CHASM/System Nexus: SignalWithStart doesn't validate cron schedules

Expected Behavior

SDK Client's SignalWithStartWorkflowExecution is usually validated by the Frontend here: service/frontend/workflow_handler.go#L2336

The SDK team is working on supporting SignalWithStartWorkflowExecution from inside a workflow, via System Nexus. The relevant system nexus validation code is here: chasm/lib/workflow/nexus_service.go#L79

I expect the server to do the same validation on signal-with-start requests in system nexus as it does in the frontend. In particular, the frontend invokes backoff.ValidateSchedule to check that cron strings are syntactically valid.

Actual Behavior

temporal-server/chasm/lib/workflow/nexus_service.go doesn't seem to invoke backoff.ValidateSchedule on cron strings. Maybe we want to move cron validation from WorkflowHandler.SignalWithStartWorkflowExecution to RequestValidator.ValidateSignalWithStartRequest?

Steps to Reproduce the Problem

Specifications

• Version:
• Platform:

Start with chasm/lib/workflow/nexus_service.go#L79 and compare its SignalWithStart validation with service/frontend/workflow_handler.go#L2336, including the existing backoff.ValidateSchedule call. Check whether RequestValidator.ValidateSignalWithStartRequest is the right shared entry point; done means System Nexus rejects syntactically invalid cron schedules like the frontend path does.