[TEP-0048] Add support for default results

Today, a Task can declare a Result but not produce it and it's execution
will still be successful whereas if a subsequent Task in a Pipeline
attempts to use that Result it will fail. This behaviour can be
confusing to the users as the Task which didn't produced the Result
passed and the follow-up Task fails. Moreover, the user of the Task will
loose faith in it as it failed to produce what it's claiming.

This patch introduces the default results, ie, Task author can now add a
default value to the results so that if their Task doesn't produce any
result then the default value can be emitted. Now, with this change, if
the Task author doesn't specifies any default value for the result and
one of the step within that Task also doesn't produces it then
the Task execution in whole will be marked as failed as it failed to
prove it's contract.

docs/pipeline-api.md

@@ -1669,7 +1669,7 @@ Used to distinguish between a single string and an array of strings.</p>
 <h3 id="tekton.dev/v1.ParamValue">ParamValue
 </h3>
 <p>
-(<em>Appears on:</em><a href="#tekton.dev/v1.Param">Param</a>, <a href="#tekton.dev/v1.ParamSpec">ParamSpec</a>, <a href="#tekton.dev/v1.PipelineResult">PipelineResult</a>, <a href="#tekton.dev/v1.PipelineRunResult">PipelineRunResult</a>, <a href="#tekton.dev/v1.TaskRunResult">TaskRunResult</a>)
+(<em>Appears on:</em><a href="#tekton.dev/v1.Param">Param</a>, <a href="#tekton.dev/v1.ParamSpec">ParamSpec</a>, <a href="#tekton.dev/v1.PipelineResult">PipelineResult</a>, <a href="#tekton.dev/v1.PipelineRunResult">PipelineRunResult</a>, <a href="#tekton.dev/v1.TaskResult">TaskResult</a>, <a href="#tekton.dev/v1.TaskRunResult">TaskRunResult</a>)
 </p>
 <div>
 <p>ResultValue is a type alias of ParamValue</p>
@@ -4476,6 +4476,22 @@ string
 <p>Description is a human-readable description of the result</p>
 </td>
 </tr>
+<tr>
+<td>
+<code>default</code><br/>
+<em>
+<a href="#tekton.dev/v1.ParamValue">
+ParamValue
+</a>
+</em>
+</td>
+<td>
+<em>(Optional)</em>
+<p>This is an alpha field. You must set the &ldquo;enable-api-fields&rdquo; feature flag to &ldquo;alpha&rdquo;
+for this field to be supported.</p>
+<p>Default is the value a result produces if no result is produced by Task</p>
+</td>
+</tr>
 </tbody>
 </table>
 <h3 id="tekton.dev/v1.TaskRunDebug">TaskRunDebug
@@ -8785,7 +8801,7 @@ Used to distinguish between a single string and an array of strings.</p>
 <h3 id="tekton.dev/v1beta1.ParamValue">ParamValue
 </h3>
 <p>
-(<em>Appears on:</em><a href="#tekton.dev/v1beta1.Param">Param</a>, <a href="#tekton.dev/v1beta1.ParamSpec">ParamSpec</a>, <a href="#tekton.dev/v1beta1.PipelineResult">PipelineResult</a>, <a href="#tekton.dev/v1beta1.PipelineRunResult">PipelineRunResult</a>, <a href="#tekton.dev/v1beta1.TaskRunResult">TaskRunResult</a>)
+(<em>Appears on:</em><a href="#tekton.dev/v1beta1.Param">Param</a>, <a href="#tekton.dev/v1beta1.ParamSpec">ParamSpec</a>, <a href="#tekton.dev/v1beta1.PipelineResult">PipelineResult</a>, <a href="#tekton.dev/v1beta1.PipelineRunResult">PipelineRunResult</a>, <a href="#tekton.dev/v1beta1.TaskResult">TaskResult</a>, <a href="#tekton.dev/v1beta1.TaskRunResult">TaskRunResult</a>)
 </p>
 <div>
 <p>ResultValue is a type alias of ParamValue</p>
@@ -12441,8 +12457,30 @@ string
 <p>Description is a human-readable description of the result</p>
 </td>
 </tr>
+<tr>
+<td>
+<code>default</code><br/>
+<em>
+<a href="#tekton.dev/v1beta1.ParamValue">
+ParamValue
+</a>
+</em>
+</td>
+<td>
+<em>(Optional)</em>
+<p>This is an alpha field. You must set the &ldquo;enable-api-fields&rdquo; feature flag to &ldquo;alpha&rdquo;
+for this field to be supported.</p>
+<p>Default is the value a result produces if no result is produced by Task</p>
+</td>
+</tr>
 </tbody>
 </table>
+<h3 id="tekton.dev/v1beta1.TaskRunConditionType">TaskRunConditionType
+(<code>string</code> alias)</h3>
+<div>
+<p>TaskRunConditionType is an enum used to store TaskRun custom conditions
+conditions such as one used in spire results verification</p>
+</div>
 <h3 id="tekton.dev/v1beta1.TaskRunDebug">TaskRunDebug
 </h3>
 <p>

docs/tasks.md

@@ -847,6 +847,46 @@ available size will less than 4096 bytes.
 As a general rule-of-thumb, if a result needs to be larger than a kilobyte, you should likely use a
 [`Workspace`](#specifying-workspaces) to store and pass it between `Tasks` within a `Pipeline`.
 
