Add helpful error message for null steps

src/Runner.Worker/ActionManager.cs

@@ -395,6 +395,12 @@ namespace GitHub.Runner.Worker
                             Trace.Info($"Action cleanup plugin: {plugin.PluginTypeName}.");
                         }
                     }
+                    else if (definition.Data.Execution.ExecutionType == ActionExecutionType.Composite && !string.IsNullOrEmpty(Environment.GetEnvironmentVariable("TESTING_COMPOSITE_ACTIONS_ALPHA")))
+                    {
+                        var compositeAction = definition.Data.Execution as CompositeActionExecutionData;
+                        Trace.Info($"Load {compositeAction.Steps.Count} action steps.");
+                        Trace.Verbose($"Details: {StringUtil.ConvertToJson(compositeAction.Steps)}");
+                    }
                     else
                     {
                         throw new NotSupportedException(definition.Data.Execution.ExecutionType.ToString());
@@ -1101,6 +1107,11 @@ namespace GitHub.Runner.Worker
                     Trace.Info($"Action plugin: {(actionDefinitionData.Execution as PluginActionExecutionData).Plugin}, no more preparation.");
                     return null;
                 }
+                else if (actionDefinitionData.Execution.ExecutionType == ActionExecutionType.Composite && !string.IsNullOrEmpty(Environment.GetEnvironmentVariable("TESTING_COMPOSITE_ACTIONS_ALPHA")))
+                {
+                    Trace.Info($"Action composite: {(actionDefinitionData.Execution as CompositeActionExecutionData).Steps}, no more preparation.");
+                    return null;
+                }
                 else
                 {
                     throw new NotSupportedException(actionDefinitionData.Execution.ExecutionType.ToString());
@@ -1211,6 +1222,7 @@ namespace GitHub.Runner.Worker
         NodeJS,
         Plugin,
         Script,
+        Composite,
     }
 
     public sealed class ContainerActionExecutionData : ActionExecutionData
@@ -1267,6 +1279,14 @@ namespace GitHub.Runner.Worker
         public override bool HasPost => false;
     }
 
+    public sealed class CompositeActionExecutionData : ActionExecutionData
+    {
+        public override ActionExecutionType ExecutionType => ActionExecutionType.Composite;
+        public override bool HasPre => false;
+        public override bool HasPost => false;
+        public List<Pipelines.ActionStep> Steps { get; set; }
+    }
+
     public abstract class ActionExecutionData
     {
         private string _initCondition = $"{Constants.Expressions.Always}()";

src/Runner.Worker/ActionManifestManager.cs

@@ -14,6 +14,7 @@ using YamlDotNet.Core;
 using YamlDotNet.Core.Events;
 using System.Globalization;
 using System.Linq;
+using Pipelines = GitHub.DistributedTask.Pipelines;
 
 namespace GitHub.Runner.Worker
 {
@@ -92,7 +93,7 @@ namespace GitHub.Runner.Worker
                             break;
 
                         case "runs":
-                            actionDefinition.Execution = ConvertRuns(context, actionPair.Value);
+                            actionDefinition.Execution = ConvertRuns(executionContext, context, actionPair.Value);
                             break;
                         default:
                             Trace.Info($"Ignore action property {propertyName}.");
@@ -294,6 +295,7 @@ namespace GitHub.Runner.Worker
         }
 
         private ActionExecutionData ConvertRuns(
+            IExecutionContext executionContext,
             TemplateContext context,
             TemplateToken inputsToken)
         {
@@ -311,6 +313,8 @@ namespace GitHub.Runner.Worker
             var postToken = default(StringToken);
             var postEntrypointToken = default(StringToken);
             var postIfToken = default(StringToken);
+            var stepsLoaded = default(List<Pipelines.ActionStep>);
+
             foreach (var run in runsMapping)
             {
                 var runsKey = run.Key.AssertString("runs key").Value;
@@ -355,6 +359,15 @@ namespace GitHub.Runner.Worker
                     case "pre-if":
                         preIfToken = run.Value.AssertString("pre-if");
                         break;
+                    case "steps":
+                        if (!string.IsNullOrEmpty(Environment.GetEnvironmentVariable("TESTING_COMPOSITE_ACTIONS_ALPHA")))
+                        {
+                            var steps = run.Value.AssertSequence("steps");
+                            var evaluator = executionContext.ToPipelineTemplateEvaluator();
+                            stepsLoaded = evaluator.LoadCompositeSteps(steps);
+                            break;
+                        }
+                        throw new Exception("You aren't supposed to be using Composite Actions yet!");
                     default:
                         Trace.Info($"Ignore run property {runsKey}.");
                         break;
@@ -402,6 +415,21 @@ namespace GitHub.Runner.Worker
                         };
                     }
                 }
+                else if (string.Equals(usingToken.Value, "composite", StringComparison.OrdinalIgnoreCase) && !string.IsNullOrEmpty(Environment.GetEnvironmentVariable("TESTING_COMPOSITE_ACTIONS_ALPHA")))
+                {
+                    if (stepsLoaded == null)
+                    {
+                        // TODO: Add a more helpful error message + including file name, etc. to show user that it's because of their yaml file
+                        throw new ArgumentNullException($"No steps provided.");
+                    }
+                    else
+                    {
+                        return new CompositeActionExecutionData()
+                        {
+                            Steps = stepsLoaded,
+                        };
+                    }
+                }
                 else
                 {
                     throw new ArgumentOutOfRangeException($"'using: {usingToken.Value}' is not supported, use 'docker' or 'node12' instead.");

