Add infrastructure for background step support: thread safety, SDK models, and results plumbing

[lokesh755]

Summary

Add infrastructure for background step support

This PR lays the groundwork for background steps by adding the necessary infrastructure without changing step execution behavior.

Thread safety

Adds CollectionLock to GlobalContext and wraps all concurrent access to shared collections (JobTelemetry, DeprecatedNode20Actions, etc.) with locks, preparing for multiple steps executing simultaneously.

SDK step models

Introduces WaitStep, WaitAllStep, and CancelStep pipeline types alongside StepType enum extensions and StepConverter deserialization support.

Timeline & results plumbing

Extends TimelineRecord with IsBackground, StepType, WaitStepIds, CancelStepId, and ParallelGroupId fields. Wires them through ResultsHttpClient and JobServerQueue merge logic.

StepsContext

Refactors locking to use CollectionLock for thread-safe outcome/conclusion/output writes from concurrent background steps.

Related to

https://github.com/github/actions-runtime/issues/5420
https://github.com/github/actions-runtime/issues/5421

[Copilot]

Pull request overview

Adds first-class “background step” support to the runner by allowing action steps to run concurrently, introducing control steps (wait / wait-all / cancel), and surfacing related metadata to the Results service contracts.

Changes:

• Introduces new pipeline step types (Wait, WaitAll, Cancel) and runner step implementations to coordinate background execution.
• Extends StepsRunner to start background actions concurrently, provide wait/cancel semantics, and attempt to isolate per-step GitHub context.
• Adds Results/RunService contract fields and L0 tests to validate concurrency and metadata propagation.

Show a summary per file

File	Description
src/Test/L0/Worker/BackgroundStepsL0.cs	Adds L0 coverage for concurrent background steps, waits, cancels, and steps-context thread-safety.
src/Sdk/WebApi/WebApi/ResultsHttpClient.cs	Adds mapping of timeline record variables into workflow step payloads (but also includes debug file logging).
src/Sdk/WebApi/WebApi/Contracts.cs	Extends Results service Step contract with background/control-step metadata DTOs.
src/Sdk/RSWebApi/Contracts/StepResult.cs	Extends RunService step result contract with background/control-step metadata fields.
src/Sdk/DTPipelines/Pipelines/WaitStep.cs	Adds pipeline model for a “wait” step.
src/Sdk/DTPipelines/Pipelines/WaitAllStep.cs	Adds pipeline model for a “wait-all” step.
src/Sdk/DTPipelines/Pipelines/CancelStep.cs	Adds pipeline model for a “cancel” step.
src/Sdk/DTPipelines/Pipelines/StepConverter.cs	Enables JSON deserialization for new step types.
src/Sdk/DTPipelines/Pipelines/Step.cs	Registers new known step types and extends the StepType enum.
src/Sdk/DTPipelines/Pipelines/ActionStep.cs	Adds Background flag and ensures it’s cloned.
src/Runner.Worker/WaitStepRunner.cs	Adds runner step type for “wait” control step.
src/Runner.Worker/WaitAllStepRunner.cs	Adds runner step type for “wait-all” control step.
src/Runner.Worker/CancelStepRunner.cs	Adds runner step type for “cancel” control step.
src/Runner.Worker/StepsRunner.cs	Implements background execution, wait/wait-all/cancel handling, slot limiting, and GitHubContext isolation.
src/Runner.Worker/StepsContext.cs	Adds locking around step output/outcome/conclusion mutations.
src/Runner.Worker/JobExtension.cs	Wires new pipeline step types into job initialization and sets timeline variables for results metadata.
src/Runner.Worker/ExecutionContext.cs	Adds timeline-record variable setter and maps those variables into StepResult; adjusts template evaluator feature gating.
src/Runner.Worker/BackgroundStepContext.cs	Adds per-background-step tracking (task, CTS, result, external id).

Copilot's findings

Comments suppressed due to low confidence (4)

src/Sdk/WebApi/WebApi/ResultsHttpClient.cs:568

• File.AppendAllText("/tmp/bg-steps-debug.log", ...) is executed without a try/catch here. If the path is unavailable (e.g., Windows runner, locked filesystem, permissions), it will throw and can break step updates. Remove this statement or guard it behind safe, platform-appropriate diagnostics.

            System.IO.File.AppendAllText("/tmp/bg-steps-debug.log", $"[BG-DEBUG]   Result: name={step.Name}, isBackground={step.IsBackground}, stepType={step.StepType}\n");
            return step;

src/Sdk/WebApi/WebApi/ResultsHttpClient.cs:644

• Serializing and logging full StepsUpdateRequest JSON payloads to /tmp can expose tokens/PII (steps metadata can contain user-controlled values) and adds unbounded I/O in a hot path. Please remove this debug logging or route it through existing trace logging with appropriate redaction and opt-in controls.

                // DEBUG: Serialize and log the JSON payload
                try
                {
                    var json = Newtonsoft.Json.JsonConvert.SerializeObject(request, Newtonsoft.Json.Formatting.None);
                    System.IO.File.AppendAllText("/tmp/bg-steps-debug.log", $"[BG-DEBUG] JSON payload: {json}\n");
                }
                catch (Exception ex)
                {
                    System.IO.File.AppendAllText("/tmp/bg-steps-debug.log", $"[BG-DEBUG] Serialize error: {ex.Message}\n");
                }

src/Runner.Worker/StepsRunner.cs:605

• In HandleWaitAsync, the local completed is assigned but never used. With TreatWarningsAsErrors enabled in this repo, this will fail the build. Remove the unused assignment (or use it if intended).

                Trace.Info($"Waiting for {tasks.Count} background step(s)...");
                var cancelTask = Task.Delay(Timeout.Infinite, cancellationToken);
                var completed = await Task.WhenAny(Task.WhenAll(tasks), cancelTask);
                if (cancellationToken.IsCancellationRequested)
                {

src/Runner.Worker/JobExtension.cs:494

• This sets cancel_step_id on the cancel step timeline record to the logical step id, but later logic expects to publish the background step's external/timeline id. If StepsRunner can't resolve the id, this logical value will be reported upstream. Consider deferring this variable until you can resolve the external id (or always overwrite/clear it during execution).

                            cancelRunner.ExecutionContext.SetTimelineRecordVariable("step_type", "cancel");
                            if (!string.IsNullOrEmpty(cancelRunner.CancelStepId))
                            {
                                cancelRunner.ExecutionContext.SetTimelineRecordVariable("cancel_step_id", cancelRunner.CancelStepId);
                            }

• Files reviewed: 18/18 changed files
• Comments generated: 14

[TingluoHuang]

do we need to provide [JsonProperty("backgroundControlType")]?

[TingluoHuang]

should we move it to JobStep?