implementing finally at the pipeline level

We can now specify a list of tasks needs to be executed just before
pipeline exits (either after finishing all non-final tasks successfully or after
a single failure)

Most useful for tasks such as report test results, cleanup cluster resources, etc

```
apiVersion: tekton.dev/v1beta1
kind: Pipeline
metadata:
  name: pipeline-with-final-tasks
spec:
  tasks:
    - name: pre-work
      taskRef:
        Name: some-pre-work
    - name: unit-test
      taskRef:
        Name: run-unit-test
      runAfter:
        - pre-work
    - name: integration-test
      taskRef:
        Name: run-integration-test
      runAfter:
        - unit-test
  finally:
    - name: cleanup-test
      taskRef:
        Name: cleanup-cluster
    - name: report-results
      taskRef:
        Name: report-test-results
```

examples/v1beta1/pipelineruns/pipelinerun-with-final-tasks.yaml

@@ -0,0 +1,236 @@
+# Task producing success by exiting with 0
+apiVersion: tekton.dev/v1beta1
+kind: Task
+metadata:
+  name: successful-task
+spec:
+  steps:
+    - name: success
+      image: ubuntu
+      script: |
+        exit 0
+---
+
+apiVersion: tekton.dev/v1beta1
+kind: Pipeline
+metadata:
+  name: pipeline-with-final-tasks
+spec:
+  tasks:
+    - name: pre-work
+      taskRef:
+        Name: successful-task
+    - name: unit-test
+      taskRef:
+        Name: successful-task
+      runAfter:
+        - pre-work
+    - name: integration-test
+      taskRef:
+        Name: successful-task
+      runAfter:
+        - unit-test
+  finally:
+    - name: cleanup-test
+      taskRef:
+        Name: successful-task
+    - name: report-results
+      taskRef:
+        Name: successful-task
+---
+
+apiVersion: tekton.dev/v1beta1
+kind: PipelineRun
+metadata:
+  name: pipelinerun-with-final-tasks
+spec:
+  pipelineRef:
+    name: pipeline-with-final-tasks
+---
+
+apiVersion: tekton.dev/v1beta1
+kind: Task
+metadata:
+  name: sum
+  annotations:
+    description: |
+      A simple task that sums the two provided integers
+spec:
+  params:
+    - name: a
+      type: string
+      default: "1"
+      description: The first integer
+    - name: b
+      type: string
+      default: "1"
+      description: The second integer
+  results:
+    - name: sum
+      description: The sum of the two provided integers
+  steps:
+    - name: sum
+      image: bash:latest
+      script: |
+        #!/usr/bin/env bash
+        echo -n $(( "$(params.a)" + "$(params.b)" )) | tee $(results.sum.path)
+---
+
+apiVersion: tekton.dev/v1beta1
+kind: Task
+metadata:
+  name: format-result
+spec:
+  params:
+    - name: result
+      type: string
+      default: "0"
+  steps:
+    - name: print-sum
+      image: ubuntu
+      script: echo "*** Result = $(params.result) ***"
+---
+
+apiVersion: tekton.dev/v1beta1
+kind: Pipeline
+metadata:
+  name: sum-and-print-pipeline
+spec:
+  params:
+    - name: a
+      type: string
+      default: "1"
+    - name: b
+      type: string
+      default: "1"
+  tasks:
+    - name: sum-inputs
+      taskRef:
+        name: sum
+      params:
+        - name: a
+          value: "$(params.a)"
+        - name: b
+          value: "$(params.b)"
+  finally:
+    - name: print-sum
+      taskRef:
+        name: format-result
+      params:
+        - name: result
+          value: "$(tasks.sum-inputs.results.sum)"
+---
+
+apiVersion: tekton.dev/v1beta1
+kind: PipelineRun
+metadata:
+  name: sum-and-print-pipeline-run
+spec:
+  pipelineRef:
+    name: sum-and-print-pipeline
+  params:
+    - name: a
+      value: "100"
+    - name: b
+      value: "1000"
+---
+
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: shared-workspace
+spec:
+  resources:
+    requests:
+      storage: 16Mi
+  volumeMode: Filesystem
+  accessModes:
+    - ReadWriteOnce
+---
+
+apiVersion: tekton.dev/v1alpha1
+kind: Task
+metadata:
+  name: clone-app-repo-to-workspace
+spec:
+  workspaces:
+    - name: shared-workspace
+  resources:
+    inputs:
+      - name: app-git
+        type: git
+        targetPath: application
+  steps:
+    - name: clone-app-repo-to-workspace
+      image: ubuntu
+      script: |
+        #!/usr/bin/env bash
+        set -xe
+        cp -avr $(resources.inputs.app-git.path)/ $(workspaces.shared-workspace.path)/
+---
+
+apiVersion: tekton.dev/v1alpha1
+kind: Task
+metadata:
+  name: cleanup-workspace
+spec:
+  workspaces:
+    - name: shared-workspace
+  steps:
+    - name: cleanup-workspace
+      image: ubuntu
+      script: |
+        #!/usr/bin/env bash
+        set -xe
+        rm -rf $(workspaces.shared-workspace.path)/application/
+---
+
+apiVersion: tekton.dev/v1alpha1
+kind: Pipeline
+metadata:
+  name: clone-into-workspace-and-cleanup-workspace
+spec:
+  resources:
+    - name: app-git
+      type: git
+  workspaces:
+    - name: shared-workspace
+  tasks:
+    - name: clone-app-source
+      taskRef:
+        name: clone-app-repo-to-workspace
+      workspaces:
+        - name: shared-workspace
+          workspace: shared-workspace
+      resources:
+        inputs:
+          - name: app-git
+            resource: app-git
+  finally:
+    - name: cleanup-workspace
+      taskRef:
+        name: cleanup-workspace
+      workspaces:
+        - name: shared-workspace
+          workspace: shared-workspace
+---
+
+apiVersion: tekton.dev/v1alpha1
+kind: PipelineRun
+metadata:
+  name: write-and-cleanup-workspace
+spec:
+  pipelineRef:
+    name: clone-into-workspace-and-cleanup-workspace
+  workspaces:
+    - name: shared-workspace
+      persistentVolumeClaim:
+        claimName: shared-workspace
+  resources:
+    - name: app-git
+      resourceSpec:
+        type: git
+        params:
+          - name: url
+            value: https://github.com/tektoncd/pipeline.git
+---