src/Runner.Worker/ExecutionContext.cs

@@ -63,7 +63,7 @@ namespace GitHub.Runner.Worker
         JobContext JobContext { get; }
 
         // Only job level ExecutionContext has JobSteps
-        Queue<IStep> JobSteps { get; }
+        List<IStep> JobSteps { get; }
 
         // Only job level ExecutionContext has PostJobSteps
         Stack<IStep> PostJobSteps { get; }
@@ -105,6 +105,7 @@ namespace GitHub.Runner.Worker
         // others
         void ForceTaskComplete();
         void RegisterPostJobStep(IStep step);
+        void RegisterNestedStep(IStep step, DictionaryContextData inputsData, int location);
     }
 
     public sealed class ExecutionContext : RunnerService, IExecutionContext
@@ -159,7 +160,7 @@ namespace GitHub.Runner.Worker
         public List<ContainerInfo> ServiceContainers { get; private set; }
 
         // Only job level ExecutionContext has JobSteps
-        public Queue<IStep> JobSteps { get; private set; }
+        public List<IStep> JobSteps { get; private set; }
 
         // Only job level ExecutionContext has PostJobSteps
         public Stack<IStep> PostJobSteps { get; private set; }
@@ -169,7 +170,6 @@ namespace GitHub.Runner.Worker
 
         public bool EchoOnActionCommand { get; set; }
 
-
         public TaskResult? Result
         {
             get
@@ -266,6 +266,20 @@ namespace GitHub.Runner.Worker
             Root.PostJobSteps.Push(step);
         }
 
+        /// <summary>
+        /// Helper function used in CompositeActionHandler::RunAsync to
+        /// add a child node, aka a step, to the current job to the Root.JobSteps based on the location. 
+        /// </summary>
+        public void RegisterNestedStep(IStep step, DictionaryContextData inputsData, int location)
+        {
+            // TODO: For UI purposes, look at figuring out how to condense steps in one node => maybe use the same previous GUID
+            var newGuid = Guid.NewGuid();
+            step.ExecutionContext = Root.CreateChild(newGuid, step.DisplayName, newGuid.ToString("N"), null, null);
+            step.ExecutionContext.ExpressionValues["inputs"] = inputsData;
+            // TODO: confirm whether not copying message contexts is safe
+            Root.JobSteps.Insert(location, step);
+        }
+
         public IExecutionContext CreateChild(Guid recordId, string displayName, string refName, string scopeName, string contextName, Dictionary<string, string> intraActionState = null, int? recordOrder = null)
         {
             Trace.Entering();
@@ -660,7 +674,7 @@ namespace GitHub.Runner.Worker
             PrependPath = new List<string>();
 
             // JobSteps for job ExecutionContext
-            JobSteps = new Queue<IStep>();
+            JobSteps = new List<IStep>();
 
             // PostJobSteps for job ExecutionContext
             PostJobSteps = new Stack<IStep>();

