Support for optional input artifacts

[mthx]

FEATURE REQUEST:

In order to support building workflows out of reusable templates, I'd like to see Argo support optional input artifacts. At the moment, failing to provide an input artifact when a step calls a template is always an error. Parameters are already optional if they have a default value.

Has this been considered?

[jessesuen]

In other words, are you asking for default field for artifacts? I think it is possible for artifacts to follow the same semantics as parameters. If there is a default value for an input artifact, then it becomes optional.

[mthx]

I was hoping for some way to say that a file was intentionally not supplied, so the container can detect it's missing and react accordingly (e.g. a shell script could omit an optional argument to a tool).

[jessesuen]

Fixed in #1277.

Example: https://github.com/argoproj/argo/blob/master/test/e2e/functional/input-artifact-optional.yaml