Document how to write a Composition Function

[negz]

What problem are you facing?

We expect that there may be multiple Composition Function runner implementations. When designing Composition Functions we realized that it was tough to find a perfect solution considering all of the tradeoffs (running functions securely, running many functions, making guarantees around starting functions quickly, etc). We should provide guidance for folks who might want to implement their own Function runner.

We have a reference Composition Function runner (xfn) implementation, and strongly typed I/O in the form of a RunFunctionRequest. We're missing (at least):

• gRPC encryption and authentication requirements.
• gRPC status code meanings.
• How to turn a RunFunctionRequest into an OCI runtime spec.
• How to pass FunctionIO to and from an OCI container.
• Expectations of the OCI containers we run (i.e. they need a baked in ENTRYPOINT)

How could Crossplane help solve your problem?

We might consider adding a specification to formally describe all these requirements.

[github-actions]

Crossplane does not currently have enough maintainers to address every issue and pull request. This issue has been automatically marked as stale because it has had no activity in the last 90 days. It will be closed in 14 days if no further activity occurs. Leaving a comment starting with /fresh will mark this issue as not stale.

[jbw976]

/fresh we are tracking this in crossplane/crossplane#3751

[negz]

This feels a little less important with crossplane/crossplane#4306, since everything will end up in the protobuf RunFunctionRequest and RunFunctionResponse. I'm going to keep this issue for now, but I think it might become more "document how to write a Function" in future.