Adds an initial implementation for conditionals

In this implementation, condition evaluations aka ConditionChecks are
backed by TaskRuns. All conditionChecks associated with a `PipelineTask`
have to succeed before the task is executed. If a ConditionCheck fails,
the PipelineTask's associated TaskRun is marked failed i.e. its
`Status.ConditionSucceeded` is False. However, the PipelineRun itself
is not marked as failed.

Signed-off-by: Dibyo Mukherjee <dibyo@google.com>

examples/pipelineruns/conditional-pipelinerun.yaml

@@ -0,0 +1,77 @@
+apiVersion: tekton.dev/v1alpha1
+kind: Condition
+metadata:
+  name: always-true
+spec:
+ check:
+    image: alpine
+    command: ["/bin/sh"]
+    args: ['-c', 'cat /workspace/prmetadata.json']
+---
+apiVersion: tekton.dev/v1alpha1
+kind: PipelineResource
+metadata:
+  name: pipeline-git
+spec:
+  type: git
+  params:
+    - name: revision
+      value: master
+    - name: url
+      value: https://github.com/tektoncd/pipeline
+---
+apiVersion: tekton.dev/v1alpha1
+kind: Task
+metadata:
+  name: list-files
+spec:
+  inputs:
+    resources:
+      - name: workspace
+        type: git
+  steps:
+    - name: run-ls
+      image: ubuntu
+      command: ["/bin/bash"]
+      args: ['-c', 'ls -al ${inputs.resources.workspace.path}']
+---
+apiVersion: tekton.dev/v1alpha1
+kind: Pipeline
+metadata:
+  name: list-files-pipeline
+spec:
+  resources:
+    - name: source-repo
+      type: git
+  tasks:
+    - name: list-files-0
+      taskRef:
+        name: list-files
+      resources:
+        inputs:
+          - name: workspace
+            resource: source-repo
+    - name: list-files-1
+      taskRef:
+        name: list-files
+      runAfter:
+        - list-files-0
+      conditions:
+        - conditionRef: "always-true"
+      resources:
+        inputs:
+          - name: workspace
+            resource: source-repo
+---
+apiVersion: tekton.dev/v1alpha1
+kind: PipelineRun
+metadata:
+  name: demo-condtional-pr
+spec:
+  pipelineRef:
+    name: list-files-pipeline
+  serviceAccount: 'default'
+  resources:
+    - name: source-repo
+      resourceRef:
+        name: pipeline-git

pkg/apis/pipeline/register.go

@@ -18,10 +18,11 @@ package pipeline
 
 // GroupName is the Kubernetes resource group name for Pipeline types.
 const (
-	GroupName            = "tekton.dev"
-	TaskLabelKey         = "/task"
-	TaskRunLabelKey      = "/taskRun"
-	PipelineLabelKey     = "/pipeline"
-	PipelineRunLabelKey  = "/pipelineRun"
-	PipelineTaskLabelKey = "/pipelineTask"
+	GroupName                    = "tekton.dev"
+	TaskLabelKey                 = "/task"
+	TaskRunLabelKey              = "/taskRun"
+	PipelineLabelKey             = "/pipeline"
+	PipelineRunLabelKey          = "/pipelineRun"
+	PipelineTaskLabelKey         = "/pipelineTask"
+	PipelineRunConditionCheckKey = "/pipelineConditionCheck"
 )

pkg/apis/pipeline/v1alpha1/condition_types.go

@@ -27,6 +27,7 @@ import (
 
 // Check that Task may be validated and defaulted.
 var _ apis.Validatable = (*Condition)(nil)
+//var _ apis.Defaultable = (*Condition)(nil)
 
 // +genclient
 // +genclient:noStatus

pkg/reconciler/v1alpha1/pipelinerun/controller.go

@@ -27,6 +27,7 @@ import (
 	"github.com/knative/pkg/tracker"
 	pipelineclient "github.com/tektoncd/pipeline/pkg/client/injection/client"
 	clustertaskinformer "github.com/tektoncd/pipeline/pkg/client/injection/informers/pipeline/v1alpha1/clustertask"
+	conditioninformer "github.com/tektoncd/pipeline/pkg/client/injection/informers/pipeline/v1alpha1/condition"
 	pipelineinformer "github.com/tektoncd/pipeline/pkg/client/injection/informers/pipeline/v1alpha1/pipeline"
 	resourceinformer "github.com/tektoncd/pipeline/pkg/client/injection/informers/pipeline/v1alpha1/pipelineresource"
 	pipelineruninformer "github.com/tektoncd/pipeline/pkg/client/injection/informers/pipeline/v1alpha1/pipelinerun"
@@ -54,6 +55,7 @@ func NewController(
 	pipelineRunInformer := pipelineruninformer.Get(ctx)
 	pipelineInformer := pipelineinformer.Get(ctx)
 	resourceInformer := resourceinformer.Get(ctx)
+	conditionInformer := conditioninformer.Get(ctx)
 	timeoutHandler := reconciler.NewTimeoutHandler(ctx.Done(), logger)
 
 	opt := reconciler.Options{
@@ -72,6 +74,7 @@ func NewController(
 		clusterTaskLister: clusterTaskInformer.Lister(),
 		taskRunLister:     taskRunInformer.Lister(),
 		resourceLister:    resourceInformer.Lister(),
+		conditionLister:   conditionInformer.Lister(),
 		timeoutHandler:    timeoutHandler,
 	}
 	impl := controller.NewImpl(c, c.Logger, pipelineRunControllerName)