src/Runner.Worker/Handlers/CompositeActionHandler.cs

@@ -0,0 +1,98 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using GitHub.DistributedTask.ObjectTemplating.Tokens;
+using GitHub.DistributedTask.Pipelines.ContextData;
+using GitHub.DistributedTask.WebApi;
+using GitHub.Runner.Common;
+using GitHub.Runner.Sdk;
+using Pipelines = GitHub.DistributedTask.Pipelines;
+
+
+namespace GitHub.Runner.Worker.Handlers
+{
+    [ServiceLocator(Default = typeof(CompositeActionHandler))]
+    public interface ICompositeActionHandler : IHandler
+    {
+        CompositeActionExecutionData Data { get; set; }
+    }
+    public sealed class CompositeActionHandler : Handler, ICompositeActionHandler
+    {
+        public CompositeActionExecutionData Data { get; set; }
+
+        public Task RunAsync(ActionRunStage stage)
+        {
+            // Validate args.
+            Trace.Entering();
+            ArgUtil.NotNull(ExecutionContext, nameof(ExecutionContext));
+            ArgUtil.NotNull(Inputs, nameof(Inputs));
+
+            var githubContext = ExecutionContext.ExpressionValues["github"] as GitHubContext;
+            ArgUtil.NotNull(githubContext, nameof(githubContext));
+
+            var tempDirectory = HostContext.GetDirectory(WellKnownDirectory.Temp);
+
+            // Resolve action steps
+            var actionSteps = Data.Steps;
+
+            // Create Context Data to reuse for each composite action step
+            var inputsData = new DictionaryContextData();
+            foreach (var i in Inputs)
+            {
+                inputsData[i.Key] = new StringContextData(i.Value);
+            }
+
+            // Add each composite action step to the front of the queue
+            int location = 0;
+            foreach (Pipelines.ActionStep aStep in actionSteps)
+            {
+                // Ex: 
+                // runs:
+                //      using: "composite"
+                //      steps:
+                //          - uses: example/test-composite@v2 (a)
+                //          - run echo hello world (b)
+                //          - run echo hello world 2 (c)
+                // 
+                // ethanchewy/test-composite/action.yaml
+                // runs:
+                //      using: "composite"
+                //      steps: 
+                //          - run echo hello world 3 (d)
+                //          - run echo hello world 4 (e)
+                // 
+                // Steps processed as follow:
+                // | a |
+                // | a | => | d |
+                // (Run step d)
+                // | a | 
+                // | a | => | e |
+                // (Run step e)
+                // | a | 
+                // (Run step a)
+                // | b | 
+                // (Run step b)
+                // | c |
+                // (Run step c)
+                // Done.
+
+                var actionRunner = HostContext.CreateService<IActionRunner>();
+                actionRunner.Action = aStep;
+                actionRunner.Stage = stage;
+                actionRunner.Condition = aStep.Condition;
+                actionRunner.DisplayName = aStep.DisplayName;
+                // TODO: Do we need to add any context data from the job message?
+                // (See JobExtension.cs ~line 236)
+
+                ExecutionContext.RegisterNestedStep(actionRunner, inputsData, location);
+                location++;
+            }
+
+            return Task.CompletedTask;
+        }
+
+    }
+}

src/Runner.Worker/Handlers/HandlerFactory.cs

@@ -66,6 +66,11 @@ namespace GitHub.Runner.Worker.Handlers
                 handler = HostContext.CreateService<IRunnerPluginHandler>();
                 (handler as IRunnerPluginHandler).Data = data as PluginActionExecutionData;
             }
+            else if (data.ExecutionType == ActionExecutionType.Composite)
+            {
+                handler = HostContext.CreateService<ICompositeActionHandler>();
+                (handler as ICompositeActionHandler).Data = data as CompositeActionExecutionData;
+            }
             else
             {
                 // This should never happen.

src/Runner.Worker/JobRunner.cs

@@ -152,7 +152,7 @@ namespace GitHub.Runner.Worker
                 {
                     foreach (var step in jobSteps)
                     {
-                        jobContext.JobSteps.Enqueue(step);
+                        jobContext.JobSteps.Add(step);
                     }
 
                     await stepsRunner.RunAsync(jobContext);

src/Runner.Worker/StepsRunner.cs

@@ -59,14 +59,15 @@ namespace GitHub.Runner.Worker
                     checkPostJobActions = true;
                     while (jobContext.PostJobSteps.TryPop(out var postStep))
                     {
-                        jobContext.JobSteps.Enqueue(postStep);
+                        jobContext.JobSteps.Add(postStep);
                     }
 
                     continue;
                 }
 
-                var step = jobContext.JobSteps.Dequeue();
-                var nextStep = jobContext.JobSteps.Count > 0 ? jobContext.JobSteps.Peek() : null;
+                var step = jobContext.JobSteps[0];
+                jobContext.JobSteps.RemoveAt(0);
+                var nextStep = jobContext.JobSteps.Count > 0 ? jobContext.JobSteps[0] : null;
 
