Add comment about not adding more signal reasons

Author: jordandcarter

agent/run_job.go

@@ -24,11 +24,23 @@ import (
 )
 
 const (
+	// Signal reasons
 	SignalReasonAgentRefused      = "agent_refused"
 	SignalReasonAgentStop         = "agent_stop"
 	SignalReasonCancel            = "cancel"
 	SignalReasonSignatureRejected = "signature_rejected"
 	SignalReasonProcessRunError   = "process_run_error"
+	// Don't add more signal reasons. If you must add a new signal reason, it must also be added to
+	// the Job::Event::SignalReason enum in the rails app.
+	//
+	// They are meant to represent the reason a job was stopped, but they've also been used to
+	// represent the reason a job wasn't started at all. This is fine but we don't want to pile more
+	// on as customers catch these signal reasons when configuring retry attempts. When we add more
+	// signal reasons we force customers to update their retry configurations to catch the new signal
+	// reasons.
+	//
+	// We should consider adding new fields 'not_run_reason' and 'not_run_details' instead of adding
+	// more signal reasons.
 )
 
 type missingKeyError struct {