Signal to self error in Workflow code

Expected Behavior

A workflow should be able to signal any workflow using a workflow ID, including itself.

Actual Behavior

Throws a UnknownExternalWorkflowExecution when attempting to signal itself.

Steps to Reproduce the Problem

1. Try the following workflow code:

func TestWorkflow(ctx workflow.Context) error {
	we := workflow.GetInfo(ctx).WorkflowExecution
	err := workflow.SignalExternalWorkflow(ctx, we.ID, we.RunID, "messages", "hello").Get(ctx, nil)
	if err != nil {
		panic(fmt.Errorf("could not signal external workflow: %v", err))
	}
	return nil
}

2. Error shows in temporal web history:
   

3. The worker logs the error:

2020/08/18 21:29:57 ERROR Workflow panic. Namespace default TaskQueue test WorkerID 465773@beast-ubuntu@ WorkflowID TestWorkflow RunID 254db1e2-ebcc-489a-9835-1f5e2e3fb8f8 PanicError could not signal external workflow: UnknownExternalWorkflowExecution PanicStack coroutine root [panic]:
myproject.com/domain/root.TestWorkflow(0x1003500, 0xc000498a00, 0x0, 0x0)
        /home/paul/myproject.com/domain/root/testworkflow.go:41 +0x17a
reflect.Value.call(0xd69c40, 0xefbd48, 0x13, 0xeaf2fa, 0x4, 0xc000486de0, 0x1, 0x1, 0xfd2270, 0xe8e420, ...)
        /usr/local/go/src/reflect/value.go:460 +0x8ab
reflect.Value.Call(0xd69c40, 0xefbd48, 0x13, 0xc000486de0, 0x1, 0x1, 0xfd2270, 0xe8e420, 0xc0004b4640)
        /usr/local/go/src/reflect/value.go:321 +0xb4
go.temporal.io/sdk/internal.(*workflowEnvironmentInterceptor).ExecuteWorkflow(0xc0004b4640, 0x1003500, 0xc000498a00, 0xc0000362c0, 0xc, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
        /home/paul/go-sdk/internal/workflow.go:370 +0x2b2
go.temporal.io/sdk/internal.(*workflowExecutor).Execute(0xc000498940, 0x1003500, 0xc000498a00, 0x0, 0xc0004f3738, 0xc68596, 0x0)
        /home/paul/go-sdk/internal/internal_worker.go:759 +0x334
go.temporal.io/sdk/internal.(*syncWorkflowDefinition).Execute.func1(0x10036c0, 0xc00009b0e0)
        /home/paul/go-sdk/internal/internal_workflow.go:491 +0xf3
^C2020/08/18 21:30:00 INFO  Stopped Worker Namespace default TaskQueue test WorkerID 465773@beast-ubuntu@

Specifications

• Version: 0.28.0
• Platform: Helm-chart

Start by reproducing the failure with workflow.SignalExternalWorkflow using the current workflow ID and run ID, then inspect the Temporal service path that handles the self-signal. Compare the worker error with the temporal web history. Done means a workflow can signal itself without UnknownExternalWorkflowExecution.