                 Trace.Info($"Processing step: DisplayName='{step.DisplayName}'");
                 ArgUtil.NotNull(step.ExecutionContext, nameof(step.ExecutionContext));
@@ -409,7 +410,11 @@ namespace GitHub.Runner.Worker
                     scope = scopesToInitialize.Pop();
                     executionContext.Debug($"Initializing scope '{scope.Name}'");
                     executionContext.ExpressionValues["steps"] = stepsContext.GetScope(scope.ParentName);
+                    // TODO: Fix this temporary workaround for Composite Actions
+                    if (!executionContext.ExpressionValues.ContainsKey("inputs") && !string.IsNullOrEmpty(Environment.GetEnvironmentVariable("TESTING_COMPOSITE_ACTIONS_ALPHA")))
+                    {
                         executionContext.ExpressionValues["inputs"] = !String.IsNullOrEmpty(scope.ParentName) ? scopeInputs[scope.ParentName] : null;
+                    }
                     var templateEvaluator = executionContext.ToPipelineTemplateEvaluator();
                     var inputs = default(DictionaryContextData);
                     try
@@ -432,7 +437,11 @@ namespace GitHub.Runner.Worker
             // Setup expression values
             var scopeName = executionContext.ScopeName;
             executionContext.ExpressionValues["steps"] = stepsContext.GetScope(scopeName);
+            // TODO: Fix this temporary workaround for Composite Actions
+            if (!executionContext.ExpressionValues.ContainsKey("inputs") && !string.IsNullOrEmpty(Environment.GetEnvironmentVariable("TESTING_COMPOSITE_ACTIONS_ALPHA")))
+            {
                 executionContext.ExpressionValues["inputs"] = string.IsNullOrEmpty(scopeName) ? null : scopeInputs[scopeName];
+            }
 
             return true;
         }

src/Runner.Worker/action_yaml.json

@@ -32,7 +32,8 @@
             "one-of": [
                 "container-runs",
                 "node12-runs",
-                "plugin-runs"
+                "plugin-runs",
+                "composite-runs"
             ]
         },
         "container-runs": {
@@ -83,6 +84,36 @@
                 }
             }
         },
+        "composite-runs": {
+            "mapping": {
+                "properties": {
+                    "using": "non-empty-string",
+                    "steps": "composite-steps"
+                }
+            }
+        },
+        "composite-steps": {
+            "context": [
+                "github",
+                "needs",
+                "strategy",
+                "matrix",
+                "secrets",
+                "steps",
+                "inputs",
+                "job",
+                "runner",
+                "env",
+                "always(0,0)",
+                "failure(0,0)",
+                "cancelled(0,0)",
+                "success(0,0)",
+                "hashFiles(1,255)"
+            ],
+            "sequence": {
+                "item-type": "any"
+            }
+        },
         "container-runs-context": {
             "context": [
                 "inputs"

src/Sdk/DTPipelines/Pipelines/ObjectTemplating/PipelineTemplateConstants.cs

@@ -65,6 +65,7 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
         public const String StepEnv = "step-env";
         public const String StepIfResult = "step-if-result";
         public const String Steps = "steps";
+        public const String StepsInTemplate = "steps-in-template";
         public const String StepsScopeInputs = "steps-scope-inputs";
         public const String StepsScopeOutputs = "steps-scope-outputs";
         public const String StepsTemplateRoot = "steps-template-root";

src/Sdk/DTPipelines/Pipelines/ObjectTemplating/PipelineTemplateConverter.cs

@@ -29,7 +29,6 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
             var evaluationResult = EvaluationResult.CreateIntermediateResult(null, ifResult);
             return evaluationResult.IsTruthy;
         }
-
         internal static Boolean? ConvertToStepContinueOnError(
             TemplateContext context,
             TemplateToken token,
@@ -264,5 +263,351 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
 