+#### Default value
+
+Results declarations (within Tasks and Pipelines) can include default values which will be used if the Result is
+not produced, for example to specify defaults for both string params and array params
+([full example](../examples/v1beta1/taskruns/alpha/default-result.yaml)) :
+
+```yaml
+apiVersion: tekton.dev/v1beta1
+kind: Task
+metadata:
+  name: task-with-default-result
+spec:
+  results:
+  - name: branch
+    default: main
+```
+
+**Note**: If a Task doesn't specify any default value for a particular `Result`
+and also fails to emit that `Result`, in that case, the following `Task` is going
+to be marked as `Failed` as specified in [TEP-0048](https://github.com/tektoncd/community/blob/main/teps/0048-task-results-without-results.md). Below is the example in which the TaskRun
+is going to fail as one of the Result wasn't produced.
+
+```yaml
+apiVersion: tekton.dev/v1beta1
+kind: TaskRun
+metadata:
+  generateName: test-tr-
+spec:
+  taskSpec:
+    results:
+      - name: result1
+        description: will be produced
+      - name: result2
+        description: will not be produced
+    steps:
+      - name: failing-step
+        image: busybox
+        script: "echo -n 123 | tee $(results.result1.path)"
+```
+
 ### Specifying `Volumes`
 
 Specifies one or more [`Volumes`](https://kubernetes.io/docs/concepts/storage/volumes/) that the `Steps` in your

examples/v1beta1/pipelineruns/alpha/pipeline-emitting-default-results.yaml

@@ -0,0 +1,72 @@
+apiVersion: tekton.dev/v1beta1
+kind: PipelineRun
+metadata:
+  name: pipelinerun-emit-default-results
+spec:
+  pipelineSpec:
+    tasks:
+      - name: task1
+        taskSpec:
+          results:
+            - name: array-results
+              type: array
+              description: The array results
+              default:
+              - "1"
+              - "2"
+              - "3"
+          steps:
+            - name: write-array
+              image: bash:latest
+              script: |
+                #!/usr/bin/env bash
+                echo -n "this script won't emit array result"
+      - name: task2
+        taskSpec:
+          results:
+            - name: object-results
+              type: object
+              description: The object results
+              properties:
+                foo: { type: string }
+                hello: { type: string }
+              default:
+                foo: bar
+                hello: world
+          steps:
+            - name: write-array
+              image: bash:latest
+              script: |
+                #!/usr/bin/env bash
+                echo -n "this script won't emit object result"
+    results:
+      - name: array-results
+        type: array
+        description: whole array
+        value: $(tasks.task1.results.array-results[*])
+      - name: array-results-from-array-indexing-and-object-elements
+        type: array
+        description: whole array
+        value:
+          [
+            "$(tasks.task1.results.array-results[0])",
+            "$(tasks.task2.results.object-results.foo)",
+          ]
+      - name: array-indexing-results
+        type: string
+        description: array element
+        value: $(tasks.task1.results.array-results[1])
+      - name: object-results
+        type: object
+        description: whole object
+        value: $(tasks.task2.results.object-results[*])
+      - name: object-results-from-array-indexing-and-object-elements
+        type: object
+        description: whole object
+        value:
+          key1: $(tasks.task1.results.array-results[1])
+          key2: $(tasks.task2.results.object-results.hello)
+      - name: object-element
+        type: string
+        description: object element
+        value: $(tasks.task2.results.object-results.foo)

examples/v1beta1/taskruns/alpha/default-result.yaml

@@ -0,0 +1,33 @@
+apiVersion: tekton.dev/v1beta1
+kind: TaskRun
+metadata:
+  generateName: default-result-run-
+spec:
+  taskSpec:
+    results:
+      - name: result1
+        description: will be produced
+      - name: string-result
+        description: will not be produced by step but has default value
+        default: "string value"
+      - name: array-result
+        description: default array result will be produced
+        default:
+          - "foo"
+          - "bar"
+      - name: eight-results
+        type: object
+        properties:
+          IMAGE_URL: {type: string}
+          IMAGE_DIGEST: {type: string}
+        default:
+          IMAGE_URL: "default.com"
+          IMAGE_DIGEST: "defaultsha"
+    steps:
+      - name: failing-step
+        onError: continue
+        image: busybox
+        script: |
+          echo -n 123 | tee $(results.result1.path)
+          exit 1
+          echo -n 456 | tee $(results.string-result.path)

pkg/apis/pipeline/v1/result_types.go

@@ -32,6 +32,13 @@ type TaskResult struct {
 	// Description is a human-readable description of the result
 	// +optional
 	Description string `json:"description,omitempty"`
+
+	// This is an alpha field. You must set the "enable-api-fields" feature flag to "alpha"
+	// for this field to be supported.
+	//
+	// Default is the value a result produces if no result is produced by Task
+	// +optional
+	Default *ResultValue `json:"default,omitempty"`
 }
 
