Process snapshot tokens (#3135)

* Added Snapshot TemplateToken to AgentJobRequestMessage

* WIP for processing the snapshot token

* Changed snapshot post job step condition to Success, added comments

* Refactored snapshot post-job step

* Added evaluation of snapshot token to retrieve image name

* Added snapshot to workflow schema

* Fixed linter error

* Migrated snapshot logic to new SnapshotOperationProvider

* Fixed linter error

* Fixed linter errors

* Fixed linter error

* Fixed linter errors

* Updated L0 tests

* Fixed linter errors

* Added new JobExtensionL0 tests for snapshot post-job step

* Added JobExtensionL0 test case for snapshot mappings

* Added SnapshotOperationProviderL0 tests

* Enabled nullable types for SnapshotOperationProvider and its tests

* Added more assertions to SnapshotOperationProviderL0 tests

* Fixed linter errors

* Made sure TestHostContexts are disposed of properlyh in SnapshotOperationProviderL0 tests

* Resolved PR comments

* Fixed formatting

* Removed redundant reference

* Addressed PR comments

src/Sdk/DTPipelines/Pipelines/AgentJobRequestMessage.cs

@@ -43,6 +43,7 @@ namespace GitHub.DistributedTask.Pipelines
             TemplateToken jobOutputs,
             IList<TemplateToken> defaults,
             ActionsEnvironmentReference actionsEnvironment,
+            TemplateToken snapshot,
             String messageType = JobRequestMessageTypes.PipelineAgentJobRequest)
         {
             this.MessageType = messageType;
@@ -57,6 +58,7 @@ namespace GitHub.DistributedTask.Pipelines
             this.Workspace = workspaceOptions;
             this.JobOutputs = jobOutputs;
             this.ActionsEnvironment = actionsEnvironment;
+            this.Snapshot = snapshot;
             m_variables = new Dictionary<String, VariableValue>(variables, StringComparer.OrdinalIgnoreCase);
             m_maskHints = new List<MaskHint>(maskHints);
             m_steps = new List<JobStep>(steps);
@@ -237,6 +239,13 @@ namespace GitHub.DistributedTask.Pipelines
             set;
         }
 
+        [DataMember(EmitDefaultValue = false)]
+        public TemplateToken Snapshot
+        {
+            get;
+            set;
+        }
+
         /// <summary>
         /// Gets the collection of variables associated with the current context.
         /// </summary>

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

@@ -29,6 +29,7 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
         public const String Id = "id";
         public const String If = "if";
         public const String Image = "image";
+        public const String ImageName = "image-name";
         public const String Include = "include";
         public const String Inputs = "inputs";
         public const String Job = "job";
@@ -60,6 +61,7 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
         public const String Services = "services";
         public const String Shell = "shell";
         public const String Skipped = "skipped";
+        public const String Snapshot = "snapshot";
         public const String StepEnv = "step-env";
         public const String StepIfResult = "step-if-result";
         public const String StepWith = "step-with";

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

@@ -346,6 +346,39 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
             return result;
         }
 
+        internal static Snapshot ConvertToJobSnapshotRequest(TemplateContext context, TemplateToken token)
+        {
+            string imageName = null;
+            if (token is StringToken snapshotStringLiteral)
+            {
+                imageName = snapshotStringLiteral.Value;
+            }
+            else
+            {
+                var snapshotMapping = token.AssertMapping($"{PipelineTemplateConstants.Snapshot}");
+                foreach (var snapshotPropertyPair in snapshotMapping)
+                {
+                    var propertyName = snapshotPropertyPair.Key.AssertString($"{PipelineTemplateConstants.Snapshot} key");
+                    switch (propertyName.Value)
+                    {
+                        case PipelineTemplateConstants.ImageName:
+                            imageName = snapshotPropertyPair.Value.AssertString($"{PipelineTemplateConstants.Snapshot} {propertyName}").Value;
+                            break;
+                        default:
+                            propertyName.AssertUnexpectedValue($"{PipelineTemplateConstants.Snapshot} key");
+                            break;
+                    }
+                }
+            }
+
+            if (String.IsNullOrEmpty(imageName))
+            {
+                return null;
+            }
+
+            return new Snapshot(imageName);
+        }
+
         private static ActionStep ConvertToStep(
             TemplateContext context,
             TemplateToken stepsItem,

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

@@ -370,6 +370,32 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
             return result;
         }
 
+        public Snapshot EvaluateJobSnapshotRequest(TemplateToken token,
+            DictionaryContextData contextData,
+            IList<IFunctionInfo> expressionFunctions)
+        {
+            var result = default(Snapshot);
+
+            if (token != null && token.Type != TokenType.Null)
+            {
+                var context = CreateContext(contextData, expressionFunctions);
+                try
+                {
+                    token = TemplateEvaluator.Evaluate(context, PipelineTemplateConstants.Snapshot, token, 0, null, omitHeader: true);
+                    context.Errors.Check();
+                    result = PipelineTemplateConverter.ConvertToJobSnapshotRequest(context, token);
+                }
+                catch (Exception ex) when (!(ex is TemplateValidationException))
+                {
+                    context.Errors.Add(ex);
+                }
+
+                context.Errors.Check();
+            }
+
+            return result;
+        }
+
         private TemplateContext CreateContext(
             DictionaryContextData contextData,
             IList<IFunctionInfo> expressionFunctions,

src/Sdk/DTPipelines/Pipelines/Snapshot.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Runtime.Serialization;
+
+namespace GitHub.DistributedTask.Pipelines
+{
+    [DataContract]
+    public class Snapshot
+    {
+        public Snapshot(string imageName)
+        {
+            ImageName = imageName;
+        }
+
+        [DataMember(EmitDefaultValue = false)]
+        public String ImageName { get; set; }
+    }
+}

src/Sdk/DTPipelines/workflow-v1.0.json

@@ -71,7 +71,8 @@
           "env": "job-env",
           "outputs": "job-outputs",
           "defaults": "job-defaults",
-          "steps": "steps"
+          "steps": "steps",
+          "snapshot": "snapshot"
         }
       }
     },
@@ -155,6 +156,24 @@
       }
     },
 
+    "snapshot": {
+      "one-of": [
+        "non-empty-string",
+        "snapshot-mapping"
+      ]
+    },
+
+    "snapshot-mapping": {
+      "mapping": {
+        "properties": {
+          "image-name": {
+            "type": "non-empty-string",
+            "required": true
+          }
+        }
+      }
+    },
+
     "runs-on": {
       "context": [
         "github",