updating doc to add note on task results

pipeline exits with failure if receiving task can not resolve task results
from dependent task.

docs/pipelines.md

@@ -540,6 +540,13 @@ params:
     value: "$(tasks.checkout-source.results.commit)"
 ```
 
+**Note:** If `checkout-source` exits successfully without initializing `commit` `Result`,
+the receiving `Task` fails and causes the `Pipeline` to fail with `InvalidTaskResultReference`:
+
+```
+unable to find result referenced by param 'foo' in 'task';: Could not find result with name 'commit' for task run 'checkout-source'
+```
+
 In the snippet below, a `WhenExpression` is provided its value from the `exists` `Result` emitted by the
 `check-file` `Task`. Tekton will make sure that the `check-file` `Task` runs before this one.
 

pkg/reconciler/pipelinerun/pipelinerun.go

@@ -100,6 +100,9 @@ const (
 	// ReasonCouldntCancel indicates that a PipelineRun was cancelled but attempting to update
 	// all of the running TaskRuns as cancelled failed.
 	ReasonCouldntCancel = "PipelineRunCouldntCancel"
+	// ReasonInvalidTaskResultReference indicates a task result was declared
+	// but was not initialized by that task
+	ReasonInvalidTaskResultReference = "InvalidTaskResultReference"
 )
 
 // Reconciler implements controller.Reconciler for Configuration resources.
@@ -548,8 +551,8 @@ func (c *Reconciler) runNextSchedulableTask(ctx context.Context, pr *v1beta1.Pip
 
 	resolvedResultRefs, err := resources.ResolveResultRefs(pipelineRunFacts.State, nextRprts)
 	if err != nil {
-		logger.Infof("Failed to resolve all task params for %q with error %v", pr.Name, err)
-		pr.Status.MarkFailed(ReasonFailedValidation, err.Error())
+		logger.Infof("Failed to resolve task result reference for %q with error %v", pr.Name, err)
+		pr.Status.MarkFailed(ReasonInvalidTaskResultReference, err.Error())
 		return controller.NewPermanentError(err)
 	}