 // TaskRunResult used to describe the results of a task

pkg/apis/pipeline/v1/swagger.json

@@ -1615,6 +1615,10 @@
         "name"
       ],
       "properties": {
+        "default": {
+          "description": "This is an alpha field. You must set the \"enable-api-fields\" feature flag to \"alpha\" for this field to be supported.\n\nDefault is the value a result produces if no result is produced by Task",
+          "$ref": "#/definitions/v1.ParamValue"
+        },
         "description": {
           "description": "Description is a human-readable description of the result",
           "type": "string"

pkg/apis/pipeline/v1beta1/result_conversion.go

@@ -26,4 +26,22 @@ func (r *TaskResult) convertFrom(ctx context.Context, source v1.TaskResult) {
 		properties[k] = PropertySpec{Type: ParamType(v.Type)}
 	}
 	r.Properties = properties
+	if source.Default != nil {
+		r.Default = &ParamValue{
+			Type: ParamType(source.Default.Type),
+		}
+
+		if source.Default.StringVal != "" {
+			r.Default.StringVal = source.Default.StringVal
+		}
+
+		if len(source.Default.ArrayVal) != 0 {
+			r.Default.ArrayVal = source.Default.ArrayVal
+		}
+
+		if len(source.Default.ObjectVal) != 0 {
+			r.Default.ObjectVal = source.Default.ObjectVal
+		}
+
+	}
 }

pkg/apis/pipeline/v1beta1/result_types.go

@@ -32,6 +32,13 @@ type TaskResult struct {
 	// Description is a human-readable description of the result
 	// +optional
 	Description string `json:"description,omitempty"`
+
+	// This is an alpha field. You must set the "enable-api-fields" feature flag to "alpha"
+	// for this field to be supported.
+	//
+	// Default is the value a result produces if no result is produced by Task
+	// +optional
+	Default *ResultValue `json:"default,omitempty"`
 }
 
 // TaskRunResult used to describe the results of a task

pkg/apis/pipeline/v1beta1/result_validation.go

@@ -27,6 +27,11 @@ func (tr TaskResult) Validate(ctx context.Context) (errs *apis.FieldError) {
 	if !resultNameFormatRegex.MatchString(tr.Name) {
 		return apis.ErrInvalidKeyName(tr.Name, "name", fmt.Sprintf("Name must consist of alphanumeric characters, '-', '_', and must start and end with an alphanumeric character (e.g. 'MyName',  or 'my-name',  or 'my_name', regex used for validation is '%s')", ResultNameFormat))
 	}
+	// Default results in alpha feature
+	if tr.Default != nil {
+		errs = errs.Also(version.ValidateEnabledAPIFields(ctx, "default results", config.AlphaAPIFields))
+		return errs
+	}
 	// Array and Object is alpha feature
 	if tr.Type == ResultsTypeArray || tr.Type == ResultsTypeObject {
 		errs := validateObjectResult(tr)

pkg/apis/pipeline/v1beta1/result_validation_test.go

@@ -67,6 +67,15 @@ func TestResultsValidate(t *testing.T) {
 			Properties:  map[string]v1beta1.PropertySpec{"hello": {Type: v1beta1.ParamTypeString}},
 		},
 		apiFields: "alpha",
+	}, {
+		name: "valid result with default value",
+		Result: v1beta1.TaskResult{
+			Name:        "MY-RESULT",
+			Type:        v1beta1.ResultsTypeString,
+			Description: "my great result",
+			Default:     &v1beta1.ParamValue{StringVal: "string value"},
+		},
+		apiFields: "alpha",
 	}}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -161,6 +170,18 @@ func TestResultsValidateError(t *testing.T) {
 			Message: "missing field(s)",
 			Paths:   []string{"MY-RESULT.properties"},
 		},
+	}, {
+		name: "valid result with default value",
+		Result: v1beta1.TaskResult{
+			Name:        "MY-RESULT",
+			Type:        v1beta1.ResultsTypeString,
+			Description: "my great result",
+			Default:     &v1beta1.ParamValue{StringVal: "string value"},
+		},
+		apiFields: "stable",
+		expectedError: apis.FieldError{
+			Message: "default results requires \"enable-api-fields\" feature gate to be \"alpha\" but it is \"stable\"",
+		},
 	}}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

pkg/apis/pipeline/v1beta1/swagger.json

@@ -2572,6 +2572,10 @@
         "name"
       ],
       "properties": {
+        "default": {
+          "description": "This is an alpha field. You must set the \"enable-api-fields\" feature flag to \"alpha\" for this field to be supported.\n\nDefault is the value a result produces if no result is produced by Task",
+          "$ref": "#/definitions/v1beta1.ParamValue"
+        },
         "description": {
           "description": "Description is a human-readable description of the result",
           "type": "string"