Add earlier validation of pipeline result references

[ghost]

Feature request

At the moment we fail a PipelineRun with InvalidTaskResultReference when we're scheduling the next TaskRuns to execute. This can happen after some tasks in the pipeline have already executed, which takes time. It should be possible to inspect all referenced Tasks before a PipelineRun starts and confirm that any references to Task Results from variables in the Pipeline are not misspelled or otherwise invalid. This would result in a shorter time to failure and a shorter time to the user realizing their mistake.

Use case

When a user submits a PipelineRun they could receive an earlier error message if any variables in the Pipeline refer to results that simply don't exist on the tasks being referenced.

More Details

We'll need to decide if we're happy to pay whatever cost there is holding back the PipelineRun's start of execution until the reconciler has fetched and checked every referenced Task and its results.

Turns out we already fetch all the tasks as part of each PipelineRun reconcile. So it should just be a case of validating the relationships between them on the first reconcile.

[bobcatfish]

Was discussing with @jerop today and I think it's totally worth it! We could probably add it in such away that it only happens once per Run

[pritidesai]

Just to be clear, this validation is also needed for a task result reference in a PipelineTask in addition to pipeline results.

We have tasks section validation making sure referenced task exist but not validating the result reference is defined in the task's result section.

[ghost]

Another use-case for reading all the Tasks in a Pipeline at startup time: It would be useful to check that a workspace marked "optional" at the pipeline level is indeed also "optional" in all the Tasks that want to consume that workspace. Because if not that should probably be a validation error.

[souleb]

can work on that

[ghost]

Hey @souleb, thanks for jumping in. I'm halfway through implementing but will unassign myself and ping you if I'm not able to finish.