             return result;
         }
+
+        //Note: originally was List<Step> but we need to change to List<ActionStep> to use the "Inputs" attribute
+        internal static List<ActionStep> ConvertToSteps(
+            TemplateContext context,
+            TemplateToken steps)
+        {
+            var stepsSequence = steps.AssertSequence($"job {PipelineTemplateConstants.Steps}");
+
+            var result = new List<ActionStep>();
+            foreach (var stepsItem in stepsSequence)
+            {
+                var step = ConvertToStep(context, stepsItem);
+                if (step != null) // step = null means we are hitting error during step conversion, there should be an error in context.errors
+                {
+                    if (step.Enabled)
+                    {
+                        result.Add(step);
+                    }
+                }
+            }
+
+            return result;
+        }
+
+        private static ActionStep ConvertToStep(
+            TemplateContext context,
+            TemplateToken stepsItem)
+        {
+            var step = stepsItem.AssertMapping($"{PipelineTemplateConstants.Steps} item");
+            var continueOnError = default(ScalarToken);
+            var env = default(TemplateToken);
+            var id = default(StringToken);
+            var ifCondition = default(String);
+            var ifToken = default(ScalarToken);
+            var name = default(ScalarToken);
+            var run = default(ScalarToken);
+            var scope = default(StringToken);
+            var timeoutMinutes = default(ScalarToken);
+            var uses = default(StringToken);
+            var with = default(TemplateToken);
+            var workingDir = default(ScalarToken);
+            var path = default(ScalarToken);
+            var clean = default(ScalarToken);
+            var fetchDepth = default(ScalarToken);
+            var lfs = default(ScalarToken);
+            var submodules = default(ScalarToken);
+            var shell = default(ScalarToken);
+
+            foreach (var stepProperty in step)
+            {
+                var propertyName = stepProperty.Key.AssertString($"{PipelineTemplateConstants.Steps} item key");
+
+                switch (propertyName.Value)
+                {
+                    case PipelineTemplateConstants.Clean:
+                        clean = stepProperty.Value.AssertScalar($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.Clean}");
+                        break;
+
+                    case PipelineTemplateConstants.ContinueOnError:
+                        ConvertToStepContinueOnError(context, stepProperty.Value, allowExpressions: true); // Validate early if possible
+                        continueOnError = stepProperty.Value.AssertScalar($"{PipelineTemplateConstants.Steps} {PipelineTemplateConstants.ContinueOnError}");
+                        break;
+
+                    case PipelineTemplateConstants.Env:
+                        ConvertToStepEnvironment(context, stepProperty.Value, StringComparer.Ordinal, allowExpressions: true); // Validate early if possible
+                        env = stepProperty.Value;
+                        break;
+
+                    case PipelineTemplateConstants.FetchDepth:
+                        fetchDepth = stepProperty.Value.AssertScalar($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.FetchDepth}");
+                        break;
+
+                    case PipelineTemplateConstants.Id:
+                        id = stepProperty.Value.AssertString($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.Id}");
+                        if (!NameValidation.IsValid(id.Value, true))
+                        {
+                            context.Error(id, $"Step id {id.Value} is invalid. Ids must start with a letter or '_' and contain only alphanumeric characters, '-', or '_'");
+                        }
+                        break;
+
+                    case PipelineTemplateConstants.If:
+                        ifToken = stepProperty.Value.AssertScalar($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.If}");
+                        break;
+
+                    case PipelineTemplateConstants.Lfs:
+                        lfs = stepProperty.Value.AssertScalar($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.Lfs}");
+                        break;
+
+                    case PipelineTemplateConstants.Name:
+                        name = stepProperty.Value.AssertScalar($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.Name}");
+                        break;
+
+                    case PipelineTemplateConstants.Path:
+                        path = stepProperty.Value.AssertScalar($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.Path}");
+                        break;
+
+                    case PipelineTemplateConstants.Run:
+                        run = stepProperty.Value.AssertScalar($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.Run}");
+                        break;
+
+                    case PipelineTemplateConstants.Shell:
+                        shell = stepProperty.Value.AssertScalar($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.Shell}");
+                        break;
+
+                    case PipelineTemplateConstants.Scope:
+                        scope = stepProperty.Value.AssertString($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.Scope}");
+                        break;
+
+                    case PipelineTemplateConstants.Submodules:
+                        submodules = stepProperty.Value.AssertScalar($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.Submodules}");
+                        break;
+
+                    case PipelineTemplateConstants.TimeoutMinutes:
+                        ConvertToStepTimeout(context, stepProperty.Value, allowExpressions: true); // Validate early if possible
+                        timeoutMinutes = stepProperty.Value.AssertScalar($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.TimeoutMinutes}");
+                        break;
+
+                    case PipelineTemplateConstants.Uses:
+                        uses = stepProperty.Value.AssertString($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.Uses}");
+                        break;
+
+                    case PipelineTemplateConstants.With:
+                        ConvertToStepInputs(context, stepProperty.Value, allowExpressions: true); // Validate early if possible
+                        with = stepProperty.Value;
+                        break;
+
+                    case PipelineTemplateConstants.WorkingDirectory:
+                        workingDir = stepProperty.Value.AssertScalar($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.WorkingDirectory}");
+                        break;
+
+                    default:
+                        propertyName.AssertUnexpectedValue($"{PipelineTemplateConstants.Steps} item key"); // throws
+                        break;
+                }
+            }
+
+            // Fixup the if-condition
+            var isDefaultScope = String.IsNullOrEmpty(scope?.Value);
+            ifCondition = ConvertToIfCondition(context, ifToken, false, isDefaultScope);
+
+            if (run != null)
+            {
+                var result = new ActionStep
+                {
+                    ScopeName = scope?.Value,
+                    ContextName = id?.Value,
+                    ContinueOnError = continueOnError,
+                    DisplayNameToken = name,
+                    Condition = ifCondition,
+                    TimeoutInMinutes = timeoutMinutes,
+                    Environment = env,
+                    Reference = new ScriptReference(),
+                };
+
+                var inputs = new MappingToken(null, null, null);
+                inputs.Add(new StringToken(null, null, null, PipelineConstants.ScriptStepInputs.Script), run);
+
+                if (workingDir != null)
+                {
+                    inputs.Add(new StringToken(null, null, null, PipelineConstants.ScriptStepInputs.WorkingDirectory), workingDir);
