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.

cmd/controller/main.go

@@ -114,6 +114,7 @@ func main() {
 	taskRunInformer := pipelineInformerFactory.Tekton().V1alpha1().TaskRuns()
 	resourceInformer := pipelineInformerFactory.Tekton().V1alpha1().PipelineResources()
 	podInformer := kubeInformerFactory.Core().V1().Pods()
+	conditionInformer := pipelineInformerFactory.Tekton().V1alpha1().Conditions()
 
 	pipelineInformer := pipelineInformerFactory.Tekton().V1alpha1().Pipelines()
 	pipelineRunInformer := pipelineInformerFactory.Tekton().V1alpha1().PipelineRuns()
@@ -140,6 +141,7 @@ func main() {
 		clusterTaskInformer,
 		taskRunInformer,
 		resourceInformer,
+		conditionInformer,
 		timeoutHandler,
 	)
 	// Build all of our controllers, with the clients constructed above.

examples/pipelineruns/conditional-pipelinerun.yaml

@@ -0,0 +1,79 @@
+apiVersion: tekton.dev/v1alpha1
+kind: Condition
+metadata:
+  name: always-true
+spec:
+ check:
+    image: ubuntu
+    command: ["/bin/bash"]
+    args: ['-c', 'exit 0']
+---
+apiVersion: tekton.dev/v1alpha1
+kind: Condition
+metadata:
+  name: always-false
+spec:
+  params:
+  check:
+    image: ubuntu
+    command: ["/bin/bash"]
+    args: ['-c', 'exit 1']
+---
+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
+      taskRef:
+        name: list-files
+      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"
 )