pkg/apis/pipeline/v1alpha1/pipeline_conversion.go

@@ -51,6 +51,14 @@ func (source *PipelineSpec) ConvertTo(ctx context.Context, sink *v1beta1.Pipelin
 			}
 		}
 	}
+	if len(source.Finally) > 0 {
+		sink.Finally = make([]v1beta1.FinalPipelineTask, len(source.Finally))
+		for i := range source.Finally {
+			if err := source.Finally[i].ConvertTo(ctx, &sink.Finally[i]); err != nil {
+				return err
+			}
+		}
+	}
 	return nil
 }
 
@@ -72,6 +80,21 @@ func (source *PipelineTask) ConvertTo(ctx context.Context, sink *v1beta1.Pipelin
 	return nil
 }
 
+func (source *FinalPipelineTask) ConvertTo(ctx context.Context, sink *v1beta1.FinalPipelineTask) error {
+	sink.Name = source.Name
+	sink.TaskRef = source.TaskRef
+	if source.TaskSpec != nil {
+		sink.TaskSpec = &v1beta1.TaskSpec{}
+		if err := source.TaskSpec.ConvertTo(ctx, sink.TaskSpec); err != nil {
+			return err
+		}
+	}
+	sink.Resources = source.Resources
+	sink.Params = source.Params
+	sink.Workspaces = source.Workspaces
+	return nil
+}
+
 // ConvertFrom implements api.Convertible
 func (sink *Pipeline) ConvertFrom(ctx context.Context, obj apis.Convertible) error {
 	switch source := obj.(type) {
@@ -96,6 +119,14 @@ func (sink *PipelineSpec) ConvertFrom(ctx context.Context, source v1beta1.Pipeli
 			}
 		}
 	}
+	if len(source.Finally) > 0 {
+		sink.Finally = make([]FinalPipelineTask, len(source.Finally))
+		for i := range source.Finally {
+			if err := sink.Finally[i].ConvertFrom(ctx, source.Finally[i]); err != nil {
+				return err
+			}
+		}
+	}
 	return nil
 }
 