+                }
+
+                if (shell != null)
+                {
+                    inputs.Add(new StringToken(null, null, null, PipelineConstants.ScriptStepInputs.Shell), shell);
+                }
+
+                result.Inputs = inputs;
+
+                return result;
+            }
+            else
+            {
+                uses.AssertString($"{PipelineTemplateConstants.Steps} item {PipelineTemplateConstants.Uses}");
+                var result = new ActionStep
+                {
+                    ScopeName = scope?.Value,
+                    ContextName = id?.Value,
+                    ContinueOnError = continueOnError,
+                    DisplayNameToken = name,
+                    Condition = ifCondition,
+                    TimeoutInMinutes = timeoutMinutes,
+                    Inputs = with,
+                    Environment = env,
+                };
+
+                if (uses.Value.StartsWith("docker://", StringComparison.Ordinal))
+                {
+                    var image = uses.Value.Substring("docker://".Length);
+                    result.Reference = new ContainerRegistryReference { Image = image };
+                }
+                else if (uses.Value.StartsWith("./") || uses.Value.StartsWith(".\\"))
+                {
+                    result.Reference = new RepositoryPathReference
+                    {
+                        RepositoryType = PipelineConstants.SelfAlias,
+                        Path = uses.Value
+                    };
+                }
+                else
+                {
+                    var usesSegments = uses.Value.Split('@');
+                    var pathSegments = usesSegments[0].Split(new[] { '/', '\\' }, StringSplitOptions.RemoveEmptyEntries);
+                    var gitRef = usesSegments.Length == 2 ? usesSegments[1] : String.Empty;
+
+                    if (usesSegments.Length != 2 ||
+                        pathSegments.Length < 2 ||
+                        String.IsNullOrEmpty(pathSegments[0]) ||
+                        String.IsNullOrEmpty(pathSegments[1]) ||
+                        String.IsNullOrEmpty(gitRef))
+                    {
+                        // todo: loc
+                        context.Error(uses, $"Expected format {{org}}/{{repo}}[/path]@ref. Actual '{uses.Value}'");
+                    }
+                    else
+                    {
+                        var repositoryName = $"{pathSegments[0]}/{pathSegments[1]}";
+                        var directoryPath = pathSegments.Length > 2 ? String.Join("/", pathSegments.Skip(2)) : String.Empty;
+
+                        result.Reference = new RepositoryPathReference
+                        {
+                            RepositoryType = RepositoryTypes.GitHub,
+                            Name = repositoryName,
+                            Ref = gitRef,
+                            Path = directoryPath,
+                        };
+                    }
+                }
+
+                return result;
+            }
+        }
+
+        /// <summary>
+        /// When empty, default to "success()".
+        /// When a status function is not referenced, format as "success() &amp;&amp; &lt;CONDITION&gt;".
+        /// </summary>
+        private static String ConvertToIfCondition(
+            TemplateContext context,
+            TemplateToken token,
+            Boolean isJob,
+            Boolean isDefaultScope)
+        {
+            String condition;
+            if (token is null)
+            {
+                condition = null;
+            }
+            else if (token is BasicExpressionToken expressionToken)
+            {
+                condition = expressionToken.Expression;
+            }
+            else
+            {
+                var stringToken = token.AssertString($"{(isJob ? "job" : "step")} {PipelineTemplateConstants.If}");
+                condition = stringToken.Value;
+            }
+
+            if (String.IsNullOrWhiteSpace(condition))
+            {
+                return $"{PipelineTemplateConstants.Success}()";
+            }
+
+            var expressionParser = new ExpressionParser();
+            var functions = default(IFunctionInfo[]);
+            var namedValues = default(INamedValueInfo[]);
+            if (isJob)
+            {
+                namedValues = s_jobIfNamedValues;
+                // TODO: refactor into seperate functions
+                // functions = PhaseCondition.FunctionInfo;
+            }
+            else
+            {
+                namedValues = isDefaultScope ? s_stepNamedValues : s_stepInTemplateNamedValues;
+                functions = s_stepConditionFunctions;
+            }
+
+            var node = default(ExpressionNode);
+            try
+            {
+                node = expressionParser.CreateTree(condition, null, namedValues, functions) as ExpressionNode;
+            }
+            catch (Exception ex)
+            {
+                context.Error(token, ex);
+                return null;
+            }
+
+            if (node == null)
+            {
+                return $"{PipelineTemplateConstants.Success}()";
+            }
+
+            var hasStatusFunction = node.Traverse().Any(x =>
+            {
+                if (x is Function function)
+                {
+                    return String.Equals(function.Name, PipelineTemplateConstants.Always, StringComparison.OrdinalIgnoreCase) ||
+                        String.Equals(function.Name, PipelineTemplateConstants.Cancelled, StringComparison.OrdinalIgnoreCase) ||
+                        String.Equals(function.Name, PipelineTemplateConstants.Failure, StringComparison.OrdinalIgnoreCase) ||
+                        String.Equals(function.Name, PipelineTemplateConstants.Success, StringComparison.OrdinalIgnoreCase);
+                }
+
+                return false;
+            });
+
+            return hasStatusFunction ? condition : $"{PipelineTemplateConstants.Success}() && ({condition})";
+        }
+
+        private static readonly INamedValueInfo[] s_jobIfNamedValues = new INamedValueInfo[]
+        {
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.GitHub),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Needs),
+        };
+        private static readonly INamedValueInfo[] s_stepNamedValues = new INamedValueInfo[]
+        {
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Strategy),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Matrix),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Steps),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.GitHub),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Job),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Runner),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Env),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Needs),
+        };
+        private static readonly INamedValueInfo[] s_stepInTemplateNamedValues = new INamedValueInfo[]
+        {
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Strategy),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Matrix),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Steps),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Inputs),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.GitHub),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Job),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Runner),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Env),
+            new NamedValueInfo<NoOperationNamedValue>(PipelineTemplateConstants.Needs),
+        };
+        private static readonly IFunctionInfo[] s_stepConditionFunctions = new IFunctionInfo[]
+        {
+            new FunctionInfo<NoOperation>(PipelineTemplateConstants.Always, 0, 0),
+            new FunctionInfo<NoOperation>(PipelineTemplateConstants.Cancelled, 0, 0),
+            new FunctionInfo<NoOperation>(PipelineTemplateConstants.Failure, 0, 0),
+            new FunctionInfo<NoOperation>(PipelineTemplateConstants.Success, 0, 0),
+            new FunctionInfo<NoOperation>(PipelineTemplateConstants.HashFiles, 1, Byte.MaxValue),
+        };
     }
 }

