Emit events from the PipelineRun controller

Emit events:
- Pipeline Start
- Pipeline Started Running
- Various Error Conditions, Cancel, Timeout

Emit all events through the events.go module.
Align and simplify the reconcile structure to have clear points
for error handling and emitting events.

Added test for the normal reconcile to completion case.
Added more event checks to existing tests.

docs/events.md

@@ -27,13 +27,21 @@ No events are emitted for `Conditions` today (https://github.com/tektoncd/pipeli
   successfully, including post-steps injected by Tekton.
 - `Failed`: this is triggered if the `TaskRun` is completed, but not successfully.
   Causes of failure may be: one the steps failed, the `TaskRun` was cancelled or
-  the `TaskRun` timed out.
+  the `TaskRun` timed out. `Failed` events are also triggered in case the `TaskRun`
+  cannot be executed at all because of validation issues.
 
 ## PipelineRuns
 
 `PipelineRun` events are generated for the following `Reasons`:
+- `Started`: this is triggered the first time the `PipelineRun` is picked by the
+  reconciler from its work queue, so it only happens if web-hook validation was
+  successful. Note that this event does not imply that a step started executing,
+  as pipeline, task and bound resource validation must be successful first.
+- `Running`: this is triggered when the `PipelineRun` passes validation and
+  actually starts running.
 - `Succeeded`: this is triggered once all `Tasks` reachable via the DAG are
   executed successfully.
 - `Failed`: this is triggered if the `PipelineRun` is completed, but not
   successfully. Causes of failure may be: one the `Tasks` failed or the
-  `PipelineRun` was cancelled.
+  `PipelineRun` was cancelled or timed out. `Failed` events are also triggered
+  in case the `PipelineRun` cannot be executed at all because of validation issues.

pkg/reconciler/pipelinerun/controller.go

@@ -67,6 +67,7 @@ func NewController(namespace string, images pipeline.Images) func(context.Contex
 			KubeClientSet:     kubeclientset,
 			PipelineClientSet: pipelineclientset,
 			Logger:            logger,
+			Recorder:          controller.GetEventRecorder(ctx),
 		}
 
 		c := &Reconciler{