutionContext.CancelToken();
+                    entry.Cts.Cancel();
+                    tasks.Add(entry.Task);
+                }
+            }
+
+            if (tasks.Count > 0)
+            {
+                try
+                {
+                    await Task.WhenAll(tasks).WaitAsync(TimeSpan.FromSeconds(graceSeconds));
+                }
+                catch (TimeoutException)
+                {
+                    Trace.Info($"Some background steps did not terminate within {graceSeconds}s grace period.");
+                }
+            }
+        }
+
+        private void FlushDeferredState(string stepId)
+        {
+            if (_backgroundSteps.TryGetValue(stepId, out var entry))
+            {
+                entry.Step.ExecutionContext.FlushDeferredOutputs();
+                entry.Step.ExecutionContext.FlushDeferredEnvironment();
+                entry.Step.ExecutionContext.FlushDeferredOutcomeConclusion();
+                Trace.Info($"Flushed deferred state for background step '{stepId}'.");
+            }
+        }
+
+        private TaskResult CompleteWaitedSteps(IEnumerable<string> stepIds)
+        {
+            var result = TaskResult.Succeeded;
+            foreach (var id in stepIds)
+            {
+                FlushDeferredState(id);
+                _waitedStepIds.Add(id);
+                if (_backgroundSteps.TryGetValue(id, out var entry) && entry.Step.ExecutionContext.Result == TaskResult.Failed)
+                {
+                    result = TaskResult.Failed;
+                }
+            }
+            return result;
+        }
+    }
+}

src/Runner.Worker/ControlFlowStepData.cs

@@ -0,0 +1,21 @@
+using System;
+
+namespace GitHub.Runner.Worker
+{
+    /// <summary>
+    /// Pure data for control-flow steps (wait, wait-all, cancel).
+    /// Type uses Pipelines.BackgroundControlTypes string constants.
+    /// </summary>
+    public sealed class ControlFlowStepData
+    {
+        public string Type { get; set; }
+        public Guid StepId { get; set; }
+        public string StepName { get; set; }
+
+        // Target step IDs (for wait: steps to wait for; for cancel: steps to cancel)
+        public string[] StepIds { get; set; }
+
+        // Parallel group ID for grouping steps in the UI
+        public string ParallelGroupId { get; set; }
+    }
+}