src/Sdk/DTPipelines/Pipelines/ObjectTemplating/PipelineTemplateEvaluator.cs

@@ -159,6 +159,32 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
             return result;
         }
 
+        public List<ActionStep> LoadCompositeSteps(
+            TemplateToken token
+        )
+        {
+            var result = default(List<ActionStep>);
+            if (token != null && token.Type != TokenType.Null)
+            {
+                var context = CreateContext(null, null, setMissingContext: false);
+                // TODO: we might want to to have a bool to prevent it from filling in with missing context w/ dummy variables
+                try
+                {
+                    token = TemplateEvaluator.Evaluate(context, PipelineTemplateConstants.StepsInTemplate, token, 0, null, omitHeader: true);
+                    context.Errors.Check();
+                    result = PipelineTemplateConverter.ConvertToSteps(context, token);
+                }
+                catch (Exception ex) when (!(ex is TemplateValidationException))
+                {
+                    context.Errors.Add(ex);
+                }
+
+                context.Errors.Check();
+            }
+            return result;
+        }
+
+
         public Dictionary<String, String> EvaluateStepEnvironment(
             TemplateToken token,
             DictionaryContextData contextData,
@@ -400,7 +426,8 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
         private TemplateContext CreateContext(
             DictionaryContextData contextData,
             IList<IFunctionInfo> expressionFunctions,
-            IEnumerable<KeyValuePair<String, Object>> expressionState = null)
+            IEnumerable<KeyValuePair<String, Object>> expressionState = null,
+            bool setMissingContext = true)
         {
             var result = new TemplateContext
             {
@@ -449,6 +476,8 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
             //   - Evaluating early when all referenced contexts are available, even though all allowed
             //     contexts may not yet be available. For example, evaluating step display name can often
             //     be performed early.
+            if (setMissingContext)
+            {
                 foreach (var name in s_expressionValueNames)
                 {
                     if (!result.ExpressionValues.ContainsKey(name))
@@ -463,6 +492,7 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
                         result.ExpressionFunctions.Add(new FunctionInfo<NoOperation>(name, 0, Int32.MaxValue));
                     }
                 }
+            }
 
             // Add state
             if (expressionState != null)

src/Sdk/DTPipelines/Pipelines/PipelineConstants.cs

@@ -94,5 +94,12 @@ namespace GitHub.DistributedTask.Pipelines
             public static readonly String Resources = "resources";
             public static readonly String All = "all";
         }
+
+        public static class ScriptStepInputs
+        {
+            public static readonly String Script = "script";
+            public static readonly String WorkingDirectory = "workingDirectory";
+            public static readonly String Shell = "shell";
+        }
     }
 }

src/Test/L0/Worker/StepsRunnerL0.cs

@@ -80,7 +80,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 {
                     _ec.Object.Result = null;
 
