Add doc for break feature for loop

[pugangxa]

The feature to support break in pipelineloop is already implemented. This is to add the document for that.

Can refer to the code

kfp-tekton/tekton-catalog/pipeline-loops/pkg/reconciler/pipelinelooprun/pipelinelooprun.go

Lines 495 to 514 in c835d14

	lastLoopTask := ""
	for key, val := range run.ObjectMeta.Labels {
	if key == "last-loop-task" {
	lastLoopTask = val
	}
	}
	if lastLoopTask != "" {
	skippedTaskList := pr.Status.SkippedTasks
	for _, task := range skippedTaskList {
	if task.Name == lastLoopTask {
	// Mark run successful and stop the loop pipelinerun
	run.Status.MarkRunSucceeded(pipelineloopv1alpha1.PipelineLoopRunReasonSucceeded.String(),
	"PipelineRuns completed successfully with the conditions are met")
	run.Status.Results = []runv1alpha1.RunResult{{
	Name: "condition",
	Value: "pass",
	}}
	}
	}
	}

I just run the example https://github.com/kubeflow/kfp-tekton/blob/master/tekton-catalog/pipeline-loops/examples/pipelinespec-with-run-condition.yaml and it works well, you can see the loop is defined to run 3 times but break when condition met while just run 2 times.

➜  examples git:(master) ✗ k describe run simplepipelinelooprun
Name:         simplepipelinelooprun
Namespace:    gang-test
Labels:       custom.tekton.dev/pipelineLoop=conditionloop
              last-loop-task=task-fail
Annotations:  kubectl.kubernetes.io/last-applied-configuration:
                {"apiVersion":"custom.tekton.dev/v1alpha1","kind":"PipelineLoop","metadata":{"annotations":{},"name":"conditionloop","namespace":"gang-tes...
API Version:  tekton.dev/v1alpha1
Kind:         Run
......................
Events:
  Type    Reason     Age                    From              Message
  ----    ------     ----                   ----              -------
  Normal  Started    2m36s (x2 over 2m36s)  run-pipelineloop
  Normal  Running    2m36s (x2 over 2m36s)  run-pipelineloop  Iterations completed: 0
  Normal  Running    2m10s                  run-pipelineloop  Iterations completed: 1
  Normal  Succeeded  2m5s (x2 over 2m5s)    run-pipelineloop  PipelineRuns completed successfully with the conditions are met

[pugangxa]

@Udiknedormin I have added the document for break feature for loop, please let me know if something unclear.
Note:

• Now there's no DSL interface in dsl.ParallelFor, may need enhance.
• Only one task can be specified now, may need enhance. Before remember we considered to specify it in CRD and support list but seems it's enough with current design.
• For parallelism it will just break and stop starting new pipelineruns but the ones that already started will continue running.

[Tomcli]

@pugangxa @Udiknedormin is this PR ready to merge?

For DSL support on this feature, we probably need to know some workflow use cases. If we just want to experiment with it, we can use the task annotations as the feature flag to enable breaks.

[pugangxa]

@pugangxa @Udiknedormin is this PR ready to merge?

For DSL support on this feature, we probably need to know some workflow use cases. If we just want to experiment with it, we can use the task annotations as the feature flag to enable breaks.

Yes there's requirements to support break in loop and this is to show the ability of that in pipeline loop now, I think we can let it go currently and enhance later when requirement is more clear, thanks.

[Tomcli]

/lgtm
/approve

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