@@ -116,3 +147,18 @@ func (sink *PipelineTask) ConvertFrom(ctx context.Context, source v1beta1.Pipeli
 	sink.Workspaces = source.Workspaces
 	return nil
 }
+
+func (sink *FinalPipelineTask) ConvertFrom(ctx context.Context, source v1beta1.FinalPipelineTask) error {
+	sink.Name = source.Name
+	sink.TaskRef = source.TaskRef
+	if source.TaskSpec != nil {
+		sink.TaskSpec = &TaskSpec{}
+		if err := sink.TaskSpec.ConvertFrom(ctx, source.TaskSpec); err != nil {
+			return err
+		}
+	}
+	sink.Resources = source.Resources
+	sink.Params = source.Params
+	sink.Workspaces = source.Workspaces
+	return nil
+}

pkg/apis/pipeline/v1alpha1/pipeline_defaults.go

@@ -42,4 +42,12 @@ func (ps *PipelineSpec) SetDefaults(ctx context.Context) {
 	for i := range ps.Params {
 		ps.Params[i].SetDefaults(ctx)
 	}
+	for _, ft := range ps.Finally {
+		if ft.TaskRef.Kind == "" {
+			ft.TaskRef.Kind = NamespacedTaskKind
+		}
+		if ft.TaskSpec != nil {
+			ft.TaskSpec.SetDefaults(ctx)
+		}
+	}
 }

pkg/apis/pipeline/v1alpha1/pipeline_types.go

@@ -43,6 +43,10 @@ type PipelineSpec struct {
 	// Results are values that this pipeline can output once run
 	// +optional
 	Results []PipelineResult `json:"results,omitempty"`
+	// Finally declares the list of Tasks that execute just before leaving the Pipeline
+	// i.e. either after all Tasks are finished executing successfully
+	// or after a failure which would result in ending the Pipeline
+	Finally []FinalPipelineTask `json:"finally,omitempty"`
 }
 
 // PipelineResult used to describe the results of a pipeline
@@ -150,6 +154,35 @@ type PipelineTask struct {
 	Timeout *metav1.Duration `json:"timeout,omitempty"`
 }
 
+// FinalPipelineTask defines a final task in a Pipeline, passing inputs from both
+// Params and from the output of non-final tasks.
+type FinalPipelineTask struct {
+	// Name is the name of this task within the context of a Pipeline.
+	Name string `json:"name,omitempty"`
+
+	// TaskRef is a reference to a task definition.
+	// +optional
+	TaskRef *TaskRef `json:"taskRef,omitempty"`
+
+	// TaskSpec is specification of a task
+	// +optional
+	TaskSpec *TaskSpec `json:"taskSpec,omitempty"`
+
+	// Resources declares the resources given to this task as inputs and
+	// outputs.
+	// +optional
+	Resources *PipelineTaskResources `json:"resources,omitempty"`
+
+	// Parameters declares parameters passed to this task.
+	// +optional
+	Params []Param `json:"params,omitempty"`
+
+	// Workspaces maps workspaces from the pipeline spec to the workspaces
+	// declared in the Task.
+	// +optional
+	Workspaces []WorkspacePipelineTaskBinding `json:"workspaces,omitempty"`
+}
+
 func (pt PipelineTask) HashKey() string {
 	return pt.Name
 }