-                    _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(variableSet.Select(x => x.Object).ToList()));
+                    _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(variableSet.Select(x => x.Object).ToList()));
 
                     // Act.
                     await _stepsRunner.RunAsync(jobContext: _ec.Object);
@@ -115,7 +115,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 {
                     _ec.Object.Result = null;
 
-                    _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(variableSet.Select(x => x.Object).ToList()));
+                    _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(variableSet.Select(x => x.Object).ToList()));
 
                     // Act.
                     await _stepsRunner.RunAsync(jobContext: _ec.Object);
@@ -154,7 +154,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 {
                     _ec.Object.Result = null;
 
-                    _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(variableSet.Steps.Select(x => x.Object).ToList()));
+                    _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(variableSet.Steps.Select(x => x.Object).ToList()));
 
                     // Act.
                     await _stepsRunner.RunAsync(jobContext: _ec.Object);
@@ -208,7 +208,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 {
                     _ec.Object.Result = null;
 
-                    _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(variableSet.Steps.Select(x => x.Object).ToList()));
+                    _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(variableSet.Steps.Select(x => x.Object).ToList()));
 
                     // Act.
                     await _stepsRunner.RunAsync(jobContext: _ec.Object);
@@ -287,7 +287,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 {
                     _ec.Object.Result = null;
 
-                    _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(variableSet.Steps.Select(x => x.Object).ToList()));
+                    _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(variableSet.Steps.Select(x => x.Object).ToList()));
 
                     // Act.
                     await _stepsRunner.RunAsync(jobContext: _ec.Object);
@@ -330,7 +330,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 {
                     _ec.Object.Result = null;
 
-                    _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(variableSet.Step.Select(x => x.Object).ToList()));
+                    _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(variableSet.Step.Select(x => x.Object).ToList()));
 
                     // Act.
                     await _stepsRunner.RunAsync(jobContext: _ec.Object);
@@ -361,7 +361,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 {
                     _ec.Object.Result = null;
 
-                    _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(variableSet.Select(x => x.Object).ToList()));
+                    _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(variableSet.Select(x => x.Object).ToList()));
 
                     // Act.
                     await _stepsRunner.RunAsync(jobContext: _ec.Object);
@@ -391,7 +391,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 {
                     _ec.Object.Result = null;
 
-                    _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(variableSet.Select(x => x.Object).ToList()));
+                    _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(variableSet.Select(x => x.Object).ToList()));
 
                     // Act.
                     await _stepsRunner.RunAsync(jobContext: _ec.Object);
@@ -417,7 +417,7 @@ namespace GitHub.Runner.Common.Tests.Worker
 
                 _ec.Object.Result = null;
 
-                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new[] { step1.Object }));
+                _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(new[] { step1.Object }));
 
                 // Act.
                 await _stepsRunner.RunAsync(jobContext: _ec.Object);
@@ -455,7 +455,7 @@ namespace GitHub.Runner.Common.Tests.Worker
 
                 _ec.Object.Result = null;
 
-                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new[] { step1.Object, step2.Object }));
+                _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(new[] { step1.Object, step2.Object }));
 
                 // Act.
                 await _stepsRunner.RunAsync(jobContext: _ec.Object);
@@ -493,7 +493,7 @@ namespace GitHub.Runner.Common.Tests.Worker
 
                 _ec.Object.Result = null;
 
-                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new[] { step1.Object, step2.Object }));
+                _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(new[] { step1.Object, step2.Object }));
 
                 // Act.
                 await _stepsRunner.RunAsync(jobContext: _ec.Object);
@@ -524,7 +524,7 @@ namespace GitHub.Runner.Common.Tests.Worker
 
                 _ec.Object.Result = null;
 
-                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new[] { step1.Object, step2.Object, step3.Object }));
+                _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(new[] { step1.Object, step2.Object, step3.Object }));
 
                 // Act.
                 await _stepsRunner.RunAsync(jobContext: _ec.Object);
@@ -560,7 +560,7 @@ namespace GitHub.Runner.Common.Tests.Worker
 
                 _ec.Object.Result = null;
 
-                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new[] { step1.Object, step2.Object, step3.Object }));
+                _ec.Setup(x => x.JobSteps).Returns(new List<IStep>(new[] { step1.Object, step2.Object, step3.Object }));
 
                 // Act.
                 await _stepsRunner.RunAsync(jobContext: _ec.Object);